LED for alternator

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Re: LED for alternator

Postby trailmaster » Mon 27 Nov 2017 7:11 pm

Hey lozzy

I’ve managed to buy a 2w 150 ohms resistor. Thanks for calculating that for me.

So the one weay dioad is protecting the LED from reverse voltage-as if someone would connect the battery terminals wrong, is that correct or is it protecting something from the alternate side?
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Re: LED for alternator

Postby Fat Fingers » Mon 27 Nov 2017 9:49 pm

for once what lossy says is basically correct .
Your warning light can be anything from 2 to 21watts ..the higher the wattage the sooner it starts charging . So if we take a 6watt bulb it draws 0.5 A so just use a 24 ohm resistor and put the led across it with its series resistor ...of course its not as good as a filament bulb as it does not glow down to as low a voltage so will not indicate alternator faults like a real bulb.
If someone connects the battery the wrong way your alternator is instantly fuuuckkked anyway so forget that senario
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Re: LED for alternator

Postby lozzy » Wed 29 Nov 2017 1:01 pm

trailmaster wrote: So the one weay dioad is protecting the LED from reverse voltage-as if someone would connect the battery terminals wrong, is that correct or is it protecting something from the alternate side?


Yes, but if its wired inappropriately it might not save the LED if the LED's connections are accidentally reversed which can easily happen when installing etc, here's an explanation as to why, vehicles and alternators etc aside....

....Protection diode, series connection....First, LED in this explanation, this means the light emitting diode itself, LED means the light emitting diode and any built in limiting resistor etc...

...So, you have a 12 volt battery and a LED that say has a maximum reverse voltage rating of 5 volts, you connect the LED's -ve connection to the -ve terminal of the battery, you connect the LED's +ve connection to the cathode of a series protection diode, you connect the anode of the series protection diode to the +ve terminal of the battery, the protection diode is now forward biased (anode connected to +ve supply) and so is the LED, current now flows and the LED lights up, no problem.

Now reverse the connections to the battery, no current can flow because the protection diode is reversed biased (anode connected to -ve supply) the LED is protected.

Now with the connections to the battery connected correctly again reverse the LED connections, current can still flow because the protection diode is still connected to the battery correctly and is forward biased (anode connected to +ve supply) but so far as the LED is concerned it's polarity has been reversed (anode connected to -ve supply) this can lead to the demise of the LED because its maximum reverse voltage rating of 5 volts has now been exceeded by approx. 7 volts, avalanche current will flow....not good.

So, if you fit a protection diode in series with the LED make sure its a permanent part of the LED assembly's wiring before you connect the said wiring to the 12 volt supply, the LED will be protected that way.

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Re: LED for alternator

Postby Fat Fingers » Thu 30 Nov 2017 10:43 am

KISS
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Re: LED for alternator

Postby Ralph » Sat 02 Dec 2017 6:13 pm

ROW 300Tdi & Td5 warning light panels are all LED, so can't see why it wouldn't work on a older vehicle, my warning light panel was changed to a TD5 unit ages ago with no problems
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Re: LED for alternator

Postby Fat Fingers » Sat 02 Dec 2017 9:10 pm

you also need to give consideration to if the alternator is a standard 9 diode or a computer controlled one. If its computer controlled then the warning light has no " direct connection" to the alternator but is switched or dimmed by the computer.
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Re: LED for alternator

Postby lozzy » Sun 03 Dec 2017 11:42 am

Ralph wrote:ROW 300Tdi & Td5 warning light panels are all LED, so can't see why it wouldn't work on a older vehicle, my warning light panel was changed to a TD5 unit ages ago with no problems


I'm not familiar with the 300Tdi but according to an online schematic I'm looking at the charge indicator is a bulb not an LED, it also shows a shunt resistor connected across it so that some current is supplied to the field winding in the event of a bulb failure.

In that instance (resistor across the bulb scenario) replacing the bulb with an LED might work.

http://www.legionlandrover.com/manuales ... %20tdi.pdf

Modern alternator regulators aren't designed to power a charge indicator light direct, typically the field winding is via a top side MOSFET to B+ve, phase comparator for determining alternator speed etc, all comms via bus to the mother chip.

Owt goes wrong = change chunks :)

Typical regulator chip here from NXP:

https://www.nxp.com/docs/en/data-sheet/TC80310.pdf

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Re: LED for alternator

Postby Ralph » Sun 03 Dec 2017 5:12 pm

Definitely no bulbs in the ROW 300Tdi/Td5 warning light panel.
this mine removed from the casing
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Re: LED for alternator

Postby lozzy » Sun 03 Dec 2017 5:51 pm

I would say that the current flowing through the circuit must be enough for the field winding if it's connected to a conventional alternator and works like you implied, the overall point being little current, little output, current through LED's can vary enormously depending on the LEDs design rating.

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