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Alternator Warning Light


Thrashed

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Hi Simon,

I realise that it is current that will destroy the LED and I could have mentioned the Resistor to limit the current(in my original post), but what I was trying to highlight was that as an LED only needs 1.2v across it to forward bias it, if the output from the Alternator is low the LED could still be forward biased. But as you point out the LED is likely to have a limiting resistor built in, so just need to check there is not a 1.2v drop across the LED/Resistor combined. :-)

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I understand that electrics is not everyone's cup of tea, so I won't even mention the Quantum phenomena that causes LEDs to work :)

 

With the limiting resistor built in though there should be either a 12 volt drop or nothing. If there is a 1.2 volt drop across the combination, this would equate to only 1.2 mA through the two items which "might" just barely light the LED with the modern more efficient LEDs, this is a possibility. The resistor means you can whack the full battery voltage across them to make it work.

 

An interesting subject is Physics ... :)

 

Simon.

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I understand that electrics is not everyone's cup of tea, so I won't even mention the Quantum phenomena that causes LEDs to work :)

 

With the limiting resistor built in though there should be either a 12 volt drop or nothing. If there is a 1.2 volt drop across the combination, this would equate to only 1.2 mA through the two items which "might" just barely light the LED with the modern more efficient LEDs, this is a possibility. The resistor means you can whack the full battery voltage across them to make it work.

 

An interesting subject is Physics ... :)

 

Simon.

 

Very true... you would probably only see that in a darkened room :)

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Simon, it might be worth a write up in the mag with some simple basics. I think I understand the basics, but beyond that I get lost very quickly. I understand the basics of Ohms Law, but when you start throwing resistance and diodes into the mix I start to drown. I am sure it would be helpful to a lot of people as this is always a common style of question.

 

Thanks for all your helps.

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That's an interesting idea. I could write up some articles along these lines and see if they would be accepted.

I could start with the oft-quoted "Van In Road" triangle and work from there...

 

(In case you're wondering post-4714-0-74393700-1426617381_thumb.jpg :) )

 

I need to find out who to post these too.

 

Thanks,

Simon.

Edited by Simon B
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Just thought I would up date.

 

Finally got it fixed.

 

First I tried 2 3w 1k resistors in each of the feeds into the dash led. Made no difference.

 

I then just decided to cheat and put the spare red led and piggy back of this. Covers the red led in heat shrink so that the light is not showing under the dash and it works perfectly.

 

I totally don't understand why but for now I am just happy that it works, so I can move on and get the rest finished.

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That's an interesting idea. I could write up some articles along these lines and see if they would be accepted.

I could start with the oft-quoted "Van In Road" triangle and work from there...

 

(In case you're wondering Van-In-Road.jpg :) )

 

I need to find out who to post these too.

 

Thanks,

Simon.

 

Derek is Magazine editor - http://www.rhocar.org/index.php?showuser=784

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Just thought I would up date.

 

Finally got it fixed.

 

First I tried 2 3w 1k resistors in each of the feeds into the dash led. Made no difference.

 

I then just decided to cheat and put the spare red led and piggy back of this. Covers the red led in heat shrink so that the light is not showing under the dash and it works perfectly.

 

I totally don't understand why but for now I am just happy that it works, so I can move on and get the rest finished.

 

It works, dont question it!

Edited by theduck
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Aerodynamically a bee can't fly --- just as well bees can't read either.

 

That's what everyone used to think. Turns out the bee is cleverer than that as it uses a figure 8 motion to make the wing area appear larger so it "just" manages to get airborne - Nature's helicopter :) Ever seen these land in slow motion? They don't - they tend to crash land mostly :)

 

Simon.

Edited by Simon B
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