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Sci.Electronics.Basics -> emitter followers / Darlington - need help
There are 24 messages in this thread.
You are currently looking at messages 1 to 20.
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Hello,
I'm trying to use a low-voltage signal from my PC's parallel port (<
5V) to activate the gate of a mosfet, which will switch a 9-12V supply
and lamp.
Something similar to this, under "NPN Darlington Configuration":
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/emitfol.html
When I built this, though, it seems that the mosfet ALWAYS wants to
stay on. Really weird. At first I thought I fried my transistors
somehow, but there's no short (after testing with my dmm).
I'm using a TIP31A for the first transistor, and an IRF530 as the
mosfet.
Why I'm doing it: I'd like to be able to control a small motor via my
parallel port, basically simulating a PWM. Unfortunately, the feeble
5V from the parallel port's Pin 2 (plus 2000 ohms of resistor) isn't
enough to fully open the gate mf my mosfet. So, I'm trying to amplify
the signal, so to speak.
Am I on the wrong track here? Any pointers?
Thank you,
Michael
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Author: Jonathan KirwanDate: 11:54 14-12-06
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On 14 Dec 2006 08:39:52 -0800, mrdarrett@gmail.com wrote:
>I'm trying to use a low-voltage signal from my PC's parallel port (<
>5V) to activate the gate of a mosfet, which will switch a 9-12V supply
>and lamp.
>
>Something similar to this, under "NPN Darlington Configuration":
>http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/emitfol.html
>
>When I built this, though, it seems that the mosfet ALWAYS wants to
>stay on. Really weird. At first I thought I fried my transistors
>somehow, but there's no short (after testing with my dmm).
>
>I'm using a TIP31A for the first transistor, and an IRF530 as the
>mosfet.
>
>Why I'm doing it: I'd like to be able to control a small motor via my
>parallel port, basically simulating a PWM. Unfortunately, the feeble
>5V from the parallel port's Pin 2 (plus 2000 ohms of resistor) isn't
>enough to fully open the gate mf my mosfet. So, I'm trying to amplify
>the signal, so to speak.
>
>Am I on the wrong track here? Any pointers?
Michael, did you replace the 2nd transistor (the one with its base
connected to the other's emitter) in that schematic with a mosfet?
Jon
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Jonathan Kirwan wrote:
> On 14 Dec 2006 08:39:52 -0800, mrdarrett@gmail.com wrote:
>
> >I'm trying to use a low-voltage signal from my PC's parallel port (<
> >5V) to activate the gate of a mosfet, which will switch a 9-12V supply
> >and lamp.
> >
> >Something similar to this, under "NPN Darlington Configuration":
> >http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/emitfol.html
> >
> >When I built this, though, it seems that the mosfet ALWAYS wants to
> >stay on. Really weird. At first I thought I fried my transistors
> >somehow, but there's no short (after testing with my dmm).
> >
> >I'm using a TIP31A for the first transistor, and an IRF530 as the
> >mosfet.
> >
> >Why I'm doing it: I'd like to be able to control a small motor via my
> >parallel port, basically simulating a PWM. Unfortunately, the feeble
> >5V from the parallel port's Pin 2 (plus 2000 ohms of resistor) isn't
> >enough to fully open the gate mf my mosfet. So, I'm trying to amplify
> >the signal, so to speak.
> >
> >Am I on the wrong track here? Any pointers?
>
> Michael, did you replace the 2nd transistor (the one with its base
> connected to the other's emitter) in that schematic with a mosfet?
>
> Jon
Sure did. Is that a bad idea? They are both TO-220s.
Michael
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Author: John PopelishDate: 12:59 14-12-06
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mrdarrett@gmail.com wrote:
> Hello,
>
> I'm trying to use a low-voltage signal from my PC's parallel port (<
> 5V) to activate the gate of a mosfet, which will switch a 9-12V supply
> and lamp.
>
> Something similar to this, under "NPN Darlington Configuration":
> http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/emitfol.html
>
> When I built this, though, it seems that the mosfet ALWAYS wants to
> stay on. Really weird. At first I thought I fried my transistors
> somehow, but there's no short (after testing with my dmm).
>
> I'm using a TIP31A for the first transistor, and an IRF530 as the
> mosfet.
>
> Why I'm doing it: I'd like to be able to control a small motor via my
> parallel port, basically simulating a PWM. Unfortunately, the feeble
> 5V from the parallel port's Pin 2 (plus 2000 ohms of resistor) isn't
> enough to fully open the gate mf my mosfet. So, I'm trying to amplify
> the signal, so to speak.
>
> Am I on the wrong track here? Any pointers?
I think so. I don't know what you are doing wrong that is
keeping the mosfet on all the time, but an emitter follower
provides no voltage gain, but actually loses a diode drop
off the input voltage, just to forward bias the transistor.
If I were you, I would start with a logic level mosfet (one
with a low turn on voltage that could be connected directly
to the parallel port output). Otherwise, you need a 10 to
12 volt supply and an amplifier with voltage gain, that will
increase the voltage swing available from the parallel port
to about 10 volts needed to switch a non logic level mosfet.
Such an amplifier can certainly be built with one to three
transistors, but you can also get integrated gate drivers
that accept the parallel port signals as inputs and provide
full supply swing output with a low enough resistance to
charge the gate capacitance very quickly, to provide low
loss switching.
For instance:
http://ww1.microchip.com/downloads/en/DeviceDoc/21393C.pdf
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John Popelish wrote:
> mrdarrett@gmail.com wrote:
> > Hello,
> >
> > I'm trying to use a low-voltage signal from my PC's parallel port (<
> > 5V) to activate the gate of a mosfet, which will switch a 9-12V supply
> > and lamp.
> >
> > Something similar to this, under "NPN Darlington Configuration":
> > http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/emitfol.html
> >
> > When I built this, though, it seems that the mosfet ALWAYS wants to
> > stay on. Really weird. At first I thought I fried my transistors
> > somehow, but there's no short (after testing with my dmm).
> >
> > I'm using a TIP31A for the first transistor, and an IRF530 as the
> > mosfet.
> >
> > Why I'm doing it: I'd like to be able to control a small motor via my
> > parallel port, basically simulating a PWM. Unfortunately, the feeble
> > 5V from the parallel port's Pin 2 (plus 2000 ohms of resistor) isn't
> > enough to fully open the gate mf my mosfet. So, I'm trying to amplify
> > the signal, so to speak.
> >
> > Am I on the wrong track here? Any pointers?
>
> I think so. I don't know what you are doing wrong that is
> keeping the mosfet on all the time, but an emitter follower
> provides no voltage gain, but actually loses a diode drop
> off the input voltage, just to forward bias the transistor.
>
> If I were you, I would start with a logic level mosfet (one
> with a low turn on voltage that could be connected directly
> to the parallel port output). Otherwise, you need a 10 to
> 12 volt supply and an amplifier with voltage gain, that will
> increase the voltage swing available from the parallel port
> to about 10 volts needed to switch a non logic level mosfet.
>
> Such an amplifier can certainly be built with one to three
> transistors, but you can also get integrated gate drivers
> that accept the parallel port signals as inputs and provide
> full supply swing output with a low enough resistance to
> charge the gate capacitance very quickly, to provide low
> loss switching.
>
> For instance:
> http://ww1.microchip.com/downloads/en/DeviceDoc/21393C.pdf
Ok, thanks for the info. So they make logic level mosfets, eh... less
than a buck each at Mouser.
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Author: Jonathan KirwanDate: 14:32 14-12-06
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On Thu, 14 Dec 2006 12:59:59 -0500, John Popelish <jpopelish@rica.net>
wrote:
>mrdarrett@gmail.com wrote:
>> Hello,
>>
>> I'm trying to use a low-voltage signal from my PC's parallel port (<
>> 5V) to activate the gate of a mosfet, which will switch a 9-12V supply
>> and lamp.
>>
>> Something similar to this, under "NPN Darlington Configuration":
>> http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/emitfol.html
>>
>> When I built this, though, it seems that the mosfet ALWAYS wants to
>> stay on. Really weird. At first I thought I fried my transistors
>> somehow, but there's no short (after testing with my dmm).
>>
>> I'm using a TIP31A for the first transistor, and an IRF530 as the
>> mosfet.
>>
>> Why I'm doing it: I'd like to be able to control a small motor via my
>> parallel port, basically simulating a PWM. Unfortunately, the feeble
>> 5V from the parallel port's Pin 2 (plus 2000 ohms of resistor) isn't
>> enough to fully open the gate mf my mosfet. So, I'm trying to amplify
>> the signal, so to speak.
>>
>> Am I on the wrong track here? Any pointers?
>
>I think so. I don't know what you are doing wrong that is
>keeping the mosfet on all the time, but an emitter follower
>provides no voltage gain, but actually loses a diode drop
>off the input voltage, just to forward bias the transistor.
He's got the emitter of the first transistor floating on the mosfet
gate, if I'm reading him right. When the BJT turns off, the emitter
just hangs there with little more than leakage to allow the mosfet
gate to fall. So it stays on, I think.
>If I were you, I would start with a logic level mosfet (one
>with a low turn on voltage that could be connected directly
>to the parallel port output). Otherwise, you need a 10 to
>12 volt supply and an amplifier with voltage gain, that will
>increase the voltage swing available from the parallel port
>to about 10 volts needed to switch a non logic level mosfet.
>
>Such an amplifier can certainly be built with one to three
>transistors, but you can also get integrated gate drivers
>that accept the parallel port signals as inputs and provide
>full supply swing output with a low enough resistance to
>charge the gate capacitance very quickly, to provide low
>loss switching.
>
>For instance:
>http://ww1.microchip.com/downloads/en/DeviceDoc/21393C.pdf
Also, it may be a good idea to look for isolation here. I gather this
is to drive some motor ("I'd like to be able to control a small motor
via my parallel port.") I don't know what's providing that power, but
it may be nice to galvanically isolate these two devices; the PC port
and the motor circuit.
Jon
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Jonathan Kirwan wrote:
...
> He's got the emitter of the first transistor floating on the mosfet
> gate, if I'm reading him right. When the BJT turns off, the emitter
> just hangs there with little more than leakage to allow the mosfet
> gate to fall. So it stays on, I think.
So, swap them? Have the mosfet drive my TIP31?
...
> Also, it may be a good idea to look for isolation here. I gather this
> is to drive some motor ("I'd like to be able to control a small motor
> via my parallel port.") I don't know what's providing that power, but
> it may be nice to galvanically isolate these two devices; the PC port
> and the motor circuit.
>
> Jon
I've got a pak of 8 NiMHs providing the power (~9.6V or so). Are you
saying there's a chance the parallel port could see this power (hence
the need for isolation)?
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Author: Jonathan KirwanDate: 15:14 14-12-06
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On 14 Dec 2006 11:36:54 -0800, mrdarrett@gmail.com wrote:
>Jonathan Kirwan wrote:
>...
>
>> He's got the emitter of the first transistor floating on the mosfet
>> gate, if I'm reading him right. When the BJT turns off, the emitter
>> just hangs there with little more than leakage to allow the mosfet
>> gate to fall. So it stays on, I think.
>
>So, swap them? Have the mosfet drive my TIP31?
At least the gate wouldn't float.
You have an NMOS and an NPN device. Since your parallel output is
going to be less than 5V max (unstated, but let's assume 3V max for
talking purposes to cover laptops and other evils) or close to ground
reference, and that is about all you have to work with, the mosfet
won't turn on well. It's Vto is about 3.2V, I think.
Keeping the two devices, you might try something only a little
different:
>: +9-12V
>: |
>: |
>: +9-12V \
>: | / MOTOR
>: | \
>: | /
>: \ |
>: / R1 |
>: \ 47k |
>: / |
>: | ||' M1
>: | ||< IRF530
>: ,-+------'|,
>: | |
>: R2 |/c Q1 |
>: >----/\/\---| TIP31A |
>: 4.7k |>e gnd
>: |
>: |
>: |
>: |
>: gnd
This is just off the cuff.
>...
>
>> Also, it may be a good idea to look for isolation here. I gather this
>> is to drive some motor ("I'd like to be able to control a small motor
>> via my parallel port.") I don't know what's providing that power, but
>> it may be nice to galvanically isolate these two devices; the PC port
>> and the motor circuit.
>>
>> Jon
>
>I've got a pak of 8 NiMHs providing the power (~9.6V or so). Are you
>saying there's a chance the parallel port could see this power (hence
>the need for isolation)?
Well, batteries are good. There is still some potential when the
motor feeds back energy -- inductive kickback. But try the above and
see if that helps any.
Jon
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Author: Rich GriseDate: 15:30 14-12-06
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On Thu, 14 Dec 2006 11:36:54 -0800, mrdarrett wrote:
> Jonathan Kirwan wrote:
>
>> He's got the emitter of the first transistor floating on the mosfet
>> gate, if I'm reading him right. When the BJT turns off, the emitter
>> just hangs there with little more than leakage to allow the mosfet
>> gate to fall. So it stays on, I think.
>
> So, swap them? Have the mosfet drive my TIP31?
>
>> Also, it may be a good idea to look for isolation here. I gather this
>> is to drive some motor ("I'd like to be able to control a small motor
>> via my parallel port.") I don't know what's providing that power, but
>> it may be nice to galvanically isolate these two devices; the PC port
>> and the motor circuit.
>
> I've got a pak of 8 NiMHs providing the power (~9.6V or so). Are you
> saying there's a chance the parallel port could see this power (hence
> the need for isolation)?
I think you've got it kind of upside down. The parallel port is nominally
a "TTL Output", which meand a weak pullup to something guaranteed to be
more than 2.7V, if you're sinking less than some ridiculously low amount
of current, like 4 mA or something (you could probably look it up), but
the point is, you're not getting enough drive to your darlington.
I wouldn't use a darlington. I'd use a PNP, like a 2N3906 or 2N4401or
so, driving a reasonable NPN; actually your TIP31 could work here,
since the PNP can provide arbitrary base drive:
+5V +5V +9V
| | |
[10K] | [1K]
| < e |
--[2K2]-+---| PNP +----- Mosfet gate
\ c |
| / c
+--[4K7]-------| NPN
| > e
[4K7] |
| |
GND GND
Good Luck!
Rich
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Jonathan Kirwan wrote:
> On 14 Dec 2006 11:36:54 -0800, mrdarrett@gmail.com wrote:
>
> >Jonathan Kirwan wrote:
> >...
> >
> >> He's got the emitter of the first transistor floating on the mosfet
> >> gate, if I'm reading him right. When the BJT turns off, the emitter
> >> just hangs there with little more than leakage to allow the mosfet
> >> gate to fall. So it stays on, I think.
> >
> >So, swap them? Have the mosfet drive my TIP31?
>
> At least the gate wouldn't float.
>
> You have an NMOS and an NPN device. Since your parallel output is
> going to be less than 5V max (unstated, but let's assume 3V max for
> talking purposes to cover laptops and other evils) or close to ground
> reference, and that is about all you have to work with, the mosfet
> won't turn on well. It's Vto is about 3.2V, I think.
>
> Keeping the two devices, you might try something only a little
> different:
>
> >: +9-12V
> >: |
> >: |
> >: +9-12V \
> >: | / MOTOR
> >: | \
> >: | /
> >: \ |
> >: / R1 |
> >: \ 47k |
> >: / |
> >: | ||' M1
> >: | ||< IRF530
> >: ,-+------'|,
> >: | |
> >: R2 |/c Q1 |
> >: >----/\/\---| TIP31A |
> >: 4.7k |>e gnd
> >: |
> >: |
> >: |
> >: |
> >: gnd
>
> This is just off the cuff.
That looks interesting... I think I'll try.
I actually am using a laptop. Really low voltage, especially
downstream of my 2k resistor (which I use for R2).
Why the 47k R1 though?
Thanks,
Michael
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Author: Jonathan KirwanDate: 16:47 14-12-06
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On 14 Dec 2006 13:25:21 -0800, mrdarrett@gmail.com wrote:
>Jonathan Kirwan wrote:
>> On 14 Dec 2006 11:36:54 -0800, mrdarrett@gmail.com wrote:
>>
>> >Jonathan Kirwan wrote:
>> >...
>> >
>> >> He's got the emitter of the first transistor floating on the mosfet
>> >> gate, if I'm reading him right. When the BJT turns off, the emitter
>> >> just hangs there with little more than leakage to allow the mosfet
>> >> gate to fall. So it stays on, I think.
>> >
>> >So, swap them? Have the mosfet drive my TIP31?
>>
>> At least the gate wouldn't float.
>>
>> You have an NMOS and an NPN device. Since your parallel output is
>> going to be less than 5V max (unstated, but let's assume 3V max for
>> talking purposes to cover laptops and other evils) or close to ground
>> reference, and that is about all you have to work with, the mosfet
>> won't turn on well. It's Vto is about 3.2V, I think.
>>
>> Keeping the two devices, you might try something only a little
>> different:
>>
>> >: +9-12V
>> >: |
>> >: |
>> >: +9-12V \
>> >: | / MOTOR
>> >: | \
>> >: | /
>> >: \ |
>> >: / R1 |
>> >: \ 47k |
>> >: / |
>> >: | ||' M1
>> >: | ||< IRF530
>> >: ,-+------'|,
>> >: | |
>> >: R2 |/c Q1 |
>> >: >----/\/\---| TIP31A |
>> >: 4.7k |>e gnd
>> >: |
>> >: |
>> >: |
>> >: |
>> >: gnd
>>
>> This is just off the cuff.
>
>
>That looks interesting... I think I'll try.
>
>I actually am using a laptop. Really low voltage, especially
>downstream of my 2k resistor (which I use for R2).
>
>Why the 47k R1 though?
It pulls the gate of the IRF530 up when the TIP31A is off. Otherwise,
you'd have a similar problem as before. The value of 4.7k is a guess.
You could try the 2k, as well. Should be about the same behavior, I
was just trying to lighten the load on the parallel port a little bit.
R1 will need to be smaller values for faster operation, I suppose. I
didn't look at the IRF530 capacitance; and for now, I don't care to
find out. But it will limit the turn-on rate of the IRF530, which
could be a small problem. Would need to know a lot more about what
you are wanting to try, though. And by then, there would probably be
a much better topology to use. So why bother, for now? Mostly, I'm
just wondering how that seems to work using the parts you have on
hand.
Jon
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Jonathan Kirwan wrote:
...
> >Why the 47k R1 though?
>
> It pulls the gate of the IRF530 up when the TIP31A is off. Otherwise,
> you'd have a similar problem as before. The value of 4.7k is a guess.
> You could try the 2k, as well. Should be about the same behavior, I
> was just trying to lighten the load on the parallel port a little bit.
> R1 will need to be smaller values for faster operation, I suppose. I
> didn't look at the IRF530 capacitance; and for now, I don't care to
> find out. But it will limit the turn-on rate of the IRF530, which
> could be a small problem. Would need to know a lot more about what
> you are wanting to try, though. And by then, there would probably be
> a much better topology to use. So why bother, for now? Mostly, I'm
> just wondering how that seems to work using the parts you have on
> hand.
>
> Jon
Gotcha. Thanks.
Basically just trying to learn. I'm using a lamp for now... once it
works properly, I'll use my motor, with capacitors to smooth out the
power.
I already built a 555-based PWM thingie off a schematic off google;
that works great. I just thought with a parallel port's 8 pins, I
could potentially control 8 motors simultaneously, from a DOS program
on my laptop. It's a learning exercise (and I have so much to learn!)
Michael
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Author: Jonathan KirwanDate: 17:02 14-12-06
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On Thu, 14 Dec 2006 21:47:32 GMT, I wrote:
>I didn't look at the IRF530 capacitance;
Okay. I looked. 1 nanofarad territory. That means on-times in the
small 10s of microseconds, roughly. If you aren't toggling around
faster than 1000 Hz or so, you are probably okay.
Jon
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Jonathan Kirwan wrote:
> On Thu, 14 Dec 2006 21:47:32 GMT, I wrote:
>
> >I didn't look at the IRF530 capacitance;
>
> Okay. I looked. 1 nanofarad territory. That means on-times in the
> small 10s of microseconds, roughly. If you aren't toggling around
> faster than 1000 Hz or so, you are probably okay.
>
> Jon
Oh. Uh... thanks. I'm running at 100 Hz.
:-o
Michael
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Author: EeyoreDate: 17:58 14-12-06
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mrdarrett@gmail.com wrote:
> Am I on the wrong track here?
Emitter followers don't amplify the voltage. In fact the voltage to the FET gate
will be reduced by one Vbe.
A 'logic level' gate drive FET would do the job simply.
Graham
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Author: Jonathan KirwanDate: 18:05 14-12-06
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On Thu, 14 Dec 2006 22:58:22 +0000, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:
>mrdarrett@gmail.com wrote:
>
>> Am I on the wrong track here?
>
>Emitter followers don't amplify the voltage. In fact the voltage to the FET gate
>will be reduced by one Vbe.
>
>A 'logic level' gate drive FET would do the job simply.
Yup. Unfortunately, he has a TIP31A and a IRF530, right now. ;)
Jon
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Jonathan Kirwan wrote:
> On Thu, 14 Dec 2006 22:58:22 +0000, Eeyore
> <rabbitsfriendsandrelations@hotmail.com> wrote:
>
> >mrdarrett@gmail.com wrote:
> >
> >> Am I on the wrong track here?
> >
> >Emitter followers don't amplify the voltage. In fact the voltage to the FET
gate
> >will be reduced by one Vbe.
> >
> >A 'logic level' gate drive FET would do the job simply.
>
> Yup. Unfortunately, he has a TIP31A and a IRF530, right now. ;)
>
> Jon
This will likely be rectified after my next order to Mouser. Dad needs
some parts, too... we will submit one large order to save on shipping.
Michael
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John Popelish wrote:
> mrdarrett@gmail.com wrote:
> (snip)
> > Basically just trying to learn. I'm using a lamp for now... once it
> > works properly, I'll use my motor, with capacitors to smooth out the
> > power.
>
> Not a good idea to smooth the output from a mosfet switch
> with a capacitor. The capacitor charges up to almost the
> same voltage, regardless of the pulse width. Better to put
> a diode across the motor, and let the motor's inductance
> average the pulses. This may require a higher pulse
> frequency to get a smooth rotation. If you want to try a
> capacitor across the motor, you will need some additional
> inductor between the mosfet and the parallel motor and
> capacitor, with the catch diode moved to the drain, instead
> of being across the motor.
This is actually what I'd had in mind:
http://www.cpemma.co.uk/555pwm.html
(with the parallel port and my DOS software taking the part of the 555
and variable resistor.)
Yes, I was planning on sticking a diode across the motor.
Are 100nF and 100uF caps between +12VDC and ground still a bad idea...?
Thanks,
Michael
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Author: John PopelishDate: 18:44 14-12-06
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mrdarrett@gmail.com wrote:
(snip)
> Basically just trying to learn. I'm using a lamp for now... once it
> works properly, I'll use my motor, with capacitors to smooth out the
> power.
Not a good idea to smooth the output from a mosfet switch
with a capacitor. The capacitor charges up to almost the
same voltage, regardless of the pulse width. Better to put
a diode across the motor, and let the motor's inductance
average the pulses. This may require a higher pulse
frequency to get a smooth rotation. If you want to try a
capacitor across the motor, you will need some additional
inductor between the mosfet and the parallel motor and
capacitor, with the catch diode moved to the drain, instead
of being across the motor.
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Author: John PopelishDate: 19:03 14-12-06
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mrdarrett@gmail.com wrote:
> John Popelish wrote:
>> mrdarrett@gmail.com wrote:
>> (snip)
>>> Basically just trying to learn. I'm using a lamp for now... once it
>>> works properly, I'll use my motor, with capacitors to smooth out the
>>> power.
>> Not a good idea to smooth the output from a mosfet switch
>> with a capacitor. The capacitor charges up to almost the
>> same voltage, regardless of the pulse width. Better to put
>> a diode across the motor, and let the motor's inductance
>> average the pulses. This may require a higher pulse
>> frequency to get a smooth rotation. If you want to try a
>> capacitor across the motor, you will need some additional
>> inductor between the mosfet and the parallel motor and
>> capacitor, with the catch diode moved to the drain, instead
>> of being across the motor.
>
>
> This is actually what I'd had in mind:
>
> http://www.cpemma.co.uk/555pwm.html
>
> (with the parallel port and my DOS software taking the part of the 555
> and variable resistor.)
>
> Yes, I was planning on sticking a diode across the motor.
>
> Are 100nF and 100uF caps between +12VDC and ground still a bad idea...?
Not at all. They should connect as closely as possible
between the motor diode node and the source of the mosfet
(or emitter if you use a junction transistor). This
provides a very close source of charge that keeps the pulses
out of the supply lines.
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