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'[OT] Power control help...'
1998\02\02@094046 by Jeff Cesnik

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I am designing a battery-powered device (using a 12 volt car battery)
that needs to be turned on and off via a momentary pushbutton switch and
a port pin of a microprocessor.  The micro would turn the system off if
there was no activity for 20 minutes or so.  The problem is that I need
to switch approx. 500-1000 mA at 12 volts before the regulator stage.
Does anyone have any ideas how this could be accomplished efficiently?
I want to minimize drain on the battery in the off state, and would
prefer not to use relays for reliability reasons.

BTW - This is not entirely off-topic, as the device is connected to an
RS-485 network of PICs! :)

-JC

1998\02\02@135008 by Mike Keitz

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On Mon, 2 Feb 1998 09:34:20 -0500 Jeff Cesnik <spam_OUTjcesnikTakeThisOuTspamCLARAVISTA.COM>
writes:
>I am designing a battery-powered device (using a 12 volt car battery)
>that needs to be turned on and off via a momentary pushbutton switch
>and
>a port pin of a microprocessor.  The micro would turn the system off
>if
>there was no activity for 20 minutes or so.  The problem is that I
>need
>to switch approx. 500-1000 mA at 12 volts before the regulator stage.
>Does anyone have any ideas how this could be accomplished efficiently?

A P-channel power MOSFET.  Connect the source to the +12V input, drain to
the regulator and other parts needing switched +12V, and drive the gate
to ground (using small NPN transistor, pushbutton switch, etc) to turn it
on.  There needs to be a resistor from source to gate to ensure it turns
off.  Also if the +12V input is able to 'spike' much above 12V,
protecting the gate from overvoltage would be required (use a 12V Zener
diode from gate to source, and a resistor to limit current flow through
the gate drive circuit).

>I want to minimize drain on the battery in the off state, and would
>prefer not to use relays for reliability reasons.

This circuit draws practically no current (only very small FET leakage)
in the off state, and only draws the current through the gate resistor (1
mA or so) in the on state.

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