Charging Lamp wiring

morris · #1 14th January 2013, 18:17

Hi all, looking for some guidance on wiring up a charging/battery lamp.

The diagrams for the E36 dash have a resistor in parallel with the lamp and then a diode down stream on the battery/run power side. Can anyone tell me what component ratings I need to get. I'm assuming that the diode is anything rated to greater than 14ish Volts but not sure on current as that's dependant on the resistor and lamp. Is just reading the resistance through the lamp enough to tell me to get a resistor of the same ohms plus greater than 14ish Volts?

thanks
Ian

MartinClan · #2 14th January 2013, 18:33

The resistor is there just in case thebulb fails (to maintain thecircuit). So it probably needs to be the same resistance as the bulb. As the resistance of the bulb is different when lit, you cannot use the static resistance. You need to know how many amps the bulb draws. Then R = V/I.

Chers Robin

morris · #3 14th January 2013, 22:07

ok, that makes sense. I did think I'd probably get a bad resistance reading if the lamp wasn't illuminated. Any thoughts on the diode or is it not actually necessary?

MartinClan · #4 15th January 2013, 07:11

Quote:

Originally Posted by morris

ok, that makes sense. I did think I'd probably get a bad resistance reading if the lamp wasn't illuminated. Any thoughts on the diode or is it not actually necessary?

No idea about the diode.I guess if there was one there originally you should also use one. I am a bit out of touch with such things but in the "old days" I would have used a 1N4001 which, as I remember, is a general purpose diode rated at about 1 amp.

Cheers, Robin

morris · #5 15th January 2013, 21:17

my lamps turned up today and I measured the current draw at 150mA so given the 12.48V on the battery that gives me 83 ohms and a power rating of 1.8W so I guess that I can use those values for the diode too. Thanks for the help

morris · #6 16th January 2013, 17:32

changed my mind on this. If you put a resistor of the same rating in parallel to the lamp it just means that you'll draw twice the current to light the lamp under normal conditions. What you need is a much higher rating parallel resistor so that when lit the lamp pulls most of the current, when blown the resistor allows the exciter on the Alternator to work but still only allows minimall current to flow and protect it. Working on the assumption that the field windings in the alternator are pretty low resistance and only need minimal current to work until the alternator starts providing its own current I'm going for 500 ohm in parallel to the lamp and a 1N4001 diode as you suggested. this way there'll be about 180mA max going down the wire under normal conditions (until the lamp goes out) and more like 26mA if the bulb goes. I had a good look around the web trying to get an independant value for that resistor and 500 ohms kept popping up as the standard.

Part of the problem with this is there's 1, 2, 3 and 4 wire alternators so trying to understand how mine works was the big hurdle and now I think I understand that, I think I know what the lamp needs. Technically the alternator would still work even without the exciter feed through the lamp but you'd have to rev the engine a bit to get it going

Chris Cussen · #7 17th January 2013, 06:09

Once the alternator has come 'on line' no current flows through the bulb.
I would assume the diode it to stop the light coming on if the volatge at the exciter end of the alternator becomes higher that the normal output voltage.

Different alternator (being Ford based) but I originally had just an LED as my ignition warning lamp, and the alternator never came on line as the there wasn't sufficient current being drawn through the LED to excite the alternator. I bodged it by putting a bulb in parallel with the LED, and hid the bulb behind the dash. So I'm not sure that the altenator will come on line unless there is something connected to the exciter.

morris · #8 17th January 2013, 11:24

I think you're probably right. I was reading some stuff on single wire (the fat charge one) alternators and I think people bodge them by just pulling a feed to the exciter direct from it though I guess you'd drain the battery after a while.

I measured the resistance through the the field coils on the alternator last night (D+ to ground) and it came in at about 800ohms so should draw around 9mA when the bulb blows and 14mA when the bulb is working so really small currents to get the thing working