FIA master switch resistor

[Roger Ford]

Can anyone explain the purpose (and method of operation) of the ballast resistor attached to the FIA switch?

 

Does it connect the live circuit to earth when (and only when) the switch is off?

[Paul Deslandes]

Roger

 

when the switch is turned to 'off' it switches the resistor across the alternator output to act as a load so that, if operated whilst the engine is running and the alternator is generating an output, the alternator still has a load to dump its output into. Otherwise large surges can be generated that will destroy the rectifier and regulator pack.

 

Paul

 

 

[Roger Ford]

Thanks - that makes sense.

[Myles]

Reminds me I need a spare for the Westie - does anyone either; a) Have a spare kicking around that they don't need or, b)know somewhere I can get one that doesn't involve a postage charge many-times that of the resistor itself?

 

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[andy_h]

You don't have to have a resistor but it is recommended.

 

If you don't then leaving the lights on when you turn the engine off does the same thing.

 

 

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[Roger Ford]

Got mine from Vehicle Wiring Products (second one down). As far as I can tell, it's identical to the one originally fitted by Caterham. Postage £3.95+VAT.

 

Edited by - Roger Ford on 22 Nov 2009 10:01:01

[Paul Deslandes]

Maplin do 7 Watt and 10 Watt resistors that would be suitable. I'm not sure what the CC supplied value is but 3.3 or 4.7 Ohm would be ok. Its only there to provide a load for a few seconds while the alternator stops spinning. You could probably use an old headlight bulb but not sure about having one floating around under the dash.

 

 

[Roger Ford]

Sorry, Myles - you just wanted the resistor. I got a new one with my new switch but I'll be re-using the old one - so you're welcome to it. Blatmail me your address.

[john milner]

I was thinking of buying a cut-out switch as way of reducing the battery drain when not in use and as a secondary security feature.

Is the resistor wired in such a way as to run the battery down?

[Paul Deslandes]

Depending on precisely how its wired, the switch isolates the battery and disconnects the ignition so there is no drain from the vehicle circuits on the battery when the switch is in 'off' mode. When the switch is turned 'off' and disconnected from the battery, the resistor (roughly 5 Ohms) is connected across the alternator output so that there isn't a sudden 'no-load' voltage spike that might damage the alternator regulator or rectifiers. You would achieve the same result by leaving the headlights on to provide a load to the alternator but the resistor guarantees that the load is there when needed. So, no, the resistor doesn't drain the battery.

 

 

[Mavic82]

All that... but the primary purpose of the resistor on an FIA switch is to ensure the engine stops when you turn the master switch off. Just disconecting the battery is not enough to stop the engine as the alternator will keep the engine running.

[Paul Deslandes]

The FIA switch doesn't just disconnect the battery , by a second set of contacts it also disconnects the supply to the ignition circuits etc and its that disconnection that stops the engine. A third set of contacts that close in the 'off' position switch the resistor across the alternator output. A 5 Ohm 'ish resistor isn't going to bother the alternator too much as it will only put a 2 to 3 Amp load on its output, compared to a capacity of 30 Amps plus. Its there to stop the voltage surge. If the switch only disconnected the battery then, yes the engine would keep running due to the supply to the ignition and ancillaries being maintained by the alternator.

 

 

[Mark Widdup]

Im about to fit an FIA battery cut off to my Porsche 912 for classic car rallies. This car has a generator rather than an alternator. Do any of the electricians out there know whether I still need to kill the ignition and coil side of things as well as the battery cable to kill the engine or is this only relevant on alternators.

 

Thanks.

 

Mark

[Paul Deslandes]

I would think so, otherwise the generator will still be powering the electrics and the engine won't stop. Whether you still need the resistor and 'shunt' switch that drops the resistor across the ignition side of the switch will depend on whether you have an old style electro-mechanical regulator or a solid state, electronic one. I would have thought most likely the latter , in which case I think I'd include the switched resistor to protect it from surges and spikes when you throw the switch.