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VTX and NCC Operating Voltages


These notes refer to the last issue of VTX controllers. So if you controller is older, the notes may be sequentially less ‘correct’.

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. See Pro, Scoota and NCC series controllers. Internal power supply and protection circuitry for a description of how the powering works.

This means that essentially the same controller may be used on 12, 24, 36 or 48v with only relatively minor changes so, for instance, conversion of a 12v controller to 24v is simple. 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 12v, 24v and 36v versions use the same components.

VTX controllers

The VTX controller is not specifically here covered yet, but the principles are identical to the NCC. See also VTX series, additional notes.

Use on wrong voltage

If you use, for instance, a 24v controller on 12v, it will probably appear to work OK. However there is a distinct possibility the relays will not pull in properly. If this occurs, relay contact resistance is increased and if the controller is operated at high current with the relays not fully pulled in the contacts will overheat and burn out.

If a 12v controller is used on 24v, the relay coil current will be twice its design value, so coil heating will be increased by a factor of 4. It is likely that sustained use will eventually damage the relay coil, which will then fail.

For more understanding read the rest of this page.

On other voltages, for instance 18v or 30v the critical factor is the relay coil operating voltage, the manufacturers data sheet for the relays is available.

Components to check

In general the changes and components to check on all controllers are:

  • 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 coil.

This resistor is fitted across two large copper lands on the component side of the circuit board, indicated by arrows in the diagram below. The link (also on the component side), indicated by the third arrow must be cut before fitting the resistor (or the resistor will be bridged out).


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.

Minimum operating voltage

The internal supply is 9.1v. The current source requires at least 1v to operate, so if the supply voltage at the controller’s terminals falls below 10v, the internal 9.1v supply will drop, reducing the value of the current limit.

Remember that the controller is chopping the battery current at 20kHz so the battery voltage cannot be read on a normal meter (as this will average the current/no current voltages) but you must look at it with an oscilloscope to see what it is when current it being drawn, at the troughs of the 20kHz squarewave voltage which will be present. Also – your battery lead inductance and resistance will add to voltage drops

These are the reasons we do not like high current 12v controllers!

NCC 60 and NCC 70

For 24v controllers

  • To operate at 36v, use a 180 Ohm 1 watt.
  • To operate at 48v, use a 390 Ohm 2 watt.
    Note that 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

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

  • For 24v, use a 82 Ohm or 100 Ohm 2 watt.
  • For 36v, use a 180 Ohm 4 watt
  • For 48v, use a 390 Ohm 4 watt.
    Note the 48v version has several transistors, MOSFETs and main capacitors upgraded.

Replacement relays

You can order replacement relays – the same relay is used on the NCC70 and the Pro-120. It may also be used on the VTX (it is slightly smaller than the original).

Replacing the relays on the NCC-70 is relatively easy as the board in single-sided.
However – be warned that it is extremely difficult to remove the relays from the Pro-120 without damaging the board as the holes are plated through and the high thermal mass around the relays makes desoldering extremely difficult.


NCC 35

The same system is used on the NCC 35 but the relay is smaller so resistor values are different. Unfortunately the original relays (Schrack) were discontinued so we have been forced to use other makes: the coil resistances do vary!

  • 12v coil is around 150 Ohms
    • For 24v, use a 150 Ohm 1 watt
    • For 36v, use a 270 or 330 Ohm 2 watt
    • For 48v, use a 470 Ohm 3 watt
  • 24v coil is around 650 Ohms
    • For 36v, use a 270 to 330 Ohm
    • For 48v, use a 560 to 680 Ohm

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.
  • (ST)P60N6. A 60v device, OK for use on 12, 24, 36v.
  • RFP-70N06. A 60v device, OK for use on 12, 24, 36v.
  • (ST)P75 NE7. 70v for 48v controllers.

Series current source

The current source on 12v and 24v controllers is a TO92 transistor delivering about 27 milliamps. At 36v this would drops some 27v so would dissipate about 736mW – which is just within the ratings of most modern TO92 transistors.

The BC556 and BC327 (which may be fitted) have been uprated over the years, but on older controllers they may not take this dissipation, so should be replaced by a TO26 or other metal cased transistor, capable of dissipating up to 1 watt.

This is really a factory-fit and is probably not suitable for retro-fitting! Unfortunately the older NCCs are now so old that we cannot offer this support so if you want to convert it is at your own risk.

The modern NCC (older Mk 1s and Mk 2) has space to fit a TO220 power transistor (TIP42) which is needed for 48v operation.

Main Capacitor operating voltage

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

Undervoltage cutback

This feature is not fitted to NCC series controllers.

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 56v.

Other components

Generally on later NCCs other components are rated suitable for 48v operation but older controller may have had transistors fitted which were not suitable for 48v operation.

VTX series, additional notes

48v operation VTX controllers are not manufactured for 48v operation and should not be converted

12v operation Controllers date coded before 08a (January 2008)(see date Codes) are not recommended for conversion to 12v. If the supply voltage ever falls below about 8v, the controller can fail catastrophically. Of course in a well maintained system the battery should never get that low. Three surface-mount resistor values were changed in 2008 to prevent this.

It is also possible that regenerative braking may be poor with a converted controller. A resistor value change is then necessary.