Home › Lessons › Power, transformers and three-phase › Reverse-polarity protection with a P-MOSFET

Reverse-polarity protection with a P-MOSFET

Power, transformers and three-phase · Intermediate · about 15 minutes · part of Electronics Technician

Connect a battery backwards and the electronics die. A diode in series protects but wastes about 0.9 V. A P-MOSFET wired “backwards” protects too: at switch-on its body diode conducts, the gate is then 10 V below the source and the channel shorts out the diode. Reversed, the gate is above the source and it stays off.

Reverse-polarity protection with a P-MOSFET: the circuit on the simulator's bench

Open this lesson in the labFree, in your browser, no account needed.

How it works

The Zener clamps V_GS to −10 V (the gate oxide is rated ±20 V). The channel is then about 201 mΩ, so at 1 A it drops 201 mV, against ≈ 0.9 V for a rectifier diode.

Try it on the bench

  1. Set V_bat to −12 V: nothing happens, which is the point.

Parts on the bench

In the course

Electronics Technician, module “Power supplies”.

More on power, transformers and three-phase