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Power Systems

Sun 17 April 2016

Preventing Reverse Polarity

The easiest way to prevent a battery from getting hooked up backwards in your robot is to key your power connectors to they can only be installed one way. However, if people are making their own, sometimes this isn't good enough. A better way is to design your circuit so it prevents this.


Using a diode is one easy way to prevent current in your system from flowing in the wrong direction. However, they have a forward voltage drop which both reduces the amount of available battery voltage and wastes power.

Part Voltage Drop at 2A Power at 2A
Diode (1N5400) 0.85V 1.7W
Schottky Diode 0.55V 1.1W
PNP MOSFET R_DS(ON)=26 mOhm to 2A * .026 ohm = 52 mV 1.04mW

Using a PNP MOSFET is better because it lowers your battery voltage less and wastes less power.

This video does a great job of explaining this a little more. It also explains why you might have to include a resistor and zener diode to clamp the voltage, depending on the MOSFET you select.

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