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Sci.Electronics.Basics -> Driving Dual MOSFETs From A NPN Transistor

There are 26 messages in this thread.
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Author: Michael
Date: 18:49 12-12-06

Hi,

I'm working on a circuit that uses a NPN transistor to drive (i think that's
the right word) two MOSFETs. Could someone please explain how to work out
the theory behind it? For example what value resistors I need between the
gates and emitter, etc. (i presume I need some). I'd like to be able to work
it out myself as opposed to asking you guys for a 'solution' but sadly I'm
struggling.....

So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
introduction, transistor introduction but to no avail....If it really just a
case of wiring the MOSFETs' gates together and connecting them directly to
the emitter? Any 'rules' to doing this sort of thing?

Thanks In Advance,

Michael

Author: Phil Allison
Date: 18:54 12-12-06

"Michael"

>
> I'm working on a circuit that uses a NPN transistor to drive (i think
> that's the right word) two MOSFETs. Could someone please explain how to
> work out the theory behind it? For example what value resistors I need
> between the gates and emitter, etc. (i presume I need some). I'd like to
> be able to work it out myself as opposed to asking you guys for a
> 'solution' but sadly I'm struggling.....
>
> So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
> introduction, transistor introduction but to no avail....If it really just
> a case of wiring the MOSFETs' gates together and connecting them directly
> to the emitter? Any 'rules' to doing this sort of thing?

** You must supply more info.

Are the MOSFETS wired in parallel to increase current ?

Are the MOSFETS switching or linear types ?

Is you circuit switching or linear ?

Etc.

........ Phil

Author: jasen
Date: 14:04 13-12-06

On 2006-12-12, Michael <mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq>
wrote:
> Hi,

> I'm working on a circuit that uses a NPN transistor to drive (i think that's
> the right word) two MOSFETs. Could someone please explain how to work out
> the theory behind it? For example what value resistors I need between the
> gates and emitter, etc. (i presume I need some). I'd like to be able to work
> it out myself as opposed to asking you guys for a 'solution' but sadly I'm
> struggling.....

what you describe suggests that you have the NPN transistor manipulating the
voltage on the gates of the mosfets, that effects their resistance.

not much more can be said without seeing the types of transistors and how it's
all hooked up.

> So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
> introduction, transistor introduction but to no avail....If it really just a
> case of wiring the MOSFETs' gates together and connecting them directly to
> the emitter? Any 'rules' to doing this sort of thing?

plenty of rules, the more info you give the better.

--

Bye.
Jasen

Author: Michael
Date: 15:53 15-12-06

"jasen" <jasen@free.net.nz> wrote in message
news:elpis9$k2$2@jasen.is-a-geek.org...
> On 2006-12-12, Michael
> <mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
>> Hi,
>
>> I'm working on a circuit that uses a NPN transistor to drive (i think
>> that's
>> the right word) two MOSFETs. Could someone please explain how to work
>> out
>> the theory behind it? For example what value resistors I need between the
>> gates and emitter, etc. (i presume I need some). I'd like to be able to
>> work
>> it out myself as opposed to asking you guys for a 'solution' but sadly
>> I'm
>> struggling.....
>
> what you describe suggests that you have the NPN transistor manipulating
> the
> voltage on the gates of the mosfets, that effects their resistance.
>
> not much more can be said without seeing the types of transistors and how
> it's
> all hooked up.
>
>> So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
>> introduction, transistor introduction but to no avail....If it really
>> just a
>> case of wiring the MOSFETs' gates together and connecting them directly
>> to
>> the emitter? Any 'rules' to doing this sort of thing?
>
> plenty of rules, the more info you give the better.
>
>
>
> --
>
> Bye.
> Jasen

Hi,

Sorry for not getting back sooner,

I've drawn you a schematic and uploaded it to:
http://www.mhims.co.uk/MOSFET%20Schematic.png

I'm trying to work out a suitable transistor (Q3) to use and it's base
resistor (R1) value... From what I've read in the mosfets datasheet
(http://www.rapidonline.com/netalogue/specs/47-0530.pdf) 2-4V needs to be
applied to the gates so that's where I get the 3V value from and the 36V
figure is from the onboard batteries (it's for a robot).

Where should I go from here?

Thanks,

Michael

Author: PeteS
Date: 16:47 15-12-06

Michael wrote:
> "jasen" <jasen@free.net.nz> wrote in message
> news:elpis9$k2$2@jasen.is-a-geek.org...
>> On 2006-12-12, Michael
>> <mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
>>> Hi,
>>> I'm working on a circuit that uses a NPN transistor to drive (i think
>>> that's
>>> the right word) two MOSFETs. Could someone please explain how to work
>>> out
>>> the theory behind it? For example what value resistors I need between the
>>> gates and emitter, etc. (i presume I need some). I'd like to be able to
>>> work
>>> it out myself as opposed to asking you guys for a 'solution' but sadly
>>> I'm
>>> struggling.....
>> what you describe suggests that you have the NPN transistor manipulating
>> the
>> voltage on the gates of the mosfets, that effects their resistance.
>>
>> not much more can be said without seeing the types of transistors and how
>> it's
>> all hooked up.
>>
>>> So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
>>> introduction, transistor introduction but to no avail....If it really
>>> just a
>>> case of wiring the MOSFETs' gates together and connecting them directly
>>> to
>>> the emitter? Any 'rules' to doing this sort of thing?
>> plenty of rules, the more info you give the better.
>>
>>
>>
>> --
>>
>> Bye.
>> Jasen
>
> Hi,
>
> Sorry for not getting back sooner,
>
> I've drawn you a schematic and uploaded it to:
> http://www.mhims.co.uk/MOSFET%20Schematic.png
>
> I'm trying to work out a suitable transistor (Q3) to use and it's base
> resistor (R1) value... From what I've read in the mosfets datasheet
> (http://www.rapidonline.com/netalogue/specs/47-0530.pdf) 2-4V needs to be
> applied to the gates so that's where I get the 3V value from and the 36V
> figure is from the onboard batteries (it's for a robot).
>
> Where should I go from here?
>
> Thanks,
>
> Michael
>
>

Looking at this, you could do the following:

1. Replace Q3 with a logic level MOSFET (the venerable VN2222LL would be
perfect), and change the sense completely. As shown, the circuit really
won't work that well.

Either use a PNP with emitter to positive rail or use a P-channel MOSFET
(Source to positive rail). If you need to drive with 3V or 3.3V logic,
then there are solutions. The simplest fix here (assuming 3.3V logic)
and the fact you need 5V for gate drive (see 3) is to

a. Remove Q3 emitter from the gates and tie it to ground
b. Remove Q3 collector from supply, and pull it to the supply via a 10k
resistor
c. Connect a 2N3096 with collector to gates, base to Q3 collector via a
1k resistor, emitter to 5V

2. Add a pulldown from the MOSFET gates of 10k or so. There has to be a
DC return, and a resistor to ground is the simplest.

3. You need to provide a minimum of 4V to the gates of Q1 and Q2. This
is the only way you are guaranteed to exceed Vgs(th). Better would be
5V, so change the power supplied to the collector [drain if you take the
advice in 1] to that level.
Note your TTL circuitry could be operating on 3V and the VN2222LL would
work fine as it would if it were a bipolar device.

4. I assume M1 and M2 are motors. As inductive loads, you need to put
diodes across them; anode to the MOSFET drain, cathode to positive power
for the motor. If you don't, the MOSFETS will operate once, and at the
first turn off, they will be toast (literally).

Cheers

PeteS

Author: Michael
Date: 17:09 15-12-06

"PeteS" <peter.smith8380@ntlworld.com> wrote in message
news:DJEgh.12108$Qa6.9135@newsfe6-gui.ntli.net...
> Michael wrote:
>> "jasen" <jasen@free.net.nz> wrote in message
>> news:elpis9$k2$2@jasen.is-a-geek.org...
>>> On 2006-12-12, Michael
>>> <mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
>>>> Hi,
>>>> I'm working on a circuit that uses a NPN transistor to drive (i think
>>>> that's
>>>> the right word) two MOSFETs. Could someone please explain how to work
>>>> out
>>>> the theory behind it? For example what value resistors I need between
>>>> the
>>>> gates and emitter, etc. (i presume I need some). I'd like to be able to

>>>> work
>>>> it out myself as opposed to asking you guys for a 'solution' but sadly
>>>> I'm
>>>> struggling.....
>>> what you describe suggests that you have the NPN transistor manipulating
>>> the
>>> voltage on the gates of the mosfets, that effects their resistance.
>>>
>>> not much more can be said without seeing the types of transistors and
>>> how it's
>>> all hooked up.
>>>
>>>> So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
>>>> introduction, transistor introduction but to no avail....If it really
>>>> just a
>>>> case of wiring the MOSFETs' gates together and connecting them directly

>>>> to
>>>> the emitter? Any 'rules' to doing this sort of thing?
>>> plenty of rules, the more info you give the better.
>>>
>>>
>>>
>>> --
>>>
>>> Bye.
>>> Jasen
>>
>> Hi,
>>
>> Sorry for not getting back sooner,
>>
>> I've drawn you a schematic and uploaded it to:
>> http://www.mhims.co.uk/MOSFET%20Schematic.png
>>
>> I'm trying to work out a suitable transistor (Q3) to use and it's base
>> resistor (R1) value... From what I've read in the mosfets datasheet
>> (http://www.rapidonline.com/netalogue/specs/47-0530.pdf) 2-4V needs to be
>> applied to the gates so that's where I get the 3V value from and the 36V
>> figure is from the onboard batteries (it's for a robot).
>>
>> Where should I go from here?
>>
>> Thanks,
>>
>> Michael
>
> Looking at this, you could do the following:
>
> 1. Replace Q3 with a logic level MOSFET (the venerable VN2222LL would be
> perfect), and change the sense completely. As shown, the circuit really
> won't work that well.
>
> Either use a PNP with emitter to positive rail or use a P-channel MOSFET
> (Source to positive rail). If you need to drive with 3V or 3.3V logic,
> then there are solutions. The simplest fix here (assuming 3.3V logic) and
> the fact you need 5V for gate drive (see 3) is to
>
> a. Remove Q3 emitter from the gates and tie it to ground
> b. Remove Q3 collector from supply, and pull it to the supply via a 10k
> resistor
> c. Connect a 2N3096 with collector to gates, base to Q3 collector via a 1k
> resistor, emitter to 5V
>
> 2. Add a pulldown from the MOSFET gates of 10k or so. There has to be a DC
> return, and a resistor to ground is the simplest.
>
> 3. You need to provide a minimum of 4V to the gates of Q1 and Q2. This is
> the only way you are guaranteed to exceed Vgs(th). Better would be 5V, so
> change the power supplied to the collector [drain if you take the advice
> in 1] to that level.
> Note your TTL circuitry could be operating on 3V and the VN2222LL would
> work fine as it would if it were a bipolar device.
>
> 4. I assume M1 and M2 are motors. As inductive loads, you need to put
> diodes across them; anode to the MOSFET drain, cathode to positive power
> for the motor. If you don't, the MOSFETS will operate once, and at the
> first turn off, they will be toast (literally).
>
>
>
> Cheers
>
> PeteS

Ok thanks Pete....

Just to make sure I understand you correctly...are these right?:
http://www.mhims.co.uk/MOSFET%20Schematic2.png
OR
http://www.mhims.co.uk/MOSFET%20Schematic3.png

Michael

Author: PeteS
Date: 17:29 15-12-06

Michael wrote:
> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
> news:DJEgh.12108$Qa6.9135@newsfe6-gui.ntli.net...
>> Michael wrote:
>>> "jasen" <jasen@free.net.nz> wrote in message
>>> news:elpis9$k2$2@jasen.is-a-geek.org...
>>>> On 2006-12-12, Michael
>>>> <mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
>>>>> Hi,
>>>>> I'm working on a circuit that uses a NPN transistor to drive (i
think
>>>>> that's
>>>>> the right word) two MOSFETs. Could someone please explain how to
work
>>>>> out
>>>>> the theory behind it? For example what value resistors I need
between
>>>>> the
>>>>> gates and emitter, etc. (i presume I need some). I'd like to be
able to
>>>>> work
>>>>> it out myself as opposed to asking you guys for a 'solution' but
sadly
>>>>> I'm
>>>>> struggling.....
>>>> what you describe suggests that you have the NPN transistor
manipulating
>>>> the
>>>> voltage on the gates of the mosfets, that effects their resistance.
>>>>
>>>> not much more can be said without seeing the types of transistors and
>>>> how it's
>>>> all hooked up.
>>>>
>>>>> So far I've googled: driving mosfet, switching mosfet, mosfet,
mosfet
>>>>> introduction, transistor introduction but to no avail....If it
really
>>>>> just a
>>>>> case of wiring the MOSFETs' gates together and connecting them
directly
>>>>> to
>>>>> the emitter? Any 'rules' to doing this sort of thing?
>>>> plenty of rules, the more info you give the better.
>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Bye.
>>>> Jasen
>>> Hi,
>>>
>>> Sorry for not getting back sooner,
>>>
>>> I've drawn you a schematic and uploaded it to:
>>> http://www.mhims.co.uk/MOSFET%20Schematic.png
>>>
>>> I'm trying to work out a suitable transistor (Q3) to use and it's base
>>> resistor (R1) value... From what I've read in the mosfets datasheet
>>> (http://www.rapidonline.com/netalogue/specs/47-0530.pdf) 2-4V needs to be
>>> applied to the gates so that's where I get the 3V value from and the 36V
>>> figure is from the onboard batteries (it's for a robot).
>>>
>>> Where should I go from here?
>>>
>>> Thanks,
>>>
>>> Michael
>> Looking at this, you could do the following:
>>
>> 1. Replace Q3 with a logic level MOSFET (the venerable VN2222LL would be
>> perfect), and change the sense completely. As shown, the circuit really
>> won't work that well.
>>
>> Either use a PNP with emitter to positive rail or use a P-channel MOSFET
>> (Source to positive rail). If you need to drive with 3V or 3.3V logic,
>> then there are solutions. The simplest fix here (assuming 3.3V logic) and
>> the fact you need 5V for gate drive (see 3) is to
>>
>> a. Remove Q3 emitter from the gates and tie it to ground
>> b. Remove Q3 collector from supply, and pull it to the supply via a 10k
>> resistor
>> c. Connect a 2N3096 with collector to gates, base to Q3 collector via a 1k
>> resistor, emitter to 5V
>>
>> 2. Add a pulldown from the MOSFET gates of 10k or so. There has to be a DC
>> return, and a resistor to ground is the simplest.
>>
>> 3. You need to provide a minimum of 4V to the gates of Q1 and Q2. This is
>> the only way you are guaranteed to exceed Vgs(th). Better would be 5V, so
>> change the power supplied to the collector [drain if you take the advice
>> in 1] to that level.
>> Note your TTL circuitry could be operating on 3V and the VN2222LL would
>> work fine as it would if it were a bipolar device.
>>
>> 4. I assume M1 and M2 are motors. As inductive loads, you need to put
>> diodes across them; anode to the MOSFET drain, cathode to positive power
>> for the motor. If you don't, the MOSFETS will operate once, and at the
>> first turn off, they will be toast (literally).
>>
>>
>>
>> Cheers
>>
>> PeteS
>
> Ok thanks Pete....
>
> Just to make sure I understand you correctly...are these right?:
> http://www.mhims.co.uk/MOSFET%20Schematic2.png
> OR
> http://www.mhims.co.uk/MOSFET%20Schematic3.png
>
> Michael
>
>
>

In http://www.mhims.co.uk/MOSFET%20Schematic2.png

1. Change Q4 to a 2N3904 or a VN2222LL

2. Remove R2 or R3 (only one needed)

3. D1 and D2 should be across the motors, not the FETs

Then we're starting to get there.

Cheers

PeteS

Author: Michael
Date: 17:41 15-12-06

"PeteS" <peter.smith8380@ntlworld.com> wrote in message
news:zlFgh.2206$v4.1702@newsfe3-win.ntli.net...
> Michael wrote:
>> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
>> news:DJEgh.12108$Qa6.9135@newsfe6-gui.ntli.net...
>>> Michael wrote:
>>>> "jasen" <jasen@free.net.nz> wrote in message
>>>> news:elpis9$k2$2@jasen.is-a-geek.org...
>>>>> On 2006-12-12, Michael
>>>>> <mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq>
wrote:
>>>>>> Hi,
>>>>>> I'm working on a circuit that uses a NPN transistor to drive (i
think
>>>>>> that's
>>>>>> the right word) two MOSFETs. Could someone please explain how
to
>>>>>> work out
>>>>>> the theory behind it? For example what value resistors I need
between
>>>>>> the
>>>>>> gates and emitter, etc. (i presume I need some). I'd like to be
able
>>>>>> to work
>>>>>> it out myself as opposed to asking you guys for a 'solution'
but
>>>>>> sadly I'm
>>>>>> struggling.....
>>>>> what you describe suggests that you have the NPN transistor
>>>>> manipulating the
>>>>> voltage on the gates of the mosfets, that effects their resistance.
>>>>>
>>>>> not much more can be said without seeing the types of transistors
and
>>>>> how it's
>>>>> all hooked up.
>>>>>
>>>>>> So far I've googled: driving mosfet, switching mosfet, mosfet,
mosfet
>>>>>> introduction, transistor introduction but to no avail....If it
really
>>>>>> just a
>>>>>> case of wiring the MOSFETs' gates together and connecting them
>>>>>> directly to
>>>>>> the emitter? Any 'rules' to doing this sort of thing?
>>>>> plenty of rules, the more info you give the better.
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>>
>>>>> Bye.
>>>>> Jasen
>>>> Hi,
>>>>
>>>> Sorry for not getting back sooner,
>>>>
>>>> I've drawn you a schematic and uploaded it to:
>>>> http://www.mhims.co.uk/MOSFET%20Schematic.png
>>>>
>>>> I'm trying to work out a suitable transistor (Q3) to use and it's base
>>>> resistor (R1) value... From what I've read in the mosfets datasheet
>>>> (http://www.rapidonline.com/netalogue/specs/47-0530.pdf) 2-4V needs to
>>>> be applied to the gates so that's where I get the 3V value from and the

>>>> 36V figure is from the onboard batteries (it's for a robot).
>>>>
>>>> Where should I go from here?
>>>>
>>>> Thanks,
>>>>
>>>> Michael
>>> Looking at this, you could do the following:
>>>
>>> 1. Replace Q3 with a logic level MOSFET (the venerable VN2222LL would be
>>> perfect), and change the sense completely. As shown, the circuit really
>>> won't work that well.
>>>
>>> Either use a PNP with emitter to positive rail or use a P-channel MOSFET
>>> (Source to positive rail). If you need to drive with 3V or 3.3V logic,
>>> then there are solutions. The simplest fix here (assuming 3.3V logic)
>>> and the fact you need 5V for gate drive (see 3) is to
>>>
>>> a. Remove Q3 emitter from the gates and tie it to ground
>>> b. Remove Q3 collector from supply, and pull it to the supply via a 10k
>>> resistor
>>> c. Connect a 2N3096 with collector to gates, base to Q3 collector via a
>>> 1k resistor, emitter to 5V
>>>
>>> 2. Add a pulldown from the MOSFET gates of 10k or so. There has to be a
>>> DC return, and a resistor to ground is the simplest.
>>>
>>> 3. You need to provide a minimum of 4V to the gates of Q1 and Q2. This
>>> is the only way you are guaranteed to exceed Vgs(th). Better would be
>>> 5V, so change the power supplied to the collector [drain if you take the
>>> advice in 1] to that level.
>>> Note your TTL circuitry could be operating on 3V and the VN2222LL would
>>> work fine as it would if it were a bipolar device.
>>>
>>> 4. I assume M1 and M2 are motors. As inductive loads, you need to put
>>> diodes across them; anode to the MOSFET drain, cathode to positive power
>>> for the motor. If you don't, the MOSFETS will operate once, and at the
>>> first turn off, they will be toast (literally).
>>>
>>>
>>>
>>> Cheers
>>>
>>> PeteS
>>
>> Ok thanks Pete....
>>
>> Just to make sure I understand you correctly...are these right?:
>> http://www.mhims.co.uk/MOSFET%20Schematic2.png
>> OR
>> http://www.mhims.co.uk/MOSFET%20Schematic3.png
>>
>> Michael
>>
>>
>>
>
> In http://www.mhims.co.uk/MOSFET%20Schematic2.png
>
> 1. Change Q4 to a 2N3904 or a VN2222LL
>
> 2. Remove R2 or R3 (only one needed)
>
> 3. D1 and D2 should be across the motors, not the FETs
>
> Then we're starting to get there.
>
> Cheers
>
> PeteS
Thanks, so this ok then?:
http://www.mhims.co.uk/MOSFET%20Schematic4.png

Cheers,

Michael

Author: PeteS
Date: 17:48 15-12-06

Michael wrote:
> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
> news:zlFgh.2206$v4.1702@newsfe3-win.ntli.net...
>> Michael wrote:
>>> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
>>> news:DJEgh.12108$Qa6.9135@newsfe6-gui.ntli.net...
>>>> Michael wrote:
>>>>> "jasen" <jasen@free.net.nz> wrote in message
>>>>> news:elpis9$k2$2@jasen.is-a-geek.org...
>>>>>> On 2006-12-12, Michael
>>>>>>
<mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
>>>>>>> Hi,
>>>>>>> I'm working on a circuit that uses a NPN transistor to
drive (i think
>>>>>>> that's
>>>>>>> the right word) two MOSFETs. Could someone please explain
how to
>>>>>>> work out
>>>>>>> the theory behind it? For example what value resistors I
need between
>>>>>>> the
>>>>>>> gates and emitter, etc. (i presume I need some). I'd like
to be able
>>>>>>> to work
>>>>>>> it out myself as opposed to asking you guys for a
'solution' but
>>>>>>> sadly I'm
>>>>>>> struggling.....
>>>>>> what you describe suggests that you have the NPN transistor
>>>>>> manipulating the
>>>>>> voltage on the gates of the mosfets, that effects their
resistance.
>>>>>>
>>>>>> not much more can be said without seeing the types of
transistors and
>>>>>> how it's
>>>>>> all hooked up.
>>>>>>
>>>>>>> So far I've googled: driving mosfet, switching mosfet,
mosfet, mosfet
>>>>>>> introduction, transistor introduction but to no avail....If
it really
>>>>>>> just a
>>>>>>> case of wiring the MOSFETs' gates together and connecting
them
>>>>>>> directly to
>>>>>>> the emitter? Any 'rules' to doing this sort of thing?
>>>>>> plenty of rules, the more info you give the better.
>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>>
>>>>>> Bye.
>>>>>> Jasen
>>>>> Hi,
>>>>>
>>>>> Sorry for not getting back sooner,
>>>>>
>>>>> I've drawn you a schematic and uploaded it to:
>>>>> http://www.mhims.co.uk/MOSFET%20Schematic.png
>>>>>
>>>>> I'm trying to work out a suitable transistor (Q3) to use and it's
base
>>>>> resistor (R1) value... From what I've read in the mosfets datasheet

>>>>> (http://www.rapidonline.com/netalogue/specs/47-0530.pdf) 2-4V needs
to
>>>>> be applied to the gates so that's where I get the 3V value from and
the
>>>>> 36V figure is from the onboard batteries (it's for a robot).
>>>>>
>>>>> Where should I go from here?
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Michael
>>>> Looking at this, you could do the following:
>>>>
>>>> 1. Replace Q3 with a logic level MOSFET (the venerable VN2222LL would
be
>>>> perfect), and change the sense completely. As shown, the circuit really

>>>> won't work that well.
>>>>
>>>> Either use a PNP with emitter to positive rail or use a P-channel
MOSFET
>>>> (Source to positive rail). If you need to drive with 3V or 3.3V logic,
>>>> then there are solutions. The simplest fix here (assuming 3.3V logic)
>>>> and the fact you need 5V for gate drive (see 3) is to
>>>>
>>>> a. Remove Q3 emitter from the gates and tie it to ground
>>>> b. Remove Q3 collector from supply, and pull it to the supply via a 10k

>>>> resistor
>>>> c. Connect a 2N3096 with collector to gates, base to Q3 collector via a

>>>> 1k resistor, emitter to 5V
>>>>
>>>> 2. Add a pulldown from the MOSFET gates of 10k or so. There has to be a

>>>> DC return, and a resistor to ground is the simplest.
>>>>
>>>> 3. You need to provide a minimum of 4V to the gates of Q1 and Q2. This
>>>> is the only way you are guaranteed to exceed Vgs(th). Better would be
>>>> 5V, so change the power supplied to the collector [drain if you take
the
>>>> advice in 1] to that level.
>>>> Note your TTL circuitry could be operating on 3V and the VN2222LL would

>>>> work fine as it would if it were a bipolar device.
>>>>
>>>> 4. I assume M1 and M2 are motors. As inductive loads, you need to put
>>>> diodes across them; anode to the MOSFET drain, cathode to positive
power
>>>> for the motor. If you don't, the MOSFETS will operate once, and at the
>>>> first turn off, they will be toast (literally).
>>>>
>>>>
>>>>
>>>> Cheers
>>>>
>>>> PeteS
>>> Ok thanks Pete....
>>>
>>> Just to make sure I understand you correctly...are these right?:
>>> http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>> OR
>>> http://www.mhims.co.uk/MOSFET%20Schematic3.png
>>>
>>> Michael
>>>
>>>
>>>
>> In http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>
>> 1. Change Q4 to a 2N3904 or a VN2222LL
>>
>> 2. Remove R2 or R3 (only one needed)
>>
>> 3. D1 and D2 should be across the motors, not the FETs
>>
>> Then we're starting to get there.
>>
>> Cheers
>>
>> PeteS
> Thanks, so this ok then?:
> http://www.mhims.co.uk/MOSFET%20Schematic4.png
>
> Cheers,
>
> Michael
>
>
Your 2N3904 shows a PNP - it should be an NPN :)

Apart from that, you'll need some decoupling - put a 0.1uF cap from the
5V rail. The amount necessary on the 36V rail depends on the motors.

The DC ratings:

Use 10V or better rated for the 5V rail
Use 63V electrolytics on the 36V rail.

As a first cut, I would use 47uF on the 36V rail.

Cheers

PeteS

PS : I hope you are learning something :)

Author: Chris
Date: 17:49 15-12-06

Michael wrote:
> "jasen" <jasen@free.net.nz> wrote in message
> news:elpis9$k2$2@jasen.is-a-geek.org...
> > On 2006-12-12, Michael
> > <mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
> >> Hi,
> >
> >> I'm working on a circuit that uses a NPN transistor to drive (i think
> >> that's
> >> the right word) two MOSFETs. Could someone please explain how to work
> >> out
> >> the theory behind it? For example what value resistors I need between the
> >> gates and emitter, etc. (i presume I need some). I'd like to be able to
> >> work
> >> it out myself as opposed to asking you guys for a 'solution' but sadly
> >> I'm
> >> struggling.....
> >
> > what you describe suggests that you have the NPN transistor manipulating
> > the
> > voltage on the gates of the mosfets, that effects their resistance.
> >
> > not much more can be said without seeing the types of transistors and how
> > it's
> > all hooked up.
> >
> >> So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
> >> introduction, transistor introduction but to no avail....If it really
> >> just a
> >> case of wiring the MOSFETs' gates together and connecting them directly
> >> to
> >> the emitter? Any 'rules' to doing this sort of thing?
> >
> > plenty of rules, the more info you give the better.
> >
> >
> >
> > --
> >
> > Bye.
> > Jasen
>
> Hi,
>
> Sorry for not getting back sooner,
>
> I've drawn you a schematic and uploaded it to:
> http://www.mhims.co.uk/MOSFET%20Schematic.png
>
> I'm trying to work out a suitable transistor (Q3) to use and it's base
> resistor (R1) value... From what I've read in the mosfets datasheet
> (http://www.rapidonline.com/netalogue/specs/47-0530.pdf) 2-4V needs to be
> applied to the gates so that's where I get the 3V value from and the 36V
> figure is from the onboard batteries (it's for a robot).
>
> Where should I go from here?
>
> Thanks,
>
> Michael

Hi, Michael. I'd guess you're overthinking this a bit.

First, according to the data sheet (ST page 4/18), the gate threshold
voltage of the MOSFET is between 2V and 4V. That means that, with a
gate voltage of less than 2V, 100% of test devices will be OFF. At a
gate voltage of 4V, 100% of test units will be ON. It gets a little
more complex than that, though, because it kind of depends on what you
mean by "ON". You'll have to look at the graphs for that info.

If you look at the graph titled, Figure 8. Static drain-source on
resistance (ST p. 7/18), you'll see that the juicy Rds(on) spec of 0.02
ohms is with a Vgs of 10V. Since you've got a 36V supply to play with,
that should be easy. You can do this with a couple of resistors like
this (view in fixed font or M$ Notepad):

36V
+ 36V 36V 36V 36V
| + + + +
.-. | | | |
10K| | 36V | | | |
| | + / \ - / \ -
'-' | ( M ) ^ ( M ) ^
| |< \_/ | \_/ |
o-| | | | |
| |\ | | | |
.-. | 2 X STB55NF06 o----' o----'
10K| | .-. | |
| | | |6.8K ||-+ ||-+
'-' | | ___ ||<- ||<-
TTL Signal | '-' .----|___|--||-+ .---||-+
___ |/ | | 100 | | |
>-|___|-| o----o === | ===
10K |> | | GND | GND
| .-. | ___ |
| | | '----|___|----------'
=== | |2.7K 100
GND '-'
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

This is somewhat easier than your original circuit, because you don't
have to supply another regulated voltage. It would be even easier if
you could live with an active low TTL signal (TTL low = ON) instead of
the active high signal shown in your application, but there it is.

When the TTL signal is high (typically 3.5V unloaded), it will source
1/2mA or so into the base of the first transistor, turning it on. That
will turn on the second transistor, making about 10VDC available to
turn on the two MOSFETs. The 100 ohm resistors are typically used to
isolate the gates and help avoid oscillation. Be sure to use diodes
rated for the motor current, so they don't smoke to save the MOSFETs on
turn-off. Be sure to use transistors rated for the voltage (such as
the 2N4401 NPN / 2N4403 PNP).

Hope this has been of help. Not the most elegant thing, but it will
work very well.

Good luck
Chris

Author: Michael
Date: 18:08 15-12-06

"PeteS" <peter.smith8380@ntlworld.com> wrote in message
news:4DFgh.7858$493.6519@newsfe4-gui.ntli.net...
> Michael wrote:
>> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
>> news:zlFgh.2206$v4.1702@newsfe3-win.ntli.net...
>>> Michael wrote:
>>>> "PeteS" <peter.smith8380@ntlworld.com> wrote in message

>>>> news:DJEgh.12108$Qa6.9135@newsfe6-gui.ntli.net...
>>>>> Michael wrote:
>>>>>> "jasen" <jasen@free.net.nz> wrote in message
>>>>>> news:elpis9$k2$2@jasen.is-a-geek.org...
>>>>>>> On 2006-12-12, Michael
>>>>>>>
<mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
>>>>>>>> Hi,
>>>>>>>> I'm working on a circuit that uses a NPN transistor to
drive (i
>>>>>>>> think that's
>>>>>>>> the right word) two MOSFETs. Could someone please
explain how to
>>>>>>>> work out
>>>>>>>> the theory behind it? For example what value resistors
I need
>>>>>>>> between the
>>>>>>>> gates and emitter, etc. (i presume I need some). I'd
like to be
>>>>>>>> able to work
>>>>>>>> it out myself as opposed to asking you guys for a
'solution' but
>>>>>>>> sadly I'm
>>>>>>>> struggling.....
>>>>>>> what you describe suggests that you have the NPN transistor

>>>>>>> manipulating the
>>>>>>> voltage on the gates of the mosfets, that effects their
resistance.
>>>>>>>
>>>>>>> not much more can be said without seeing the types of
transistors
>>>>>>> and how it's
>>>>>>> all hooked up.
>>>>>>>
>>>>>>>> So far I've googled: driving mosfet, switching mosfet,
mosfet,
>>>>>>>> mosfet
>>>>>>>> introduction, transistor introduction but to no
avail....If it
>>>>>>>> really just a
>>>>>>>> case of wiring the MOSFETs' gates together and
connecting them
>>>>>>>> directly to
>>>>>>>> the emitter? Any 'rules' to doing this sort of thing?
>>>>>>> plenty of rules, the more info you give the better.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>>
>>>>>>> Bye.
>>>>>>> Jasen
>>>>>> Hi,
>>>>>>
>>>>>> Sorry for not getting back sooner,
>>>>>>
>>>>>> I've drawn you a schematic and uploaded it to:
>>>>>> http://www.mhims.co.uk/MOSFET%20Schematic.png
>>>>>>
>>>>>> I'm trying to work out a suitable transistor (Q3) to use and
it's
>>>>>> base resistor (R1) value... From what I've read in the mosfets
>>>>>> datasheet
(http://www.rapidonline.com/netalogue/specs/47-0530.pdf)
>>>>>> 2-4V needs to be applied to the gates so that's where I get the
3V
>>>>>> value from and the 36V figure is from the onboard batteries
(it's for
>>>>>> a robot).
>>>>>>
>>>>>> Where should I go from here?
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> Michael
>>>>> Looking at this, you could do the following:
>>>>>
>>>>> 1. Replace Q3 with a logic level MOSFET (the venerable VN2222LL
would
>>>>> be perfect), and change the sense completely. As shown, the circuit

>>>>> really won't work that well.
>>>>>
>>>>> Either use a PNP with emitter to positive rail or use a P-channel
>>>>> MOSFET (Source to positive rail). If you need to drive with 3V or
3.3V
>>>>> logic, then there are solutions. The simplest fix here (assuming
3.3V
>>>>> logic) and the fact you need 5V for gate drive (see 3) is to
>>>>>
>>>>> a. Remove Q3 emitter from the gates and tie it to ground
>>>>> b. Remove Q3 collector from supply, and pull it to the supply via a

>>>>> 10k resistor
>>>>> c. Connect a 2N3096 with collector to gates, base to Q3 collector
via
>>>>> a 1k resistor, emitter to 5V
>>>>>
>>>>> 2. Add a pulldown from the MOSFET gates of 10k or so. There has to
be
>>>>> a DC return, and a resistor to ground is the simplest.
>>>>>
>>>>> 3. You need to provide a minimum of 4V to the gates of Q1 and Q2.
This
>>>>> is the only way you are guaranteed to exceed Vgs(th). Better would
be
>>>>> 5V, so change the power supplied to the collector [drain if you
take
>>>>> the advice in 1] to that level.
>>>>> Note your TTL circuitry could be operating on 3V and the VN2222LL
>>>>> would work fine as it would if it were a bipolar device.
>>>>>
>>>>> 4. I assume M1 and M2 are motors. As inductive loads, you need to
put
>>>>> diodes across them; anode to the MOSFET drain, cathode to positive
>>>>> power for the motor. If you don't, the MOSFETS will operate once,
and
>>>>> at the first turn off, they will be toast (literally).
>>>>>
>>>>>
>>>>>
>>>>> Cheers
>>>>>
>>>>> PeteS
>>>> Ok thanks Pete....
>>>>
>>>> Just to make sure I understand you correctly...are these right?:
>>>> http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>>> OR
>>>> http://www.mhims.co.uk/MOSFET%20Schematic3.png
>>>>
>>>> Michael
>>>>
>>>>
>>>>
>>> In http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>>
>>> 1. Change Q4 to a 2N3904 or a VN2222LL
>>>
>>> 2. Remove R2 or R3 (only one needed)
>>>
>>> 3. D1 and D2 should be across the motors, not the FETs
>>>
>>> Then we're starting to get there.
>>>
>>> Cheers
>>>
>>> PeteS
>> Thanks, so this ok then?:
>> http://www.mhims.co.uk/MOSFET%20Schematic4.png
>>
>> Cheers,
>>
>> Michael
> Your 2N3904 shows a PNP - it should be an NPN :)
>
> Apart from that, you'll need some decoupling - put a 0.1uF cap from the 5V
> rail. The amount necessary on the 36V rail depends on the motors.
>
> The DC ratings:
>
> Use 10V or better rated for the 5V rail
> Use 63V electrolytics on the 36V rail.
>
> As a first cut, I would use 47uF on the 36V rail.
>
> Cheers
>
> PeteS
>
> PS : I hope you are learning something :)

lol, I appreciate the help :-)

This better?:
http://www.mhims.co.uk/MOSFET%20Schematic5.png

Where do the values of 10K and 1K for the resistors come from? I presuming
they aren't 'rigid' values.....

Cheers,

Michael

Author: PeteS
Date: 18:11 15-12-06

Michael wrote:
> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
> news:4DFgh.7858$493.6519@newsfe4-gui.ntli.net...
>> Michael wrote:
>>> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
>>> news:zlFgh.2206$v4.1702@newsfe3-win.ntli.net...
>>>> Michael wrote:
>>>>> "PeteS" <peter.smith8380@ntlworld.com> wrote in
message
>>>>> news:DJEgh.12108$Qa6.9135@newsfe6-gui.ntli.net...
>>>>>> Michael wrote:
>>>>>>> "jasen" <jasen@free.net.nz> wrote in
message
>>>>>>> news:elpis9$k2$2@jasen.is-a-geek.org...
>>>>>>>> On 2006-12-12, Michael
>>>>>>>>
<mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
>>>>>>>>> Hi,
>>>>>>>>> I'm working on a circuit that uses a NPN transistor
to drive (i
>>>>>>>>> think that's
>>>>>>>>> the right word) two MOSFETs. Could someone please
explain how to
>>>>>>>>> work out
>>>>>>>>> the theory behind it? For example what value
resistors I need
>>>>>>>>> between the
>>>>>>>>> gates and emitter, etc. (i presume I need some).
I'd like to be
>>>>>>>>> able to work
>>>>>>>>> it out myself as opposed to asking you guys for a
'solution' but
>>>>>>>>> sadly I'm
>>>>>>>>> struggling.....
>>>>>>>> what you describe suggests that you have the NPN
transistor
>>>>>>>> manipulating the
>>>>>>>> voltage on the gates of the mosfets, that effects their
resistance.
>>>>>>>>
>>>>>>>> not much more can be said without seeing the types of
transistors
>>>>>>>> and how it's
>>>>>>>> all hooked up.
>>>>>>>>
>>>>>>>>> So far I've googled: driving mosfet, switching
mosfet, mosfet,
>>>>>>>>> mosfet
>>>>>>>>> introduction, transistor introduction but to no
avail....If it
>>>>>>>>> really just a
>>>>>>>>> case of wiring the MOSFETs' gates together and
connecting them
>>>>>>>>> directly to
>>>>>>>>> the emitter? Any 'rules' to doing this sort of
thing?
>>>>>>>> plenty of rules, the more info you give the better.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>>
>>>>>>>> Bye.
>>>>>>>> Jasen
>>>>>>> Hi,
>>>>>>>
>>>>>>> Sorry for not getting back sooner,
>>>>>>>
>>>>>>> I've drawn you a schematic and uploaded it to:
>>>>>>> http://www.mhims.co.uk/MOSFET%20Schematic.png
>>>>>>>
>>>>>>> I'm trying to work out a suitable transistor (Q3) to use
and it's
>>>>>>> base resistor (R1) value... From what I've read in the
mosfets
>>>>>>> datasheet
(http://www.rapidonline.com/netalogue/specs/47-0530.pdf)
>>>>>>> 2-4V needs to be applied to the gates so that's where I get
the 3V
>>>>>>> value from and the 36V figure is from the onboard batteries
(it's for
>>>>>>> a robot).
>>>>>>>
>>>>>>> Where should I go from here?
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> Michael
>>>>>> Looking at this, you could do the following:
>>>>>>
>>>>>> 1. Replace Q3 with a logic level MOSFET (the venerable VN2222LL
would
>>>>>> be perfect), and change the sense completely. As shown, the
circuit
>>>>>> really won't work that well.
>>>>>>
>>>>>> Either use a PNP with emitter to positive rail or use a
P-channel
>>>>>> MOSFET (Source to positive rail). If you need to drive with 3V
or 3.3V
>>>>>> logic, then there are solutions. The simplest fix here
(assuming 3.3V
>>>>>> logic) and the fact you need 5V for gate drive (see 3) is to
>>>>>>
>>>>>> a. Remove Q3 emitter from the gates and tie it to ground
>>>>>> b. Remove Q3 collector from supply, and pull it to the supply
via a
>>>>>> 10k resistor
>>>>>> c. Connect a 2N3096 with collector to gates, base to Q3
collector via
>>>>>> a 1k resistor, emitter to 5V
>>>>>>
>>>>>> 2. Add a pulldown from the MOSFET gates of 10k or so. There has
to be
>>>>>> a DC return, and a resistor to ground is the simplest.
>>>>>>
>>>>>> 3. You need to provide a minimum of 4V to the gates of Q1 and
Q2. This
>>>>>> is the only way you are guaranteed to exceed Vgs(th). Better
would be
>>>>>> 5V, so change the power supplied to the collector [drain if you
take
>>>>>> the advice in 1] to that level.
>>>>>> Note your TTL circuitry could be operating on 3V and the
VN2222LL
>>>>>> would work fine as it would if it were a bipolar device.
>>>>>>
>>>>>> 4. I assume M1 and M2 are motors. As inductive loads, you need
to put
>>>>>> diodes across them; anode to the MOSFET drain, cathode to
positive
>>>>>> power for the motor. If you don't, the MOSFETS will operate
once, and
>>>>>> at the first turn off, they will be toast (literally).
>>>>>>
>>>>>>
>>>>>>
>>>>>> Cheers
>>>>>>
>>>>>> PeteS
>>>>> Ok thanks Pete....
>>>>>
>>>>> Just to make sure I understand you correctly...are these right?:
>>>>> http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>>>> OR
>>>>> http://www.mhims.co.uk/MOSFET%20Schematic3.png
>>>>>
>>>>> Michael
>>>>>
>>>>>
>>>>>
>>>> In http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>>>
>>>> 1. Change Q4 to a 2N3904 or a VN2222LL
>>>>
>>>> 2. Remove R2 or R3 (only one needed)
>>>>
>>>> 3. D1 and D2 should be across the motors, not the FETs
>>>>
>>>> Then we're starting to get there.
>>>>
>>>> Cheers
>>>>
>>>> PeteS
>>> Thanks, so this ok then?:
>>> http://www.mhims.co.uk/MOSFET%20Schematic4.png
>>>
>>> Cheers,
>>>
>>> Michael
>> Your 2N3904 shows a PNP - it should be an NPN :)
>>
>> Apart from that, you'll need some decoupling - put a 0.1uF cap from the 5V
>> rail. The amount necessary on the 36V rail depends on the motors.
>>
>> The DC ratings:
>>
>> Use 10V or better rated for the 5V rail
>> Use 63V electrolytics on the 36V rail.
>>
>> As a first cut, I would use 47uF on the 36V rail.
>>
>> Cheers
>>
>> PeteS
>>
>> PS : I hope you are learning something :)
>
> lol, I appreciate the help :-)
>
> This better?:
> http://www.mhims.co.uk/MOSFET%20Schematic5.png
>
> Where do the values of 10K and 1K for the resistors come from? I presuming
> they aren't 'rigid' values.....
>
> Cheers,
>
> Michael
>
>
Looks good, but to know the proper values for the decoupling/mass
storage for the motors I would need more information about the motors.

You are correct, they are not rigid values. Let's take R4 / R5

When Q5 turns on, there will be a few 10s of milliamps of current to
charge the gates, and assuming a worst case hfe of 10, a milliamp or so
of base current into Q5 during this time. It will stabilise at about
500uA, with a base current of 1/10 of that max.

R4 + R5 provides that current into the base. We could probably increase
the values significantly, but the trick of 'make it work' errs on the
side of plenty of current for bipolar devices.

Hope that helps too!

Cheers

PeteS

Author: Michael
Date: 18:36 15-12-06

"PeteS" <peter.smith8380@ntlworld.com> wrote in message
news:yYFgh.6822$Dr3.3269@newsfe2-gui.ntli.net...
> Michael wrote:
>> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
>> news:4DFgh.7858$493.6519@newsfe4-gui.ntli.net...
>>> Michael wrote:
>>>> "PeteS" <peter.smith8380@ntlworld.com> wrote in message

>>>> news:zlFgh.2206$v4.1702@newsfe3-win.ntli.net...
>>>>> Michael wrote:
>>>>>> "PeteS" <peter.smith8380@ntlworld.com> wrote in
message
>>>>>> news:DJEgh.12108$Qa6.9135@newsfe6-gui.ntli.net...
>>>>>>> Michael wrote:
>>>>>>>> "jasen" <jasen@free.net.nz> wrote in
message
>>>>>>>> news:elpis9$k2$2@jasen.is-a-geek.org...
>>>>>>>>> On 2006-12-12, Michael
>>>>>>>>>
<mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
>>>>>>>>>> Hi,
>>>>>>>>>> I'm working on a circuit that uses a NPN
transistor to drive (i
>>>>>>>>>> think that's
>>>>>>>>>> the right word) two MOSFETs. Could someone
please explain how to
>>>>>>>>>> work out
>>>>>>>>>> the theory behind it? For example what value
resistors I need
>>>>>>>>>> between the
>>>>>>>>>> gates and emitter, etc. (i presume I need
some). I'd like to be
>>>>>>>>>> able to work
>>>>>>>>>> it out myself as opposed to asking you guys for
a 'solution' but
>>>>>>>>>> sadly I'm
>>>>>>>>>> struggling.....
>>>>>>>>> what you describe suggests that you have the NPN
transistor
>>>>>>>>> manipulating the
>>>>>>>>> voltage on the gates of the mosfets, that effects
their
>>>>>>>>> resistance.
>>>>>>>>>
>>>>>>>>> not much more can be said without seeing the types
of transistors
>>>>>>>>> and how it's
>>>>>>>>> all hooked up.
>>>>>>>>>
>>>>>>>>>> So far I've googled: driving mosfet, switching
mosfet, mosfet,
>>>>>>>>>> mosfet
>>>>>>>>>> introduction, transistor introduction but to no
avail....If it
>>>>>>>>>> really just a
>>>>>>>>>> case of wiring the MOSFETs' gates together and
connecting them
>>>>>>>>>> directly to
>>>>>>>>>> the emitter? Any 'rules' to doing this sort of
thing?
>>>>>>>>> plenty of rules, the more info you give the better.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>>
>>>>>>>>> Bye.
>>>>>>>>> Jasen
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> Sorry for not getting back sooner,
>>>>>>>>
>>>>>>>> I've drawn you a schematic and uploaded it to:
>>>>>>>> http://www.mhims.co.uk/MOSFET%20Schematic.png
>>>>>>>>
>>>>>>>> I'm trying to work out a suitable transistor (Q3) to
use and it's
>>>>>>>> base resistor (R1) value... From what I've read in the
mosfets
>>>>>>>> datasheet
(http://www.rapidonline.com/netalogue/specs/47-0530.pdf)
>>>>>>>> 2-4V needs to be applied to the gates so that's where I
get the 3V
>>>>>>>> value from and the 36V figure is from the onboard
batteries (it's
>>>>>>>> for a robot).
>>>>>>>>
>>>>>>>> Where should I go from here?
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>> Michael
>>>>>>> Looking at this, you could do the following:
>>>>>>>
>>>>>>> 1. Replace Q3 with a logic level MOSFET (the venerable
VN2222LL
>>>>>>> would be perfect), and change the sense completely. As
shown, the
>>>>>>> circuit really won't work that well.
>>>>>>>
>>>>>>> Either use a PNP with emitter to positive rail or use a
P-channel
>>>>>>> MOSFET (Source to positive rail). If you need to drive with
3V or
>>>>>>> 3.3V logic, then there are solutions. The simplest fix here

>>>>>>> (assuming 3.3V logic) and the fact you need 5V for gate
drive (see
>>>>>>> 3) is to
>>>>>>>
>>>>>>> a. Remove Q3 emitter from the gates and tie it to ground
>>>>>>> b. Remove Q3 collector from supply, and pull it to the
supply via a
>>>>>>> 10k resistor
>>>>>>> c. Connect a 2N3096 with collector to gates, base to Q3
collector
>>>>>>> via a 1k resistor, emitter to 5V
>>>>>>>
>>>>>>> 2. Add a pulldown from the MOSFET gates of 10k or so. There
has to
>>>>>>> be a DC return, and a resistor to ground is the simplest.
>>>>>>>
>>>>>>> 3. You need to provide a minimum of 4V to the gates of Q1
and Q2.
>>>>>>> This is the only way you are guaranteed to exceed Vgs(th).
Better
>>>>>>> would be 5V, so change the power supplied to the collector
[drain if
>>>>>>> you take the advice in 1] to that level.
>>>>>>> Note your TTL circuitry could be operating on 3V and the
VN2222LL
>>>>>>> would work fine as it would if it were a bipolar device.
>>>>>>>
>>>>>>> 4. I assume M1 and M2 are motors. As inductive loads, you
need to
>>>>>>> put diodes across them; anode to the MOSFET drain, cathode
to
>>>>>>> positive power for the motor. If you don't, the MOSFETS
will operate
>>>>>>> once, and at the first turn off, they will be toast
(literally).
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Cheers
>>>>>>>
>>>>>>> PeteS
>>>>>> Ok thanks Pete....
>>>>>>
>>>>>> Just to make sure I understand you correctly...are these
right?:
>>>>>> http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>>>>> OR
>>>>>> http://www.mhims.co.uk/MOSFET%20Schematic3.png
>>>>>>
>>>>>> Michael
>>>>>>
>>>>>>
>>>>>>
>>>>> In http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>>>>
>>>>> 1. Change Q4 to a 2N3904 or a VN2222LL
>>>>>
>>>>> 2. Remove R2 or R3 (only one needed)
>>>>>
>>>>> 3. D1 and D2 should be across the motors, not the FETs
>>>>>
>>>>> Then we're starting to get there.
>>>>>
>>>>> Cheers
>>>>>
>>>>> PeteS
>>>> Thanks, so this ok then?:
>>>> http://www.mhims.co.uk/MOSFET%20Schematic4.png
>>>>
>>>> Cheers,
>>>>
>>>> Michael
>>> Your 2N3904 shows a PNP - it should be an NPN :)
>>>
>>> Apart from that, you'll need some decoupling - put a 0.1uF cap from the
>>> 5V rail. The amount necessary on the 36V rail depends on the motors.
>>>
>>> The DC ratings:
>>>
>>> Use 10V or better rated for the 5V rail
>>> Use 63V electrolytics on the 36V rail.
>>>
>>> As a first cut, I would use 47uF on the 36V rail.
>>>
>>> Cheers
>>>
>>> PeteS
>>>
>>> PS : I hope you are learning something :)
>>
>> lol, I appreciate the help :-)
>>
>> This better?:
>> http://www.mhims.co.uk/MOSFET%20Schematic5.png
>>
>> Where do the values of 10K and 1K for the resistors come from? I
>> presuming they aren't 'rigid' values.....
>>
>> Cheers,
>>
>> Michael
> Looks good, but to know the proper values for the decoupling/mass storage
> for the motors I would need more information about the motors.
>
> You are correct, they are not rigid values. Let's take R4 / R5
>
> When Q5 turns on, there will be a few 10s of milliamps of current to
> charge the gates, and assuming a worst case hfe of 10, a milliamp or so of
> base current into Q5 during this time. It will stabilise at about 500uA,
> with a base current of 1/10 of that max.
>
> R4 + R5 provides that current into the base. We could probably increase
> the values significantly, but the trick of 'make it work' errs on the side
> of plenty of current for bipolar devices.
>
> Hope that helps too!
>
> Cheers
>
> PeteS

Ok thanks Pete, that makes sense...

The motors are pretty heavy duty and rated at 1.8Kw (I'm thinking the cap
value will need to be increased a fair amount...?)

Could you (or Chris) explain why your circuit has decoupling caps and Chris'
doesn't?

Cheers,

Michael

Author: PeteS
Date: 18:44 15-12-06

Michael wrote:
> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
> news:yYFgh.6822$Dr3.3269@newsfe2-gui.ntli.net...
>> Michael wrote:
>>> "PeteS" <peter.smith8380@ntlworld.com> wrote in message
>>> news:4DFgh.7858$493.6519@newsfe4-gui.ntli.net...
>>>> Michael wrote:
>>>>> "PeteS" <peter.smith8380@ntlworld.com> wrote in
message
>>>>> news:zlFgh.2206$v4.1702@newsfe3-win.ntli.net...
>>>>>> Michael wrote:
>>>>>>> "PeteS" <peter.smith8380@ntlworld.com>
wrote in message
>>>>>>> news:DJEgh.12108$Qa6.9135@newsfe6-gui.ntli.net...
>>>>>>>> Michael wrote:
>>>>>>>>> "jasen" <jasen@free.net.nz> wrote
in message
>>>>>>>>> news:elpis9$k2$2@jasen.is-a-geek.org...
>>>>>>>>>> On 2006-12-12, Michael
>>>>>>>>>>
<mqiqcqhqaqeqlqhqiqmqsq@qbqlquqeqyqoqnqdqeqrq.qcqoq.quqkq> wrote:
>>>>>>>>>>> Hi,
>>>>>>>>>>> I'm working on a circuit that uses a NPN
transistor to drive (i
>>>>>>>>>>> think that's
>>>>>>>>>>> the right word) two MOSFETs. Could someone
please explain how to
>>>>>>>>>>> work out
>>>>>>>>>>> the theory behind it? For example what
value resistors I need
>>>>>>>>>>> between the
>>>>>>>>>>> gates and emitter, etc. (i presume I need
some). I'd like to be
>>>>>>>>>>> able to work
>>>>>>>>>>> it out myself as opposed to asking you guys
for a 'solution' but
>>>>>>>>>>> sadly I'm
>>>>>>>>>>> struggling.....
>>>>>>>>>> what you describe suggests that you have the
NPN transistor
>>>>>>>>>> manipulating the
>>>>>>>>>> voltage on the gates of the mosfets, that
effects their
>>>>>>>>>> resistance.
>>>>>>>>>>
>>>>>>>>>> not much more can be said without seeing the
types of transistors
>>>>>>>>>> and how it's
>>>>>>>>>> all hooked up.
>>>>>>>>>>
>>>>>>>>>>> So far I've googled: driving mosfet,
switching mosfet, mosfet,
>>>>>>>>>>> mosfet
>>>>>>>>>>> introduction, transistor introduction but
to no avail....If it
>>>>>>>>>>> really just a
>>>>>>>>>>> case of wiring the MOSFETs' gates together
and connecting them
>>>>>>>>>>> directly to
>>>>>>>>>>> the emitter? Any 'rules' to doing this sort
of thing?
>>>>>>>>>> plenty of rules, the more info you give the
better.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>>
>>>>>>>>>> Bye.
>>>>>>>>>> Jasen
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> Sorry for not getting back sooner,
>>>>>>>>>
>>>>>>>>> I've drawn you a schematic and uploaded it to:
>>>>>>>>> http://www.mhims.co.uk/MOSFET%20Schematic.png
>>>>>>>>>
>>>>>>>>> I'm trying to work out a suitable transistor (Q3)
to use and it's
>>>>>>>>> base resistor (R1) value... From what I've read in
the mosfets
>>>>>>>>> datasheet
(http://www.rapidonline.com/netalogue/specs/47-0530.pdf)
>>>>>>>>> 2-4V needs to be applied to the gates so that's
where I get the 3V
>>>>>>>>> value from and the 36V figure is from the onboard
batteries (it's
>>>>>>>>> for a robot).
>>>>>>>>>
>>>>>>>>> Where should I go from here?
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>>
>>>>>>>>> Michael
>>>>>>>> Looking at this, you could do the following:
>>>>>>>>
>>>>>>>> 1. Replace Q3 with a logic level MOSFET (the venerable
VN2222LL
>>>>>>>> would be perfect), and change the sense completely. As
shown, the
>>>>>>>> circuit really won't work that well.
>>>>>>>>
>>>>>>>> Either use a PNP with emitter to positive rail or use a
P-channel
>>>>>>>> MOSFET (Source to positive rail). If you need to drive
with 3V or
>>>>>>>> 3.3V logic, then there are solutions. The simplest fix
here
>>>>>>>> (assuming 3.3V logic) and the fact you need 5V for gate
drive (see
>>>>>>>> 3) is to
>>>>>>>>
>>>>>>>> a. Remove Q3 emitter from the gates and tie it to
ground
>>>>>>>> b. Remove Q3 collector from supply, and pull it to the
supply via a
>>>>>>>> 10k resistor
>>>>>>>> c. Connect a 2N3096 with collector to gates, base to Q3
collector
>>>>>>>> via a 1k resistor, emitter to 5V
>>>>>>>>
>>>>>>>> 2. Add a pulldown from the MOSFET gates of 10k or so.
There has to
>>>>>>>> be a DC return, and a resistor to ground is the
simplest.
>>>>>>>>
>>>>>>>> 3. You need to provide a minimum of 4V to the gates of
Q1 and Q2.
>>>>>>>> This is the only way you are guaranteed to exceed
Vgs(th). Better
>>>>>>>> would be 5V, so change the power supplied to the
collector [drain if
>>>>>>>> you take the advice in 1] to that level.
>>>>>>>> Note your TTL circuitry could be operating on 3V and
the VN2222LL
>>>>>>>> would work fine as it would if it were a bipolar
device.
>>>>>>>>
>>>>>>>> 4. I assume M1 and M2 are motors. As inductive loads,
you need to
>>>>>>>> put diodes across them; anode to the MOSFET drain,
cathode to
>>>>>>>> positive power for the motor. If you don't, the MOSFETS
will operate
>>>>>>>> once, and at the first turn off, they will be toast
(literally).
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Cheers
>>>>>>>>
>>>>>>>> PeteS
>>>>>>> Ok thanks Pete....
>>>>>>>
>>>>>>> Just to make sure I understand you correctly...are these
right?:
>>>>>>> http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>>>>>> OR
>>>>>>> http://www.mhims.co.uk/MOSFET%20Schematic3.png
>>>>>>>
>>>>>>> Michael
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>> In http://www.mhims.co.uk/MOSFET%20Schematic2.png
>>>>>>
>>>>>> 1. Change Q4 to a 2N3904 or a VN2222LL
>>>>>>
>>>>>> 2. Remove R2 or R3 (only one needed)
>>>>>>
>>>>>> 3. D1 and D2 should be across the motors, not the FETs
>>>>>>
>>>>>> Then we're starting to get there.
>>>>>>
>>>>>> Cheers
>>>>>>
>>>>>> PeteS
>>>>> Thanks, so this ok then?:
>>>>> http://www.mhims.co.uk/MOSFET%20Schematic4.png
>>>>>
>>>>> Cheers,
>>>>>
>>>>> Michael
>>>> Your 2N3904 shows a PNP - it should be an NPN :)
>>>>
>>>> Apart from that, you'll need some decoupling - put a 0.1uF cap from the

>>>> 5V rail. The amount necessary on the 36V rail depends on the motors.
>>>>
>>>> The DC ratings:
>>>>
>>>> Use 10V or better rated for the 5V rail
>>>> Use 63V electrolytics on the 36V rail.
>>>>
>>>> As a first cut, I would use 47uF on the 36V rail.
>>>>
>>>> Cheers
>>>>
>>>> PeteS
>>>>
>>>> PS : I hope you are learning something :)
>>> lol, I appreciate the help :-)
>>>
>>> This better?:
>>> http://www.mhims.co.uk/MOSFET%20Schematic5.png
>>>
>>> Where do the values of 10K and 1K for the resistors come from? I
>>> presuming they aren't 'rigid' values.....
>>>
>>> Cheers,
>>>
>>> Michael
>> Looks good, but to know the proper values for the decoupling/mass storage
>> for the motors I would need more information about the motors.
>>
>> You are correct, they are not rigid values. Let's take R4 / R5
>>
>> When Q5 turns on, there will be a few 10s of milliamps of current to
>> charge the gates, and assuming a worst case hfe of 10, a milliamp or so of
>> base current into Q5 during this time. It will stabilise at about 500uA,
>> with a base current of 1/10 of that max.
>>
>> R4 + R5 provides that current into the base. We could probably increase
>> the values significantly, but the trick of 'make it work' errs on the side
>> of plenty of current for bipolar devices.
>>
>> Hope that helps too!
>>
>> Cheers
>>
>> PeteS
>
> Ok thanks Pete, that makes sense...
>
> The motors are pretty heavy duty and rated at 1.8Kw (I'm thinking the cap
> value will need to be increased a fair amount...?)
>
> Could you (or Chris) explain why your circuit has decoupling caps and Chris'
> doesn't?

Perhaps because we were trying to look at the basics first, and the
practicalities afterwards :)

Cheers

PeteS

>
> Cheers,
>
> Michael
>
>

Author: Chris
Date: 00:41 16-12-06