For a ~100mA load, should I use a small relay, or just a tra

[James W]

I have a couple of small devices that draw ~100mA each. I want to
control the power to these with a microprocessor. Each of these devices
is located remotely from the controller, and each has an onboard 5V
regulator, and can be powered from 13.8V ( This is an automotive
application). I could use a PC relay, to supply the 13.8 to these
devices, but I'm wondering, given that they have onboard regulators
(78L05), if I couldn't get by cheaper simply using a discrete transitor
(2N2907A) as a high-side switch.

Comments?


thanks

- jim


p.s. Let me just say this has to be one of the finest newsgroups in
existance. You all have been a GREAT help to me as I learn my way around
electronics.. THANKS!!!

 

[Triffid Hunter]

you'd be fine using a transistor for this. Some transistors are rated at
up to 20A!

just be aware that if you use it as a high-side switch, you can't just
hook the base resistor straight to your uC - a uC pin at 5v is more than
negative enough to turn the transistor on hard when its emitter is at 13v!
you'll need a signal transistor between the uC output and ground, then
connect the power transistor's base to this transistor's collector (all
via suitable resistors of course)

an open collector out from the uC probably wouldn't work either, as almost
all uC pins have protection diodes to both rails.

On Mon, 29 Mar 2004 11:32:50 -0800, James W
<change-to-jw22-at-trlp-dot-com@trlp.com> wrote:

I have a couple of small devices that draw ~100mA each. I want to
control the power to these with a microprocessor. Each of these devices
is located remotely from the controller, and each has an onboard 5V
regulator, and can be powered from 13.8V ( This is an automotive
application). I could use a PC relay, to supply the 13.8 to these
devices, but I'm wondering, given that they have onboard regulators
(78L05), if I couldn't get by cheaper simply using a discrete transitor
(2N2907A) as a high-side switch.

Comments?


thanks

- jim


p.s. Let me just say this has to be one of the finest newsgroups in
existance. You all have been a GREAT help to me as I learn my way around
electronics.. THANKS!!!

 

[John Popelish]

James W wrote:

I have a couple of small devices that draw ~100mA each. I want to
control the power to these with a microprocessor. Each of these devices
is located remotely from the controller, and each has an onboard 5V
regulator, and can be powered from 13.8V ( This is an automotive
application). I could use a PC relay, to supply the 13.8 to these
devices, but I'm wondering, given that they have onboard regulators
(78L05), if I couldn't get by cheaper simply using a discrete transitor
(2N2907A) as a high-side switch.

High side switches imply that the high side is shared (or well
behaved) with respect to the microprocessor. So if the loads get
their ground and power from very nearly the same source as the micro,
then the transistor will probably work fine. If the loads are quite
noisy compared to the micro supply, then isolation from that noise is
a good idea. A relay or high current opto coupler might be better in
that case.

--
John Popelish