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Converting Parking Brake to Brake Light

The 4QD series has a Parking Brake solenoid driver. Optionally controllers can be supplied from the factory with this converted to a ‘Braking’ Light driver. This page describes the modification. Warning. The parts that need to be changed for the conversion are located inaccessibly, on the base board, on both the -150/200 and the -300 base boards. This means that the modification is not suitable for retro-fitting and this page is not intended to encourage DIY modifications. If you do attempt DIY modifications be very sure that you understand what has to be done and that you have the necessary skills to undertake the task.


This page describes how to dismantle the controller. You will also need to remove the negative busbar (the thinnest one) – which also requires unsoldering the MOSFET source leads.


brake light conversion
For normal use, the two outputs ‘Stopped’ and ‘Demand Zero’ are fed into a NOR gate. Tr1 is off (output high) when the motor not stopped and/or the demand speed is not zero. Either condition powers the parking brake solenoid to disengage it. See 4QD series brake driver modification for further discussion.

To modify, the components marked œ are removed, those marked  are added. The diode marked ±is removed and replaced by a link.


A layout digram has not been prepared, at this time, for this modification. However the baseboard layout diagram may help.

Circuit Description

The brake driver can only be operated when Tr1 is turned off. Tr1 is turned on whenever the motor is Stopped, from pin 6 of the 14 way connector. In addition, the lights are turned off whenever Hi 1 or Hi2 are low: these (pins 4 and 5) are the hiside driver outputs. The hiside driver inputs are high to turn the hiside drivers on, in normal drive. Regeneration is done entirely on the lo-sides of the bride, so when both hisides are off, and the motor is going, the controller must be in braking mode: this is the logic required for operating a braking light.

Other points

The brake light driver is protected by the R47 (470 milliOhm) resistor and Tr2. Tr2 feeds a trip circuitry to switch off the controller’s power is the brake driver ever hits current limit. The current limit is somewhere over 1 amp.

Bulbs have a cold resistance which may be 20 times lower than their hot resistance, so, unless the bulb is of low wattage, the controller is likely to trip out when the brake is engaged. It is therefore advisable not to switch the bulbs direct but use a suitable relay. The driver can operate the relay coil.