Power down state [tour 13]

When the controller is powered down (ignition turned off) but with the battery still connected, different controllers behave in different ways. There are, essentially, 3 possibilities:

  • Open circuit
  • Short circuit
  • Diode

Open circuit

All semiconductor full bridge controllers (i.e. those that do not use relays for reversing) tend to switch all four legs of the full bridge off when inactive. If you want technical description of a full bridge circuit, there’s one in our circuits archive. But if you turn all four legs of a bridge off it goes open-circuit.

With the controller output open-circuit, there is no load across the motor and it is free to rotate.

The following controllers have this ‘power down state’:

  • 4QD series

Short circuit

Controllers that use a half bridge with relays for reversing, relax to an unpowered state where both relays are off and the controller’s output (therefore the motor) is shorted therough the relays. The motor is not then free to rotate as this will cause the motor to attempt to generate and this causes a current to flow in the relay contacts. This is not a problem unless you tow the machine fast behind a car when you could burn out either the motor or the reversing relays in the controller.

However – it does make it a little difficult (not impossible) to push your vehicle by hand. If this is required, fit a motor disconnect switch, Switches sold as Battery Isolator Switch are usually quite suitable.

The following controllers have this ‘power down state’:

  • DNO series
  • Pro-150
  • Pro-150


This gets a bit more technical! All controllers are, in effect, a diode (but this may is switched out of circuit in a reversing controller). A diode connected across the motor will allow it to free wheel forward, but not backwards. Forward being the direction the controller normally would turn it.

So, as you might guess, this applies to all non-reversing controllers.

The following controllers have this ‘power down state’:

  • Porter 5 and Porter 10
  • Porter 40
  • Uni

If you want to get technical as to why this diode is present, see PWM motor speed control.

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