Modifying the Pro-120 for other voltages

All controllers made by 4QD are generally wide voltage operating range: limits are the operating voltages on the MOSFETs and the relays, and the power dissipation in the current source transistor used for the internal supply.This means that essentially the same controller may be used on 12, 24, 36 or 48v with only relatively minor changes. However since this involves upgrading certain components it is not necessarily acceptable to modify a controller to work on a voltage higher than that for which it was originally specified. This particularly applies to the 48v versions where several components (mainly semiconductors) are changed but in general the 12 24 and 36v versions use the same components.

If you are interested in modifying your controller then these are the areas to consider…..

  • Relay coil operating voltage
  • MOSFET operating voltage
  • Series current source
  • Main Capacitor operating voltage
  • Undervoltage cutback (not fitted to NCC series)
  • Overvoltage clamping

Relay coil operating voltages

  • 12v controllers use a 12v coil
  • Other controllers use a 24v coil
  • 36 and 48v controllers have a resistor in series with the (24v) relay coils.

On the Pro, three separate resistors are used, one for each relay coil. These are indicated in the diagram below by three red arrows. Rp is the power relay resistor and Ra and Rb are the direction change relay resistors.

Note that these resistors were not fitted to the circuit board until issue 6: earlier 36v and 48v controllers had them fitted as a production modification so are not suitable for modification.

The early controllers only included a single resistor to feed both direction relays (which are only ever energised one at a time).

Beneath each resistor is a scratch-through track. This is left intact on the 12 and 24v controllers and only broken on the 36v and 48v ones, to engage the additional resistors.


All three resistors should be the same value:

  • For 36v, use a 180 Ohm 1 watt.
  • For 48v, a 390 Ohm 2 watt resistor is used but conversion from lowwr voltages is not practical as the 48v version has several transistors, MOSFETs and main capacitors upgraded.

The relay coil resistance is specified as 93 ohms for the 12v coil and 330 for the 24v coil (but in practise they tend to be nearer 389-390 ohms), so a resistor equal to the coil resistance will effectively double the operating voltage.

12v controllers

By putting resistors in series with the relays on a 12v controller, it also could, in theory, be made suitable for operation on other voltages. However this is not recommender: you need high power resistors and things will get quite hot.

The coil resistance of the 12v relays is about 86 ohms. So the required series resistors are:

  • For 24v, use a 82 Ohm 2 watt.
  • For 36v, use a 180 Ohm 4 watt
  • For 48v, conversion is not possible: the 48v version has several transistors, MOSFETs and main capacitors upgraded.

Manufacturer’s relay coil ratings

This is a very technical subject as the maximum allowable voltage depends on ambient temperature. Also at high currents the contacts get hot and some of this heat gets transferred to the coils, reducing the maximum allowable operating voltage in a complex fashion.

Also, if the coil is hot, then the ‘must operate’ voltage rises, because of increasing coil resistance. So this value is dependant on how long since the relay last operated and on how much current the contacts were carrying.

This all makes it impossible for us to give a definitive operating voltage range but, for most purposes, the allowable operating range will probably be something like 80% to 150% of the nominal voltage at 40°C, so the 12v coil should work from 9.5v to 18v and the 24v from say 19 to 36.

MOSFET voltage

MOSFET technology improves. So we use different MOSFETs as technology and prices alter.

MOSFETs most likely to be present are:

  • BUZ-100S. A 55v device, OK for use on 12, 24 and 36v. Do not use on 48v!
  • SPP77N06S2-12. A 60v device, OK for use on 12, 24, 36v.
  • RFP-70N06. A 60v device, OK for use on 12, 24 and 36v.
  • STP75NE75. 75v, used on 48v controllers.

Series current source

The current source on 12v, 24 and 36v controllers is a TO92 transistor delivering about 30 milliamps. At 36v this drops some 28v so dissipates about 700mW so gets very hot! On 48v it would self-destruct (over 1 watt) so we fit a TO220 transistor which is screwed onto the base as a heatsink. This is a factory-fit, is not suitable retro-fitting and is the reason the 48v version is not available as a bare board.!

Main Capacitor operating voltage

Commonly 50v, OK for 12v to 36v but not for 48v which requires 63v capacitors.

Undervoltage cutback

On 24v versions, this is disabled. On 24v versions a 3v9 zener is fitted and on 48v versions this increases to 8v2. Fine adjustment of the undervoltage cutback value can be done (as explained in the manual) by adding a resistor in series with the ignition input.

For most applications it is only important that the undervoltage cutback comes in just early enough to stop the battery voltage falling low enough to de-power the relays.

Overvoltage clamping

Earlier 12v and 24v controllers normally had a clamping zener of 36v. 36v controllers use 47v. Later 12v, 24v and 36v controllers all use 47v.
48v controllers use 72v.