Using LED ignition warning light with Alternator

[chris_cussen]

I have used a set of LEDs as the warning lights on my Cabrio build. All worked fine except the ignition warning light, which was always on. Not only that but the alternator never output any charge.
I changed the alternator (for one from a scrap yard) which I saw work on their test rig, but that one didn't work either when I got home. I could prove there wasn't any charge coming out of the alternator because once the engine was running if I pulled the fuse to the electrics the engine stopped.
Finally I tried using a bulb rather than the LED as the warning light and everything started working OK.
It seems that the warning light supplies current to energise the alternator, and the LED was not supplying sufficient current.
So I've now got a bulb wired across LED. I've included a circuit diagram for anyone else who may have this problem.

[greyV8pete]

Re: Using LED ignition warning light with Alternator

Hi Chris. Apparently when current flows through the warning bulb circuit it provides primary excitation to the field coils in the alternator which tells the system to start charging. I had this problem (no charge) when the bulb failed on my Mk 2 Cortina which I had fitted with an alternator. I guess the LED passes so little current that it is effectively the same as a blown bulb. With your revised circuit, does your LED go out when the alternator starts charging? Peter.

[b_caswell]

Re: Using LED ignition warning light with Alternator

What happens is when you turn on the ignition the current flows through the warning light to excite the alternator.Once the alternator starts to produce a current the circuit equals out ant the warning light goes out
LED will only work in one direction so it wont allow the circuit to balnce.Well something like that!!

[donsayers]

Re: Using LED ignition warning light with Alternator

Yep that's the answer, the bulb provides the current for the field (rotor) windings in the alternator. When the alternator is at rest, going too slow or faulty the output voltage is less than the battery voltage and the bulb stays on.

As the voltage increases the current through the bulb decreases, until as the output is greater than the battery voltage, then the battery starts to charge and the light goes out.


However if the bulb blows, the alternator won't start and you will never know. They didn't call Joe Lucas the prince of darkness for nothing.

The bulbs were usually 4 watt to enable them to supply the current.

On later cars where the bulb became smaller to fit in with more modern dashboards there was a low value resistor fitted in parallel with it.

A little guesstimation suggests a 33 ohm 3 to 4 watt resistor should work in PARALLEL with the LED.

Ie Maplin part no W33R or (better still) RS components Part no 206-0385

Hope that helps.

[chris_cussen]

Re: Using LED ignition warning light with Alternator

Peter. Yes the LED goes out.
And you're right too Don, you could replace the bulb I used with a resistor, but my nearest Maplin is 20 miles away and it was late on a Sunday evening when I came up with the solution!