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Forum Index : Electronics : LED-ifying a Mini dashboard....

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Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 7851
Posted: 05:09am 13 Nov 2021
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Hi all.

My latest project is a classic Mini Clubman, but the FPC circuit on the back of the dash is all broken around where it connects to the loom plug, and while you CAN get replacements, they start at $160, and I am not a sucker.  

That's a ridiculous price for a FPC, when we all know you can get PCB's made for US$5 for ten.  FPC might cost more, sure, but not $160 per unit - as I said: I am not a sucker.

So, I decided to LED-ify the dash, replacing all the bulbs with 10mm White LED's instead, as I have heaps of those here.

All the replacements will work and look find through the dash colour filters for the various things, but the one I have tripped up on, is the alternator charge light.

The circuit is so simple, I am not even going to draw it, as you can describe it:

IGNITION---->CHG BULB---->ALTERNATOR

When you turn on the ignition, there is a circuit to ground via the alternator charge light terminal, so the CHG light on the dash lights up.  When you start the engine, the CHG light terminal on the alternator floats high, turning off the bulb.

Or so I thought.

When I tested this with a LED, it lights fine with ignition on, but once you start the engine, the LED remains lit.
I SUSPECT that this is cos the alternator, which is expecting a bulb and therefore at least a few hundred milliamps of current to flow, is simply not lifting the CHG light terminal with only 20mA or so of LED current.  The alternator needs to see more current flow in the CHG light line before it will float high when spinning, in other words.

I am thinking perhaps I need a shunt resistor across the LED and its resistor - thoughts?
I can't try it today, as it is pouring with rain, so I thought I would just jot my ideas down here in a thread, and see what others think.
Smoke makes things work. When the smoke gets out, it stops!
 
Godoh
Senior Member

Joined: 26/09/2020
Location: Australia
Posts: 119
Posted: 05:56am 13 Nov 2021
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I thought that the warning light worked by getting a ground through the alternator windings, then once the alternator started charging it had 12 volts both sides so it was no longer lit.
Pete
 
Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 7851
Posted: 06:27am 13 Nov 2021
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You could well be right.
I am only assuming that was how it worked, so people who really know their auto-eclectics - would love to hear from you!  
Smoke makes things work. When the smoke gets out, it stops!
 
Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 920
Posted: 07:23am 13 Nov 2021
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Old style alternators - IGNITION---->CHG BULB---->ALTERNATOR = small current flowing in the field coil, required to bias the magnetic field slightly so the main coil will produce an output once it starts spinning, increase in voltage makes the light go out.

You will need a resistor = hot resistance of the bulb in parallel with the led, may also require a series diode so the led cannot be reverse biased and blow up.
Modern alternators have more sophisticated electronics or maybe brushless with no field coil, so the above doesn't apply.

Cheers
Mike
 
phil99

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Joined: 11/02/2018
Location: Australia
Posts: 341
Posted: 07:26am 13 Nov 2021
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Yes, a shunt resistor is the answer. 75 to 120 ohms 2W is usual. Current through the lamp or shunt provides B+ for initial excitation of the alternator. The LED stays on, as you thought, because the current is too low to get the alternator started. Once started the field diodes (D+) takeover the supply of excitation current and the light goes out as it now has 14V on both sides.
PS You can sometimes get the alternator to excite with just a LED if you rev the engine hard enough.
On some recent cars the main ECU drives the lamp and controls the alternator voltage.

EDIT We seem to have cross posted essentially the same thing.
Edited 2021-11-13 17:30 by phil99
 
CaptainBoing

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Joined: 07/09/2016
Location: United Kingdom
Posts: 1727
Posted: 07:30am 13 Nov 2021
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  Grogster said   auto-eclectics


My Nemesis!

In the past three or four years I have found myself embroiled in a lot of bike electrics (same meat, Harleys, rice-burners...) and it can be a nightmare trying to understand what the designer had in mind. Even simple circuits can be infuriating
 
Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 7851
Posted: 04:58am 14 Nov 2021
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Excellent, thanks very much chums.

So I was on the right track then, but the extra information as to how the CHG light goes via the field winding was very useful information.  I actually thought that with an alternator, the voltage regulator controlled the CHG light, so I've learnt a little something new.  I will try a shunt resistor and series diode just to cover all the bases.

@ CaptainBoing: eclectics was a typo!  
Smoke makes things work. When the smoke gets out, it stops!
 
Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 7851
Posted: 04:37am 15 Nov 2021
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This is what I have now, and it is working fine, so I will design this in so the dash is fully LED, and I should never need to change a blown bulb ever again for the life of the car.





If anyone sees anything fundamentally wrong with this arrangement, please let me know before I glue it all to the back of the dash!  
Smoke makes things work. When the smoke gets out, it stops!
 
phil99

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Joined: 11/02/2018
Location: Australia
Posts: 341
Posted: 06:12am 15 Nov 2021
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That would be perfect. On Bosch alternators your ALT goes to D+, for Lucas it's IND.
 
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