Helpful Hints



A little time spent getting things right can greatly improve your old cars lights.


Current (Amps) = Power (Watts) divided by Volts.

(Remember also if you halve the voltage you double the current so a 6 volt system will use twice the current of a 12 volt system.)

Example. Dynamo rating = 120 watts. Vehicle voltage = 12 volts


So, 120 divided by 12 = 10 amps


That’s 10 amps to run lights, engine etc


Use the same theory to then work out how many amps each bulb will use.


12 volt headlamp bulb rated 36 watt (typical pre war car)


36 divided by 12 = 3 amps


12 volt tail or side lamp bulb rated 3 watts (typical)


3 divided by 12 = ¼ watt


So, 2 headlamp bulbs, 2 sidelight bulbs (on with headlamps), 2 tail lamp bulbs all on together would require


3 + 3 + ¼ + ¼ + ¼ + ¼ = 7 amps


Add on 1 or 2 amps for the coil, dash lamps and 2 amps or more for the mechanical solenoid(S) in your old regulator/cutout to waste and you’ve reached the maximum 10 amps. Pre war cars only had one rear lamp remember so by adding another to satisfy the modern MOT test we use up any spare amps.


So, having done away with the nasty inefficient mechanical device and fitted an efficient electronic device we now have more current to play with and more importantly that current is better controlled and now so much more usable.



Fit 3 watt tail lamp bulbs today if you can (they are still available) instead of the bigger 5 watt type normally available.

 Better still, LED bulbs are available for vintage and classic vehicles and not only use much less current but will almost last forever too.

A great way to save amps and it means you can also fit brighter headlamp bulbs. This is especially good for cars with 6 volt systems where double the amount of current is needed to power the same wattage bulb.





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