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Exciter circuit on Hotrod

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ShaunWalters

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Hi guys, im a diesel mechanic living in Toowoomba QLD, Australia and im doing a bit of casual wiring at the local hotrod shop, its all straight forward stuff, using the painless style kits, every wire is labled so its really cut and crimp.

the problem comes in the alternator charge light circuit, as with most people, custom builds like to use LEDs for warning lights, that works fine for most except when it comes to the Alternator charge circuit. I have wired them up as normal... 12v from ign switch throught the red LED, the to the field/exciter side of the ALT, but the LED doesent draw enough current to excite the ALT, unless you rev the snot out of it.

the easy/cheats way out is to put a 12v5w lamp in the circuit and tuck it under the dash, untill it blows and the customer doesnt know there is an extra hidden bulb they didnt know about.

I want to make the circuit run properly and use a resistor to increase the load to excite the ALT. I have done a bit of reaserch and found its quite hard to find what value resistor can replace a lamp.

any ideas? or am i being too picky?

thanks
Shaun
 
The light bulb is just a resistor that happens to provide light so replace it with a resistor.

You said the bulb was 12v5w. So we need a resistor that draws 5w at 12V.


Lets start with P = IV or I = P/V or 5W/12V = .4A

now using ohms law R=V/I = 12V/.4A = 30 Ohm

So you need a 30 Ohm 5Watt resistor to replace the bulb but we like to use a resistor with a higher wattage as it gives us some headroom in that we are not pushing it to the limit. So 30 Ohm 10Watt and I suggest wirewound.

Mouser has them for about 60 cents
https://www.mouser.com/ProductDetail/Xicon/280-CR10-30-RC/?qs=sGAEpiMZZMtbXrIkmrvidMioh4DscLtBuyFzEmKz8ZU%3d

But that will not help you much in AU. Maybe some other member can point you to a supplier that is not out of country.
 
Last edited:
Awesome, thanks for the quick reply, we have a couple of suppliers here that should have what we want.

thanks again!
Shaun

ill let ya know how it works
 
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