NCC series controllers.
Battery reversal.
We do warn 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.
Reversing the battery on an unprotected controller is not a good idea! It 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.
MOSFETs
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. The MOSFETs fuse open-circuit. Now the voltage across the board is free to rise to the reversed battery voltage.
Main Capacitor
The main capacitor is reversed. It will get hot and will get hot enough to explode in less than a minute.
Current source
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.
Zener diode
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.
Gate resistors.
The MOSFET gate resistors (10R CR16 size) don't usually fail when the MOSFETs are reversed, but if the MOSFETs fail to drain-gate short, they can go when correct polarity is restored.
Drivers
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.
Hiside drivers
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.
DCI-111
Most of our controllers use the same current source and zener as the NCCs. The DCI-11 certainly does. So, it you reverse connections to this, you will probably blow the current source and maybe the 9v1 zener diode. The circuit of this is available via our circuits section where there is also a technical description. Current source is at top right of the circuit.
Spare MOSFETs
These are detailed on a separate page.
Other pages with relevance to this subject
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Page's Author: Richard Torrens
Document URI: www.4qd.co.uk
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Last modified: Monday, 28-Aug-2006 21:13:24 BST
