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Reverse polarity protection [tour 26]

If you inadvertently connect the battery up the wrong way round , then depending on which model you have, the controller may be destroyed in quite a spectacular manner! Only you can decide if this is likely or not: if you are making a machine for your own use then (hopefully) it is unlikely. Or maybe you can mount the batteries so connecting them backwards is difficult?

Internal protection

Reverse polarity protection is fitted internally to these controllers:

  • DNO-5 and DNO-10
  • Pro-120 and Pro-150
  • Porter 5, Porter 10 and Porter 40
  • Uni


An external battery contactor can be easily added to the 4QD-200 / 300 and the Pro-100 / 160 / 360.

If 100% guaranteed protection is required, you can fit an external relay to any controller as shown.


When the relay is off (open contacts), current flows through the 470R resistor to charge up the main capacitors in the controller.

When the ignition switch is closed, the relay operates and shorts out the resistor, applying full power to the controller. Of course the main capacitor must have charged up adequately before you close the ignition: there must be enough voltage across the capacitor to operate the relay coil!

If the battery is reversed, the MOSFETs within the controller(s) act as diodes, shorting out the + and – supply lines. With the circuit above there will then never be more than about 1.5 v present across the internal capacitor, so the relay coil cannot operate if the battery is reversed and the reverse current is restricted to a safe value by the resistor.

The value of the resistor can be altered to suit any application.