🔀 Reserve polarity protection simple electrical circuit
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
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.
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.
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.
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.
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.
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
So, install of small protection circuit, won't take up much space on the board.