I understand that a diode allows current to flow in one direction only, and that inserting a diode in series with a power source will protect the circuit from a reverse polarity battery.

How do I calculate what capacity diode is appropriate? I want to minimize the voltage drop as the battery is a one cell lithium unit with maximum 4.2V. The device (a camera) seems to shut itself off when the battery hits 3.8V.

enter image description here

  • \$\begingroup\$ First thing you need to know is the current consumed by the device. But you seem to be in trouble as even the "low-vf" diodes are having Vf around 0.4V (st.com/en/diodes-and-rectifiers/…). \$\endgroup\$
    – Eugene Sh.
    Aug 10, 2018 at 21:10
  • \$\begingroup\$ You might want to consider using a low Vth P-channel FET as reverse polarity protection device instead of a diode. Another option would be using a diode across the input with a fuse, resettable or other. \$\endgroup\$
    – John D
    Aug 10, 2018 at 21:19
  • \$\begingroup\$ Is this from motor kickback turning off or just battery reversed? \$\endgroup\$ Aug 10, 2018 at 21:35
  • \$\begingroup\$ What kind of protection do you mean, against what? Against an operator mistake to connect it in reverse, or else? \$\endgroup\$ Aug 10, 2018 at 22:20
  • \$\begingroup\$ if you care more about the camera than the power source, you can put a a several amp diode across the red and black wire; no drop in normal operation, 99%+ drop in reverse. fuse in front would blow in that condition, if you don't like shorting batteries for long periods. \$\endgroup\$
    – dandavis
    Aug 10, 2018 at 22:53

2 Answers 2


For such a low allowable drop, you need a zero voltage drop diode. It can be done with a P-Channel enhancement mode MOSFET. E.g. a IRFD9120 is sufficient for up to 1A drain current.

(Use a multimeter and check how much current your camera draws. Check it with display on and zoom moving.)


simulate this circuit – Schematic created using CircuitLab


You don't need any diode or transistor for reverse polarity protection. The RC toy hobby industry offers a dozen of polarity-protected connectors, rated from 1 A to 100 A. You just can't plug them in reverse, mechanically. This was the solution for RC toy industry, where underage operators may be very unreliable and irresponsible.

enter image description here

  • \$\begingroup\$ This is good information. As you can see in my photo, I'm "cheating" on my battery usage by tapping into a single cell of a 3 cell RC battery, currently using just two non-polarized pins. I can use your idea by having three different cables, each wired to appropriately tap off a different cell (using the battery's balance connector). \$\endgroup\$ Aug 13, 2018 at 17:18

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