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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

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  • \$\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\$
    – Tony Stewart EE75
    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\$
    – Ale..chenski
    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

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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.)

schematic

simulate this circuit – Schematic created using CircuitLab

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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.

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  • \$\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\$
    – Mark Harrison
    Aug 13, 2018 at 17:18

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