Working out the rating of a unmarked lamp

[Kenny.s]

Will do, although it might be a few weeks or so before i actually get started on it as i still need to buy the most expensive part of the project which is this

**broken link removed**

I'm fitting this in place of my original clocks but i need to retain the centre diff display, all the other warning lights etc can be replaced with led's.

Just thought of something else,

The battery warning light needs to draw a sufficient amount of current as it makes up part of the excitation circuit for the alternator, so if i just used an led (with current limiting resistor) it would only be drawing about 25-30ma, from memory this needs to be more like 300ma.
To increase the load could i put a resistor in parallel with the led? If so how would do i work out what value i need?

Kenny

 

[StopGo]

300ma - 30ma = 270ma

V = 12V (? assuming this is true)
so R = 44 ohms

edit..
this would be the value to connect across both the LED and limiting resistor, which I think would be the best way to do it. You could connect a resistor in parallel with just the LED but then the voltage across the resistor is clamped to ~1.5V so you will need a much lower value resistor.

 

[ThermalRunaway]

Are you sure about the whole 300mA thing? I'd have thought there'd be a seperate battery warning detection circuit and that the warning light itself would have no part in it at all except to light up when there's a problem. That seems like an awful lot of wastage - a third of an amp down the pan just for the detection circuit???

By the way, I'm loving that new display!

Brian

 

[Kenny.s]

There are variations on how they work but for a two wire alternator the warning light also serves as the supply to the excitation field, when the ignition is switched on a +12v is applied to the lamp from the ignition circuit the other side is connected to warning light/excitation connection on the alternator which is currently grounded so the light comes on, when the engine is started and charging begins this ground becomes +12v (or 13.8v which is the normal running voltage on most alternators) and the light goes out. If you have too small a lamp ie current flow you'll find that you have to rev the engine in order to get the light out but it will quite often come back on once the revs fall and the field collapses.

You also get a three wire alternator which has an ignition switched live to the alternator and a warning light but i'm not 100% sure about the function of the warning light on this type, was led to believe it worked on the same principle.

Kenny

 

[ThermalRunaway]

Hmmm I'm not understanding something about that. If you're saying that the warning light has 12V applied to one side of it and 14V from the alternator is applied to the other side when the engine is running, it shouldn't matter what sort of lamp you've got on there - there'll only be a 2V drop across the lamp when the engine is running regardless of what current rating it is. So I can't understand what relevance that has. You're probably right though because I've never looked into how the battery warning system works and you seem to have done your homework on it - I'm just not understanding something about it.

My alternator uses the three wire method...

Brian

 

[ThermalRunaway]

Actually, the 12V which was originally applied to one side of the lamp from the ignition switch should also rise to 14V when the engine is running so the lamp would have 0V across it when the alternator is charging the battery, and 12V across it when it's not. Again, I can't see what relevance the current rating of the actual warning light would have.

Either way, you can always experiment. Try your LED version without the dummy load and see if it works. If it doesn't, put the dummy load in parallel and see if it fixes the problem. Do me a favour and let us know if you did need the resistor in parallel because I'd be quite interested to find out why that's the case



Brian

 

[HiTech]

Hee, hee. Dumb, heavy, gas-gussling trucks just roll the edge of their tires and wheels over and over when cornering, don't they?

That's what's needed when one desires to effectively plow snow, tow a large/heavy load, or haul a heavy load in the box section. True work trucks don't care about fuel economy.... getting the job done safely is priority one; fuel economy and rider comfort is secondary. I drive a 3/4 ton outfitted w/ 1-ton axles and chassis and a V10 motor.