Battery light

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AllanEuerby
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Joined: 19 Apr 2002, 14:17

Battery light

Post by AllanEuerby » 27 Feb 2003, 20:23

Anyone know how the battery/alternator warning light 'dumps' the coastdown current from the alternator when you switch the ignition off? - Watch the warning light when you switch off, you'll see what I mean - you can even see the pulses, so could be from the trio/field diodes.
According to my Haynes book of lies & my multimeter, there's no earth connection on the opposite side of the light to the alternator - would short with the ignition on, too, I suppose.
I need to know as the new dash I'm making has all LEDs & if the alternator is driving the light when coasting down, I'll damage it with the LED stopping this function.

RichardW
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Battery light

Post by RichardW » 27 Feb 2003, 21:48

Never see mine coast down, as it's a diesel, and when you switch off it stops dead[}:)]
But, I think the light is powered up and earths through the alternator, then when the alternator is running it puts +12V to the light which then has +12 on both sides so goes out.
I have heard of others putting LEDs in BX's, and not heard of the charge light blowing up during shutdown. You could always lash one up with feed off the battery and connected to the alternator and run it for a while to see if it's OK.
Richard

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AndersDK
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Battery light

Post by AndersDK » 28 Feb 2003, 08:23

The alternator uses the low power diodes for powering the regulator module. This low power is available on a spade connector located on either on the regulator or the alternator body - or even sometimes both places.
This outlet is used to feed +12V to the warning lamp's ground side - to have the lamp go out - when engine & alternator is running.
When the alternator is not running - no feed is coming from the low power diodes - the regulator module's internal circuit grounds the feed output - and the lamp is lit.
NOTE :
The general +12V feed to the instrument panel is - by a fuse in the fusebox - fed from the battery.
This means that power drawn from the fusebox +12V feed - WILL pull down (by cable resistance = loss) the +12V feed to the instrument panel. The result is that the alternator's regulator +12V lamp feed can be quite some higher (-> 1.5V) than the instrument panels fusebox +12V feed - in effect the alternator lamp is weakly reverse polarity feeded.
This is seen on some cars by the alternator lamp still glowing weakly - even when everything is ok (except for the cable loss to the +12V consumers) - especially a problem on older CX's and some older BX's.
This must be taken into account when using solid state (semiconductor) lighting like LED's and associated electronics.
Commonly LED's will accept up to 5V reverse polarity with no harm. More than 5V may damage them.
A simple series diode in the alternator lamp feed wire may solve such problems.
[8D]

AllanEuerby
Posts: 4
Joined: 19 Apr 2002, 14:17

Battery light

Post by AllanEuerby » 01 Mar 2003, 15:20

Thankyou Anders.
I really wanted to check that it wouldn't cause any problems to the alternator with an LED - it would be the same if the warning light 'blew' I suppose.
I still can't understand how the light is lit (even if only for a second or two) when the ignition is switched off - the battery/fuse side of the light is isolated...
Anyway, I've sort of followed Richard's advice & soldered a wire onto the back of the warning ligt & I'm connecting an LED to see what happens, in parallel with the warning light (just in case).
Incidentally, Iskra alternators are cheap when bought direct - I got one for a '91 AXGT for £42 delivered (no exchange) a couple of years ago when Halfords wanted £95 exchange