Author: tedDate: 17:39 05-06-08
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I have a microcontroller and some other devices on a circuit powered
by a 5V power supply. The "other devices" consume a lot of current so
I'd like the microcontroller to turn them on only when they're
needed. I know how to do that with a transistor with its base
connected to the microcontroller. The trouble is that this would
supply the other devices with about 4.38V at most, and they really
need 5V (or very close to it). Is there any way to switch on the full
5V to those other devices on my circuit?
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Author: John FieldsDate: 18:00 05-06-08
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On Thu, 5 Jun 2008 14:39:40 -0700 (PDT), ted <strnbrg59@gmail.com>
wrote:
>I have a microcontroller and some other devices on a circuit powered
>by a 5V power supply. The "other devices" consume a lot of current so
>I'd like the microcontroller to turn them on only when they're
>needed. I know how to do that with a transistor with its base
>connected to the microcontroller. The trouble is that this would
>supply the other devices with about 4.38V at most, and they really
>need 5V (or very close to it). Is there any way to switch on the full
>5V to those other devices on my circuit?
---
View in Courier:
Use a logic-level P-Channel MOSFET with as low an Rds as you can
afford and turn it on with a logic low.
+5V>--------+
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____ S
µCIO>-----G PCH
D
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[LOAD]
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GND>--------+
or a relay:
+5V>--------+--------+--------+
| | |
|K | OC
[DIODE] [COIL]- - -|
| | NO O-> |
+--------+ |
| |
C |
µCIO>-----B NPN [LOAD]
E |
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GND>--------+-------------+
JF
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Author: David L. JonesDate: 18:52 05-06-08
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"ted" <strnbrg59@gmail.com> wrote in message
news:2e36ce49-023b-40e8-ab2d-87867024c4c6@q24g2000prf.googlegroups.com...
>I have a microcontroller and some other devices on a circuit powered
> by a 5V power supply. The "other devices" consume a lot of current so
> I'd like the microcontroller to turn them on only when they're
> needed. I know how to do that with a transistor with its base
> connected to the microcontroller. The trouble is that this would
> supply the other devices with about 4.38V at most, and they really
> need 5V (or very close to it). Is there any way to switch on the full
> 5V to those other devices on my circuit?
You don't say what current you need.
Here is one device that will do that if 140mohms doesn't give you too much
drop:
http://www.linear.com/pc/productDetail.jsp?navId=LTC4411
They can be parralled if needed, but in that case you are probably better
off finding a higher current device.
Dave.
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Author: Paul CarpenterDate: 20:39 05-06-08
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In article <2e36ce49-023b-40e8-ab2d-
87867024c4c6@q24g2000prf.googlegroups.com>, strnbrg59@gmail.com says...
> I have a microcontroller and some other devices on a circuit powered
> by a 5V power supply. The "other devices" consume a lot of current so
> I'd like the microcontroller to turn them on only when they're
> needed. I know how to do that with a transistor with its base
> connected to the microcontroller. The trouble is that this would
> supply the other devices with about 4.38V at most, and they really
> need 5V (or very close to it). Is there any way to switch on the full
> 5V to those other devices on my circuit?
Alternatively, put an extra regulator in with ENABLE pin, or similar for
the heavier current supply.
Word of caution watch what happens when the 'other devices' are off and
the micro is ON, as to what power up paths and potential latch ups
you have.
--
Paul Carpenter | paul@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/>; PC Services
<http://www.pcserviceselectronics.co.uk/fonts/>; Timing Diagram Font
<http://www.gnuh8.org.uk/>; GNU H8 - compiler & Renesas H8/H8S/H8 Tiny
<http://www.badweb.org.uk/>; For those web sites you hate
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Author: JamieDate: 21:20 05-06-08
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Paul Carpenter wrote:
> In article <2e36ce49-023b-40e8-ab2d-
> 87867024c4c6@q24g2000prf.googlegroups.com>, strnbrg59@gmail.com says...
>
>>I have a microcontroller and some other devices on a circuit powered
>>by a 5V power supply. The "other devices" consume a lot of current so
>>I'd like the microcontroller to turn them on only when they're
>>needed. I know how to do that with a transistor with its base
>>connected to the microcontroller. The trouble is that this would
>>supply the other devices with about 4.38V at most, and they really
>>need 5V (or very close to it). Is there any way to switch on the full
>>5V to those other devices on my circuit?
>
>
> Alternatively, put an extra regulator in with ENABLE pin, or similar for
> the heavier current supply.
>
> Word of caution watch what happens when the 'other devices' are off and
> the micro is ON, as to what power up paths and potential latch ups
> you have.
>
A Logic Level power N/PMOS would take care of that.
The low voltage Vgs types, generally offer very good Ron specs so
very little loss will be noticed in the low scale voltage applications.
http://webpages.charter.net/jamie_5";
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Author: ChrisDate: 21:44 05-06-08
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On Jun 5, 4:39=A0pm, ted <strnbr...@gmail.com> wrote:
> I have a microcontroller and some other devices on a circuit powered
> by a 5V power supply. =A0The "other devices" consume a lot of current so
> I'd like the microcontroller to turn them on only when they're
> needed. =A0I know how to do that with a transistor with its base
> connected to the microcontroller. =A0The trouble is that this would
> supply the other devices with about 4.38V at most, and they really
> need 5V (or very close to it). =A0Is there any way to switch on the full
> 5V to those other devices on my circuit?
Hi, Ted. You've already got a driver transistor -- just add a relay,
and you're done. The relay contacts can switch the load, and are only
a few milliohms.
Cheers
Chris
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Author: Bob EldDate: 11:17 06-06-08
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"ted" <strnbrg59@gmail.com> wrote in message
news:2e36ce49-023b-40e8-ab2d-87867024c4c6@q24g2000prf.googlegroups.com...
> I have a microcontroller and some other devices on a circuit powered
> by a 5V power supply. The "other devices" consume a lot of current so
> I'd like the microcontroller to turn them on only when they're
> needed. I know how to do that with a transistor with its base
> connected to the microcontroller. The trouble is that this would
> supply the other devices with about 4.38V at most, and they really
> need 5V (or very close to it). Is there any way to switch on the full
> 5V to those other devices on my circuit?
Using a FET or a relay has already been mentioned. However, a saturated
transistor will have an on voltage, collector to emitter, far lower than
0.62 Volts. In most transistors it will be around 0.2 to 0.3 Volts.
One of the definitions of saturation is where the collector voltage falls
below the base voltage.
If it is really critical, you need to boost the regulator output a
corresponding amount to insure an exact voltage on the devices or locally
regulate it from a higher voltage. Unless this voltage is a DAC/ADC
reference or some such thing why is to so critical?
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Author: John FieldsDate: 15:44 06-06-08
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On Fri, 6 Jun 2008 08:17:42 -0700, "Bob Eld" <nsmontassoc@yahoo.com>
wrote:
>
>"ted" <strnbrg59@gmail.com> wrote in message
>news:2e36ce49-023b-40e8-ab2d-87867024c4c6@q24g2000prf.googlegroups.com...
>> I have a microcontroller and some other devices on a circuit powered
>> by a 5V power supply. The "other devices" consume a lot of current
so
>> I'd like the microcontroller to turn them on only when they're
>> needed. I know how to do that with a transistor with its base
>> connected to the microcontroller. The trouble is that this would
>> supply the other devices with about 4.38V at most, and they really
>> need 5V (or very close to it). Is there any way to switch on the full
>> 5V to those other devices on my circuit?
>
>Using a FET or a relay has already been mentioned. However, a saturated
>transistor will have an on voltage, collector to emitter, far lower than
>0.62 Volts. In most transistors it will be around 0.2 to 0.3 Volts.
---
That's if it's an NPN and he's using it as a low side driver or if
it's a PNP and he's using it as a high side driver.
From his description I'd guess that he's using an NPN as an emitter
follower and it'll _never_ go into saturation.
JF
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Author: Bob EldDate: 16:36 06-06-08
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"John Fields" <jfields@austininstruments.com> wrote in message
news:d43j441p3utgmsdt1qbbjmeh9u3fe6n5is@4ax.com...
> On Fri, 6 Jun 2008 08:17:42 -0700, "Bob Eld"
<nsmontassoc@yahoo.com>
> wrote:
>
> >
> >"ted" <strnbrg59@gmail.com> wrote in message
> >news:2e36ce49-023b-40e8-ab2d-87867024c4c6@q24g2000prf.googlegroups.com...
> >> I have a microcontroller and some other devices on a circuit powered
> >> by a 5V power supply. The "other devices" consume a lot of
current so
> >> I'd like the microcontroller to turn them on only when they're
> >> needed. I know how to do that with a transistor with its base
> >> connected to the microcontroller. The trouble is that this would
> >> supply the other devices with about 4.38V at most, and they really
> >> need 5V (or very close to it). Is there any way to switch on the full
> >> 5V to those other devices on my circuit?
> >
> >Using a FET or a relay has already been mentioned. However, a saturated
> >transistor will have an on voltage, collector to emitter, far lower than
> >0.62 Volts. In most transistors it will be around 0.2 to 0.3 Volts.
>
> ---
> That's if it's an NPN and he's using it as a low side driver or if
> it's a PNP and he's using it as a high side driver.
>
> From his description I'd guess that he's using an NPN as an emitter
> follower and it'll _never_ go into saturation.
>
> JF
Yeah, clearly an emitter follower would not be applicable. I didn't even
consider that the OP would try to go down that road.
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