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Science Forum Index » Electronics - Basics Forum » Switch on power without 0.62V drop?...
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| ted... |
 Posted: Thu Jun 05, 2008 11:39 am |
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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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| Chris... |
| Posted: Thu Jun 05, 2008 3:44 pm |
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On Jun 5, 4:39 pm, ted <strnbr... at (no spam) gmail.com> wrote:
Quote: 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 |
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| John Fields... |
| Posted: Thu Jun 05, 2008 5:00 pm |
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On Thu, 5 Jun 2008 14:39:40 -0700 (PDT), ted <strnbrg59 at (no spam) gmail.com>
wrote:
Quote: 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>--------+--------+--------+
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|K | OC
[DIODE] [COIL]- - -|
| | NO O-> |
+--------+ |
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C |
µCIO>-----B NPN [LOAD]
E |
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GND>--------+-------------+
JF |
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| David L. Jones... |
| Posted: Thu Jun 05, 2008 5:52 pm |
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"ted" <strnbrg59 at (no spam) gmail.com> wrote in message
news:2e36ce49-023b-40e8-ab2d-87867024c4c6 at (no spam) q24g2000prf.googlegroups.com...
Quote: 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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| Paul Carpenter... |
| Posted: Thu Jun 05, 2008 7:39 pm |
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Guest
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In article <2e36ce49-023b-40e8-ab2d-
87867024c4c6 at (no spam) q24g2000prf.googlegroups.com>, strnbrg59 at (no spam) gmail.com says...
Quote: 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 at (no spam) 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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| Jamie... |
| Posted: Thu Jun 05, 2008 8:24 pm |
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Paul Carpenter wrote:
Quote: In article <2e36ce49-023b-40e8-ab2d-
87867024c4c6 at (no spam) q24g2000prf.googlegroups.com>, strnbrg59 at (no spam) 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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| John Fields... |
| Posted: Fri Jun 06, 2008 2:44 pm |
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On Fri, 6 Jun 2008 08:17:42 -0700, "Bob Eld" <nsmontassoc at (no spam) yahoo.com>
wrote:
Quote:
"ted" <strnbrg59 at (no spam) gmail.com> wrote in message
news:2e36ce49-023b-40e8-ab2d-87867024c4c6 at (no spam) 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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| Bob Eld... |
| Posted: Fri Jun 06, 2008 3:36 pm |
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Guest
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"John Fields" <jfields at (no spam) austininstruments.com> wrote in message
news:d43j441p3utgmsdt1qbbjmeh9u3fe6n5is at (no spam) 4ax.com...
Quote: On Fri, 6 Jun 2008 08:17:42 -0700, "Bob Eld" <nsmontassoc at (no spam) yahoo.com
wrote:
"ted" <strnbrg59 at (no spam) gmail.com> wrote in message
news:2e36ce49-023b-40e8-ab2d-87867024c4c6 at (no spam) 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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