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Pro-120 Ignition Options

The Power relay drive circuit on the Pro-120 can be factory configured in three different ways. This page explains them.

The power relay is switched on and off by a transistor that detects the internal 9v1 supply rail: this is the rail that provides the supply for all of the drive electronics. It is configured this way so that if there is any internal fault, then the power is disconnected immediately.

The transistor switches the bottom end of the relay to battery negative. The options involve where the high end is connected.

  • Standard
  • Ignition
  • Diode


This is the ‘safest’ configuration, in that it has maximum protection against external faults. The hot end of the relay is wired to the internal B+ supply line, so the relay will not power up until the main capacitor has charged up sufficiently. The arrangement is fully described in our service section. The disadvantage of this feed is that the ignition cannot be ‘hard-wired’ – a switch must be fitted. If it is hard-wired, the capacitor may not charge up enough for the relay to pull in, because the relay coil current will flow through the trickle resistor.


The power relay coil and the parking brake feed is hard wired to the ignition line, pin B of the 6 pin connector.

This connection is advocated where the ignition switch is required to act as an emergency stop. Opening the switch will re-energise the power relay and remove power from the parking brake (which operates the parking brake).

If the controller is electronically working, it will ramp down to zero – however since this is a braking action, it will try to return energy to the battery. That cannot happen as the battery is effectively disconnected form the controller.

If however the control system has fouled to fill speed (maybe the throttle pedal is mechanically fouled, or water has entered the controller), then depowering the relay and applying the parking brake may be the only option!

Using the ignition switch in this mode while travelling normally is not safe: the parking brake is applied and these brakes are not designed to be applied while moving. The brake disc is likely to be broken.


In th -RW version, the top of the relay (and the feed to the parking brake) is fed not from the internal B+ supply rail but from the B+ input, via a diode (to maintain reverse polarity protection). In the RW version, powering up is guaranteed even with a hard-wired ignition connection.