🔋 Reserve polarity protection simple electrical circuit

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If the power supply is turned on incorrectly the device could be partially of fully damaged. Simple circuit for protection against reverse polarity connection help to give a voltage only in correct polarity case and with lowest power losses.

❕ This is the typical high-efficacy and simplest protection circuit.

Electrical schematic

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Purpose of the components:

V1 — Input voltage supply (Battery/SMPS);
Led1, R2 – Reverse polarity indicator (optional, maybe useful);
Led2, R3 – Load (your electrical circuit);
R1 — Limit gate charge current (1k-10k) and Zener diode stabilization current (Izt);
VD1 – Zener diode (limits gate-to-source voltage of transistor (typical Vgs(MAX)=20V));

Simulation

Using Microcap 12 (now free) powerful simulation software let's check how circuit works ?.

High side

  • Not recommended for high current, due to worse electrical parameter of P-channel transistors.
  • Recommended for low-current devices (like MCU);

When power supply V1 connected properly Led2 glow, when in reverse polarity Led2 doesn't glow.

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Low side

  • Utterly not recommended for devices with one power supply, where ground should be connected
  • Recommended for battery supplying device

When power supply V1 connected properly Led2 glow, when in reverse polarity Led2 doesn't glow.

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Real use example

Here is part of real electrical schematic of DC-DC converter board with some connectors for power distribution, where input power is battery 12 VDC – 24 VDC.

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Because size on this board is not constraint and input power could be high (50W — 70W), for the easiest assembly, even in case of components sourcing problems two footprints for different transistor packages placed on the schematic and correspondingly on the board.

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The board is powered by external battery with onboard 2 position connector XT60, so it's pretty hard, but possible to insert plug in socket incorrectly.

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Conclusions

Also, it's maybe very helpful to have the possibility to turn-off the protection, so place resistor 0 Ohm DNP (Do Not Place) or soldered jumper XT1 between drain and source of transistor. In which case it might be necessary? When transistor parameters not so good (especially of P-channel) ,current very high and voltage low, so for short high-current pulses voltage drop of circuit could be too low for connected load.

Modern MOSFETs have excellent electrical parameters with extremely small sizes (SOT-23 and smaller):

  • AO3400: Vgs=30V Id=5.8A
  • AO3407: Vgs=30V Id=4.3A

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So, install of small protection circuit, won't take up much space on the board.

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