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What is the simplest and best device (or circuit) to stop the flow of current in a 3.7V (2A) line when current in another 5V (2A) line is present?

Basically I want to make an UPS using a LiPo battery for a SIM800l, 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.

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  • \$\begingroup\$ An electromagnetic relay. \$\endgroup\$ – Andy aka Jul 31 '18 at 16:05
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    \$\begingroup\$ That depends on just what your requirements are. Are these two lines feeding the same thing? Or are they isolated? \$\endgroup\$ – Hearth Jul 31 '18 at 16:05
  • \$\begingroup\$ What is the application? There are chips that could do this. \$\endgroup\$ – Billy Kalfus Jul 31 '18 at 16:06
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    \$\begingroup\$ Are you sure this is not another case of XY? Because the mentioned values hint strongly at switching between an adapter/USB and battery. Which is usually done by voltage, not current. i.e. the X problem would be: a way to disconnect the output line when voltage on the input line is present. \$\endgroup\$ – Maple Jul 31 '18 at 16:29
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You can use a basic diode switch like this:

schematic

When 5 V is present, no current will flow from the 3.7 V source.

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    \$\begingroup\$ 0.6 volts (the drop across the active forward-bias diode) out of 3.7v (or outa 5v) is a significant portion. you might have to accept 15% power loss or, at best, 85% efficiency. \$\endgroup\$ – robert bristow-johnson Jul 31 '18 at 21:26
  • \$\begingroup\$ @Robert The O.P. wanted teh simplest [sic]. A 15% hit on efficiency is a fair price for simplicity in many cases. \$\endgroup\$ – Nick Alexeev Aug 1 '18 at 15:59

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