Reverse Power Control Circuit

Thread Starter

techwriter

Joined Jan 30, 2019
5
I have a circuit that operates at 3 VDC and less than 10 mA. I need to control power to the circuit in reverse. That is, the circuit is powered up when a switch opens and powered down when the switch closes. To conserve battery life the power control circuit must draw 1 uA or less when power to my circuit is shut off. Can anyone show me a way, perhaps using MOSFET switches, to do this?
 

crutschow

Joined Mar 14, 2008
27,761
Below is the LTspice simulation of a simple circuit to do what you want with one P-MOSFET and one resistor.
The P-MOSFET can be just about any logic-level type device (Vgs(th) max. <2v).
Note that the output (red trace) is on when the switch is off (green trace) and that the off battery current (yellow trace is 1µA.

1619053757297.png
 

AnalogKid

Joined Aug 1, 2013
9,362
3 M is a very high impedance, easily zapped by ambient noise. I would add a capacitor across R1 to lower the circuit's susceptibility to radiated noise. the capacitor value is a tradeoff between turn-on crispness and noise protection. 0.1 uF would yield a turn-on time of under 1/4 second.

ak
 

crutschow

Joined Mar 14, 2008
27,761
3 M is a very high impedance, easily zapped by ambient noise. I would add a capacitor across R1 to lower the circuit's susceptibility to radiated noise.
I doubt that radiated nose is a problem for this digital circuit with short leads, but adding the capacitor won't hurt.
 

Thread Starter

techwriter

Joined Jan 30, 2019
5
This is a bit unusual. Why can't you use a NO switch? Is the switch doing something else at the same time?
It has to be a sealed reed switch for a rugged environment. SPDT glass reed switches are available but if you ever looked at one closely you would be amazed they could manufacture it and wonder how reliable it is. They seal the glass end with the moveable reed biased against one reed, and with a very short distance to a third reed. The cost of crutschow's circuit is less than the added cost of a SPDT reed switch.
 

AnalogKid

Joined Aug 1, 2013
9,362
if you ever looked at one closely you would be amazed they could manufacture it and wonder how reliable it is.
Agree. I have encountered them, and once had to use one, but they always looked like a problem waiting to happen. BUT - I've never had an actual problem with one.

ak
 

Yaakov

Joined Jan 27, 2019
3,523
It has to be a sealed reed switch for a rugged environment. SPDT glass reed switches are available but if you ever looked at one closely you would be amazed they could manufacture it and wonder how reliable it is. They seal the glass end with the moveable reed biased against one reed, and with a very short distance to a third reed. The cost of crutschow's circuit is less than the added cost of a SPDT reed switch.
Reed switches are among the most reliable available. For a long time they were used in telephone switching, a high availability, high demand application. They use very little force which makes wear very little. You can find out about reliability from manufacturer's specs, you don't have to guess.

But you also don't need to use a double throw switch if that somehow disturbs you, they are available in SPST NO and NC configurations.

So I feel like I must be missing something here. I can't see why you'd go through so much trouble to use a dedicated switch opposite of its normal.
 
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