Damage from Reversed Batteries (NCC/VTX)
This page details the reverse battery damage that can occur to a VTX or NCC if the battery is connected the wrong way round.
Be warned that reversing the battery is instant death to the controller and that, if there is any chance of this event, a battery fuse or a separate relay should be fitted to give protection. However this is usually done only after bitter experience has shown the necessity! See Fuses and circuit breakers in our FAQ sheet for more information.
Boxed VTX controllers are supplied with a battery fuse so they normally survive battery reversal.
Reversing the battery on an unprotected controller is not a good idea! If the correct fue is present, the MOSFET will simply conduct, shorting out the battery and thus blowing the fuse. If the MOSFETs survive (as they will with a suitable fuse) nothing else happens.
If the MOSFETs blow, the effect is instantly damaging. However, the exact damage that occurs depends on the exact circumstances: 12v or 24v, how thick was the battery wiring and how for what length of time was the battery reversed. Additional damage may also be caused if the battery is then reconnected the correct way round!
This page is not intended to tell you how to repair such damage: rather it should show you that there’s quite a lot to do. If you wish to go ahead despite this, email us for access to the circuit diagram.
If you’ve got this far, see Pro, Scoota and NCC series controllers. Internal power supply and protection circuitry. It gives a description of the power supply system of the NCC series.
The VTX series uses very much the same circuitry – except there is an extra transistor to power the hiside drive. This, too, blows if the battery is reversed!
MOSFETs when reversed are conducting diodes. So two MOSFET diodes are connected across the battery with current limited only by the battery and the wiring to it.If a suitable battery fuse fuse is fitted this effective short circuit will blow the fuse and protect the MOSFETs and the rest of the circuitry.
If no (or an unsuitable) fuse is fitted, then the MOSFETs will fuse open-circuit, possibly cracking open. Now the voltage across the board is free to rise to the reversed battery voltage and other components will be damaged.
If some MOSFETs are replaced, but some are still faulty, then additional damage is done when the battery is reconnected.
The voltage on the main capacitor can only reverse significantly once the MOSFETs have blown. When this reverse voltage rises the capacitor will get hot – and hot enough to explode in less than a minute.
The current source is reverse biased. Transistors when reversed can stand about 11-12v across their base-emitter junctions. So on a 12v controller, they may survive, but not on 24v. The 22R current sensing resistor for this current source can get very hot – it may burn out.
When the current source is reversed, excess current flows through it and through the 9v1 zener diode which regulates the internal power. If sustained, the zener junction may melt so the zener is open circuit.
The MOSFET gate resistors (10R CR16 size) don’t fail immediately when the MOSFETs are reversed, but if the MOSFETs fail to a drain-gate short, they can go when correct polarity is restored.
If the 10R resistors have failed, the drivers (mainly the PNP on the low side) should be checked.
Gate clamp zener.
There is a 9v1 gate clamp zener: if the resistors have blown this may need checking, but is usually OK.
Should be checked: these are usually OK on a straight battery reversal but sometimes not.
Relay drivers and diodes
When reversed there is nothing to limit current through the now-forward-biased relay catching diodes and the PNP relay drivers. These will need to be replaced, though on 12v they may survive.
Parking Brake Driver
On NCC-70 and boards fitted with a parking brake, the brake catching diode and the TIP power transistor may well blow and should be tested.