Switch on power without 0.62V drop?

[ted]

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?

 

[John Fields]

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>--------+
|
____ S
ľCIO>-----G PCH
D
|
[LOAD]
|
GND>--------+

or a relay:


+5V>--------+--------+--------+
| | |
|K | OC
[DIODE] [COIL]- - -|
| | NO O-> |
+--------+ |
| |
C |
ľCIO>-----B NPN [LOAD]
E |
| |
GND>--------+-------------+

JF

 

[Paul Carpenter]

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
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<http://www.pcserviceselectronics.co.uk/fonts/> Timing Diagram Font
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<http://www.badweb.org.uk/> For those web sites you hate

 

[Jamie]

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"

 

[Chris]

On Jun 5, 4:39 pm, ted <strnbr...@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?

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

 

[John Fields]

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