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JohnD

Durability of LEDs

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I have overdrive on SofS, and to ensure I knew if the circuit was live or not, I fitted a little warning light.   Originally it was a tiny pea bulb, but when it failed, I fitted a LED.    Bought it from CPC, and it was working well, but on the tripmto Le Mans it was intermittent.   By the time I'm back, it doesn't work at all.

So I get one from Halfords, for quickness.  Fitted it this evening - and its failed.   Won't glow, even when directly connected to a test battery.

I thought LEDs were robust!   Two in succession fail, the last while I'm fitting it.  Have I been unlucky?

John

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LEDs are long lasting when used within their design ranges of supply voltage and temperature.

Nominal 12v leds can sometimes be a problem in automotive applications due to the real voltage being more like 14v. Something else to watch in the overdrive circuit is that if the LED is in the same part of the circuit as the solenoid coil then it may see up to 40v reverse voltage when the OD is switched out generated by "back EMF" as the magnetic field collapses. That has the potential to damage them.

Nick

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Where I live is a very hostile environment to "240v" LED lamps. We are rural and at the end of a long 2 phase overhead supply poles.  Because of the resistance and current loss the supply is jacked up to around 252V, falling back to around 245V at times of high consumption, but according to the meter plugged in next to my desk at night can peak as high as 256V.

Now as a country we used to have a supply standard that was 240V +/- 5%  and most electrical equipment was built for 240V and expected to cope easily with 252V.   Actually rather than me explain badly why all the imported 230V electrical stuff (especially LED Floodlights) tend to blow the brief explanation here is much better. 

https://www.schneider-electric.co.uk/en/faqs/FA144717/

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Thanks, Nick, I hoped there might be an explanation.   

The LED runs off the downstream side of the Press On- Press Off switch, so energised when on to the Overdrive relay but not in that circuit - the LED has its own earth.   Like this:

  Would that avoid the back EMF?   Wouldn't it be excluded by the relay?   If not, is there a way to protect the LED?   A diode for instance in the LED circuit, so the current can only go one way?  But an LED is a diode!

John

O-drive switch circuit.png

Edited by JohnD

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Looks like the led is wired in the relay coil circuit not the OD solenoid circuit so it won't see the back EMF from the solenoid.

It will see the back EMF from the relay coil though.  This is considered enough to pop transistors in the MS ECU and the usual way of dealing with this is to connect a diode between the coil supply and earth so that the back EMF, which is opposite polarity to the energising current is shunted directly to earth.  It is also possible to use a biggish value resistor in the same way. The Ford sourced relay I use to interface MS with my cooling fan has a resistor built in.

I do wonder whether the main issue isn't simply that the led isn't fully rated for automotive voltage though.

Nick

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Like this, Nick?

I have a couple of diodes in my 'electrics box' - the gods know why, some previous overambitious project, no doubt - would they do?   They are marked "6 A 0.5  0640", and the packet lists 10 factors! 

Vrm 50VPK

Vrm (non-rep) 100V

Vr 35V

Io 6A @ 60C

Ir 25 microAdc

Ifm(surge) 400APK

TjTstg (temp range I presume) -65 - +175C

Vf(AV) .9VPK

Ir(AV) 25 microAdc

 

If they are suitable, they are cylinders about 1cm in diameter and length, black with one end silvered.   Which end is which?

O-drive switch circuit with protective diode.png

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Err, possibly.

If pushed I was going to suggest one of the 1N400x series (which might be what you have from the description) where the final x can be any number between 1 and 7. Higher the number the higher the voltage capability. You really want something with a reverse voltage rating >100v.

I always have to google for polarity marking - or test with a meter.

Alternatively use a 1K resistor which does nearly the same job but without the complexity.

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Shouldn't it be the other way round to protect against back emf? or am I confused.

If the diode has a "band" round one end that usually corresponds to the "current shall not pass" (Cathode) end of the diode

both-diodes.png

Edited by yorkshire_spam

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I've looked it up and diodes are marked with a band next the cathode, the negative end.     The symbol's triangle (if it's a arrow head) shows the direction in which curent flows.     And the protecting diode should be orientated the opposite way to the LED.

As we think of current flowing from +ve to -ve, and cars have a negative earth, then the LED should have its cathode to earth (the last one I used conveniently had black (-ve?) and Red (+ve?) leads, that let me test it with a battery, with the black to negative)and the protecting diode its anode to earth.   Or have I got it backwards?

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@John: If you look at your drawing - the current (yes, I know - electrons 'walks' opposite, but forget that!) will go through your diode and make a short across the LED (i.e. there will only be aprox 0,7v left on the LED). That's not what you want.

Long time since I did electronics, but why not put a capacitor - it will not draw DC but send spikes down on ground.

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Yes, JMH, I drew that diode at random.      I think the LED's cathode is to earth, so the protective diode shouldd be the other way around, like this?

O-drive switch circuit with protective diode V2.png

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