13 replies to this topic

[vtec_miniestate]

Hi guys and gals

I have replaced my std clocks with a digital dash i have also added a warning light module. the only light that has not been accounted for is the alternator excite lamp, does this have to draw a certain amount of current i.e. a bulb or can it be replaced with a led?

Cheers

[bmcecosse]

The Lucas ACR alternators don't actually NEED the light - they work perfectly well without. I can't speak for other makes. The light is there to tell you the Ignition is ON, or that the alternator has stopped charging. You can certainly try an LED - can't see it doing any harm.

[vtec_miniestate]

Oh, for some reason I thought that the car needed this light. Because my mini stopped charging once and it (according to the AA) turned out to be a blown bulb and as soon as it was replaced the car started charging again? Is this not the case?

[andy159]

which digital dials did you use as im thinking of replacing mine soon

[vtec_miniestate]

Acewell ones Check out my project thread for more info

....any ideas on the light?

[dklawson]

I defer to Roy on the ACR model alternators. However, to be SAFE, you should include a warning light for any alternator you use if the car it came from used a warning light.

The lamp does not "draw" current as such in this installation. The way the light works is current flows from the ignition switch, through the lamp, through the field coils in the alternator and to earth. When you turn the key to start the engine, current flows through the lamp and alternator field coils which both powers the light and charges up the alternator field coils. Once the alternator starts producing electricity, it self-energizes its field coils. This brings the "small terminal" on the alternator up to charging system potential. Since both sides of the lamp are now at charge system potential the warning lamp goes out. (No voltage difference across the bulb).

Can you use an LED? Generally "no", at least not by itself. LEDs do not allow sufficient current to flow through themselves for the alternator field coils to charge up. HOWEVER, there is an easy work around. Start with the "normal" warning lamp configuration where a white wire goes from the ignition switch, to the lamp, out the lamp via a brown/yellow wire and finally off to the alternator. Replace the warning lamp with a 40 Ohm power resistor (something rated for at least 1 Watt). NOW get your LED lamp assembly. A plain LED will need a "dropping resistor", typically 1000 Ohms. A pre-configured LED warning lamp will already have this resistor inside. Take your LED lamp (or LED with dropping resistor) and wire it in PARALLEL with the 40 Ohm power resistor. Pay attention to the polarity or the LED will not light. With the LED wired in parallel to the 40 Watt resistor... the LED will turn on and off just like a normal incandescent light. However, the current required to power the alternator field coils on startup will pass through the 40 Watt resistor.

[Kerrin]

I have the Koso digital gauges fitted to my mini, and also have a digital voltmeter permenatly fitted to the dash.

When I connected the ignition light wire to the digi dash LED the alternator would not charge.
So I wired up a seperate ignition light bulb holder and fitted it to the dash. Hey presto alternator charge.

This is using a new 70a later style alternator.

[Dan]

Yes that's because the ACR type alternator does indeed need the lamp (or at least needs a suitably resistive 12V supply to the warning terminal) in order to switch on, sorry Roy. Several people have had similar problems on here that have been fixed just as easily. Usually when building custom wiring for a track car or buggy of some sort and so doing away with most of the standard stuff they though they wouldn't need. Doug's instructions are the way to go for the LED.

[vtec_miniestate]

Cheers Doug, that's exactly the way I was thought it would have to be done, just wanted to check I was on the right path. thanks for the info

[vtec_miniestate]

I defer to Roy on the ACR model alternators. However, to be SAFE, you should include a warning light for any alternator you use if the car it came from used a warning light.

The lamp does not "draw" current as such in this installation. The way the light works is current flows from the ignition switch, through the lamp, through the field coils in the alternator and to earth. When you turn the key to start the engine, current flows through the lamp and alternator field coils which both powers the light and charges up the alternator field coils. Once the alternator starts producing electricity, it self-energizes its field coils. This brings the "small terminal" on the alternator up to charging system potential. Since both sides of the lamp are now at charge system potential the warning lamp goes out. (No voltage difference across the bulb).

Can you use an LED? Generally "no", at least not by itself. LEDs do not allow sufficient current to flow through themselves for the alternator field coils to charge up. HOWEVER, there is an easy work around. Start with the "normal" warning lamp configuration where a white wire goes from the ignition switch, to the lamp, out the lamp via a brown/yellow wire and finally off to the alternator. Replace the warning lamp with a 40 Ohm power resistor (something rated for at least 1 Watt). NOW get your LED lamp assembly. A plain LED will need a "dropping resistor", typically 1000 Ohms. A pre-configured LED warning lamp will already have this resistor inside. Take your LED lamp (or LED with dropping resistor) and wire it in PARALLEL with the 40 Ohm power resistor. Pay attention to the polarity or the LED will not light. With the LED wired in parallel to the 40 Watt resistor... the LED will turn on and off just like a normal incandescent light. However, the current required to power the alternator field coils on startup will pass through the 40 Watt resistor.

1 other question about this is that 40 ohm 1 watt resistors seem a little hard to obtain. If i remember rightly would it be feasable to wire 4 1/4 watt resistors in parallel and use 160 ohm resistors to get the correct resistance?

[dklawson]

Yes, you can certainly parallel gang the resistors and your math is correct.

However, the 40 Ohm figure is only approximate, not carved in stone. Anything from about 30 Ohms up to 90 Ohms should work. (If I remember correctly, you can measure the current flowing through a light bulb and back calculate its impedance, and find it may be close to 90 Ohms. However, put the multimeter across the bulb directly and it will measure lower resistance.) Again, something between 30-90 Ohms will be OK as long as the Wattage is there. The parallel resistors will work also.

[vtec_miniestate]

thats brilliant thanks again for your help!

[Dan]

Are you looking at Maplin for the resistor? Use a proper component company and you should have no trouble finding a 1 Watt resistor. Try RS or Farnell.

[vtec_miniestate]

yeah looked there, farnell do it but they have a minimum £20 order