Will the driver be damaged if I connect reverse polarity?
Without protection, reversing the supply leads can cause permanent damage. A series diode drops 0.4-0.7 V; a P-channel MOSFET is low loss but needs a zener.
On a circuit with no reverse polarity protection, connecting the supply leads the wrong way round can cause permanent damage; so verify the polarity with a multimeter before connecting. If protection is to be added there are two routes. A series diode is simple but drops a continuous 0.4-0.7 V; in the 15-30 A region that means 6-20 W of heat. A P-channel MOSFET has far lower losses, but a zener between gate and source is mandatory.
What happens in the circuit when it is connected the wrong way round
In motor drivers the semiconductors in the output stage form an unwanted conduction path under reverse polarity. That path has no resistance; unless something comes in between to limit the current, a high current flows within a very short time. If the fuse blows the damage may stay limited, but a fuse is chosen to protect the cable rather than the semiconductor, and it usually blows after the semiconductor has gone. That is why "I have a fuse" does not stand in for reverse polarity protection.
Connecting the motor leads the wrong way round is a separate matter and does no damage; only the direction runs the wrong way, and it is corrected by swapping the leads. What carries the risk is mixing up the supply leads.
Comparison of the protection methods
| Criterion | Series diode | P-channel MOSFET |
|---|---|---|
| Voltage drop | A continuous 0.4-0.7 V | Very low; set by the on-resistance |
| Heat | About 6-10 W at 15 A, 12-20 W at 30 A | Far below the diode |
| Installation | One component, no extra circuit | Gate resistor and zener mandatory |
| Effect in a 12V system | The drop is torque lost outright | Negligible |
The choice is made according to the current: at small currents a series diode is practical, while in the 15-30 A region the heat and the voltage it costs become unacceptable. Whether a diode or a MOSFET is preferred is settled exactly at that threshold.
Why the zener is mandatory on a P-channel MOSFET
The source of the MOSFET is connected to the supply line and the gate to the chassis side. Because the on-resistance is very low, the loss is far below that of a diode. However, a zener between gate and source is mandatory: the VGS limits of MOSFETs are typically on the order of ±20 V, and in a 24V system tying the gate straight to chassis destroys the MOSFET. Transients in an automotive environment increase this risk. In a P-channel protection MOSFET application the zener and the series gate resistor are inseparable parts of the circuit.
What the protection does not cover
Reverse polarity protection guards only against connecting the poles the wrong way round. Overvoltage, transients, overcurrent and thermal wear call for separate measures; reverse polarity protection does not stand in for any of them. Whether a product has such protection is stated in the product data; where it is not stated, it should be assumed there is none and the installation done accordingly. Jump starting and welding work are a separate risk item; isolating the electronics before welding or jump starting is a matter independent of reverse connection but one that burns the same board. The protection component is itself a limit as well: a series diode or a MOSFET run above the current it was chosen for becomes the point of failure, cutting the supply line and stopping the motor.
What to do
Most of the error is prevented during installation, by a check that takes a few minutes:
- Colour-code the cable ends and label the terminals.
- Measure the polarity with a multimeter before connecting; the colour does not mean the same thing on every vehicle.
- Leave fitting the fuse as the very last step of the installation.
- If you are going to add protection on a permanent installation, take your current as the basis: a series diode at low currents, and a P-channel MOSFET with a zener at 15 A and above.
The full topic: DC Motor Driver Selection Guide