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simple battery protection for 3xAAA vs USB
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djacobow:
I've got a simple device that needs to be ultra-cheap. Think holiday decoration. It normally runs from 3xAAA batteries and runs fine on 4.5V. I'd like to add USB power to it, but want to make sure the batteries don't see the USB voltage and start to "charge" (It is not my intention to make a rechargeable device and these would be normal alkalines in most cases.)

I've came up with the circuit attached, but I see now it won't work properly because the body diode of the transistor will conduct current from USB power back to the battery. The effect is small when the batteries are new, but is big when the battery is partly discharged.

I probably would not use the transistor shown. Most likely would use a DMP1045, which is already in the BOM.

Anyway, trying to keep things cheap. I could ask people to remove the batteries when using a USB supply, but I am 100% sure that will result in hot, leaky batteries and unhappy users.


schmitt trigger:
I've succesfully used an identical circuit since 1999, but I used an IRF7205 in a SOIC package.

But the drain-source must be reversed, otherwise the intrinsic body diode may conduct under certain situations.

Haven't checked your Mosfet specs. Just make sure that it is fully enhanced at VGS= -4.5v
NiHaoMike:
As stated, flip the MOSFET around. You can also use a regular diode rather than a Schottky, in order to cut cost and/or use the voltage drop to your advantage.
djacobow:
That was it, folks! Transistor was reversed.

Yeah, the transistor I will use is pretty hard on at -4.5V.
mariush:
Wouldn't two diodes work?

For example PMEG2005CT (two diodes with common cathode per chip) , 20 cents each if you buy 100: https://www.digikey.com/product-detail/en/nexperia-usa-inc/PMEG2005CT215/1727-5188-1-ND/2531787
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