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Over voltage protection in the 4QD series

If the battery is ever disconnected (either accidentally or via an isolation switch) on a controller with regen braking, then if the controller tries to brake it will simply pump up its own supply line – until something breaks down. This will probably be catastrophic! Therefore all controllers made by 4QD incorporate over voltage protection mechanisms against this.

On the 4QD series there have been two such mechanisms (the reasons for having two are historical): an overvoltage trip and an overvoltage clamp.

Overvoltage trip

Some time around the middle of the life of issue 11 base boards, we dropped the over-voltage trip mechanism as it is not useful when overvoltage clamping is present and it can be a nuisance during bench testing (when a bench power supply is used). The trip was fitted before the overvoltage clamp was added, so if you are going to remove the trip – make certain that the board has got the overvoltage clamp fitted! Generally this will present when issue 5 (or later) baseboards are used with issue 6 (or later) control boards, though it may be retrofitted to earlier issue control boards. See 4QD issue number history.

To disconnect the overvoltage trip, simply cut one end of the diode marked OVS-Z on the baseboard layout diagram, just to the right of the 14 way connector.

Overvoltage clamp

This works by feeding a signal to the control board to increase speed – the assumption is that the overvoltage is caused by excessive braking, so speed must be increased to reduce braking.

There may be instances where the clamping level needs to be altered. The feed on the base board is a zener diode (OVC-Z) and a 3K3 resistor (OVC-R) – both indicated on the baseboard layout diagram.

The zener determines the voltage where clamping starts, and the resistor controls the sharpness of the clamping.

Clearly you should not fit components that can result in overvoltage, however the protection mechanism is only protection against fault conditions – so it could be argued that it should never be engaged. If you agree with this argument, then it follows that it can be disconnected. We should warn that controllers damaged from overvoltage will not be covered by the guarantee.