I've put together the following circuit, it's just a rechargable LED torch, just to try load sharing:


simulate this circuit – Schematic created using CircuitLab

BAT1 is a LiPo battery, 5.2 V comes from the AC adapter when the thing is being charged. There's a charging circuit connected, but I'm using a module, so I couldn't include that in the schematic, but I think it should be irrelevant as the battery is basically disconnected in this configuration.

The voltage drop on D1 was around 0.2 V, but the voltage between V1 and ground was around 3.9. The battery was at 3.7-3.8 V at the time, but like I said, it should be irrelevant.

  • What am I missing? Where could that additional 1 V voltage drop come from?
  • Is my battery safe, and completely disconnected from the load?
  • Can you recommend a test setup so I can check this without actually using a LiPo battery?

It is entirely possible that I've messed up assembling the circuit, I'll probably build another later anyway.

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    \$\begingroup\$ If the volt drop on D1 was 0.2 volts and V1 is 3.9 volts then your DC supply voltage can only be 4.1 volts. \$\endgroup\$ – Andy aka Oct 14 at 9:41
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    \$\begingroup\$ Are you sure the FET source and gate are correctly identified. If they were swapped the supply may charge the battery via D1 and FET body diode.- What is the supply voltage AT the supply? \$\endgroup\$ – Russell McMahon Oct 14 at 9:48
  • \$\begingroup\$ @Andyaka That's what I thought, for that I want to make another circuit and have a test setup (without a LiPo), but I don't exactly know how to do that. \$\endgroup\$ – wrock Oct 14 at 10:11
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    \$\begingroup\$ So, measure the total voltage and prove 5.2 volts IS 5.2 volts. \$\endgroup\$ – Andy aka Oct 14 at 10:17
  • \$\begingroup\$ @RussellMcMahon Now that you mention it, I've had failed test runs because I've set up the FET incorrectly twice (mostly because I wasn't paying enough attention). It's not impossible, that I have semi-fried the FET with that or the multiple soldering/desoldering. \$\endgroup\$ – wrock Oct 14 at 10:20

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