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I want to make an UPS using a LiPo battery for a SIM800l cellular module, I have an SIM800l module attached to a 3.7V wall adapter but I also have connected the SIM800l to a 3.7V LiPo Battery, I want to stop using the battery when there is current from the USB wall charger.

I do not want to use a standard powerbank since these always use the battery.

What about this circuit:

enter image description here

but I would like to know what MOSFET and diode should I choose for a 3.7 Volts input and 3.7 Volts output. Will a BS250P MOSFET work?

S. Feunmajer
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1 Answers1

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No, this won't work the way you want it to. The PMOS transistor won't be conducting until its gate voltage falls (at least) several volts. By the time that happens the voltage on your load is so low that the SIM800 module will not work properly.

The typical way to do this is to use two Schottky diodes. The cathodes of both are tied together and then to the load. The anode of one diode is tied to each of the voltage sources. Current will flow primarily from the source that is about 0.3V higher than the other, and there will never be a time when current does not flow from one of the sources.

Elliot Alderson
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  • Hi - FYI, something similar was already given on the OP's previous question a few hours ago. Then they made several edits to the original question and changed it into the same as this new question. IMHO the question is unclear, as I explained under the question :-( – SamGibson Jul 31 '18 at 23:38
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    @SamGibson I thought this question looked familiar. Hey, OP, don't do that! Modify your original question or, better yet, take the time to ask a complete question in the first place. – Elliot Alderson Jul 31 '18 at 23:44