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henk

Alternator and led bulb as control light?

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Posted (edited)

So I have the second alternator installed but can I use a led light as control light to check the working of the alternator or do I need to use a normal bulb? 

 

Solved: it's not a good idea

Edited by henk

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Normal bulb or an led with parallel resistor equivalent to a normal bulb.

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I'm not great on electrickery, but what about an LED bulb and a resistance (normal bulb) in series?

Mike

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In parallel, yes, that would work, in series, no.

 

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Led charge light works OK using a td5 instrument pack in a older 90 or 110 with the existing charge light resistor remaining in place, so a similar set up for the 2nd alternator should work. 

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To enlarge a bit they often have a resistor in parallel with the bulb in case the bulb blows; if you didn't the alternator may not charge with a blown bulb, as it wouldn't get the excitation to get started(!). So a led instead of a bulb should be OK if you have the resistor in parallel with it, as western says.  

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Alternators need a certain amount of current flowing into the sense terminal to get them going - the bulbs are usually ~5W - so putting an LED in there that only flows ~20mA may result in the thing not charging.

That's why you want a resistor in parallel with the LED.

Also the LED will NOT warn you of the same failure modes because it can't flow current "backwards" (being a diode and all), so it will warn of the "usual" situation (ignition on, engine  off) but not things like a dead alternator diode (ignition off, power back-feeding through warning light) etc.

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1 hour ago, FridgeFreezer said:

Also the LED will NOT warn you of the same failure modes because it can't flow current "backwards" (being a diode and all), so it will warn of the "usual" situation (ignition on, engine  off) but not things like a dead alternator diode (ignition off, power back-feeding through warning light) etc.

Yes, but pretty much every car has a diode in the circuit to prevent the alternator back feeding the switched circuits when the ignition is turned off.  In Defenders, this was adding in 1991 as the engine would keep running if there were no switched load on.

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20 hours ago, FridgeFreezer said:

Also the LED will NOT warn you of the same failure modes because it can't flow current "backwards" (being a diode and all), so it will warn of the "usual" situation (ignition on, engine  off) but not things like a dead alternator diode (ignition off, power back-feeding through warning light) etc.

Not sure about that without testing, but simple put another LED in inverted parallel, one or the other will light. Make them 2 different colours and you have that information too.

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6 hours ago, HoSS said:

Not sure about that without testing, but simple put another LED in inverted parallel, one or the other will light. Make them 2 different colours and you have that information too.

Exactly, my setup has LED's in anti-parallel and a resistor to pass the necessary current, works perfick.

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On 5/22/2020 at 11:27 PM, Bowie69 said:

In parallel, yes, that would work, in series, no.

 

Yes, thank you, Bowie69.  As I said, I'm not great on electrickery, but I do understand, now🤔!

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