Power down state

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:

• Short circuit
• Diode
• Double diode

Short circuit

Controllers that use a half bridge design with relays for reversing, relax to an unpowered state where both relays are off and the controller’s output (& therefore the motor) is shorted through 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 above a walking pace when you could burn out either the motor or the relay contacts in the controller. If this is required, fit a motor disconnect switch, switches sold as battery isolators are usually suitable. The following controllers have this ‘power down state’:

• DNO series
• VTX series
• Pro-120
• Pro-150

Diode

All controllers use mosfets which include a body diode. 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. The following controllers have this ‘power down state’:

• Porter 5 and Porter 10
• Porter 40
• Uni

Double diode

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 turn all four legs of an H-bridge controller off then you get two diodes in series with the motor.

The following controllers have this ‘power down state’:

• Pro-100 / 160 / 360
• 4QD series

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