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Coping with voltage spikes

A 12volt LED question

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Tim Stevens13/08/2020 17:45:42
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1268 forum posts

I added a tiny 12v LED to illuminate the oil gauge on an old motorcar. I have tried it and it works wonderfully, until I start the engine. It seems that voltage spikes generated by the ignition system may be the culprit, so is that likely, and what can be done about it?

The system has a big 12v battery and a small dynamo, and the ignition is electronic so the sparks may be 'better' than conventional points systems. The only other symptom which might help is that the warning light for the indicators (flashers) can be seen to flicker at low engine speeds when the flashers are not being used (but only if you look very closely) - this might be caused by the same spikes, but the LED survives.

I am sure that a solution is there - a Zener perhaps - but can you point me towards something that works - thanks

Tim

Bryan Cedar 113/08/2020 18:11:20
51 forum posts
1 photos

You have not stated what actually happens to the LED when you start the engine.

Speedy Builder513/08/2020 18:40:12
2111 forum posts
146 photos

I have a WALT led indicator on a 6volt Austin 7. Some times it works and other times it D'ISNEY.

Speedy Builder513/08/2020 18:41:17
2111 forum posts
146 photos

Tim - a bulb ??

Tim Stevens13/08/2020 18:41:39
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1268 forum posts

Nothing happens. It just ceases to produce light. It is not in a position where I can stare into the beam while pressing 'go'. The diode, when removed, looks exactly like a new one - no connection failure, not even anything visibly different about the gismo in the plastic blob.

But I can send a failed one if this would help diagnosis. I have several ...

Cheers, Tim

Brian Sweeting13/08/2020 18:49:14
444 forum posts
1 photos

At low engine/dynamo revolutions the regulator will be busy switching on and off to balance the battery/load.

Do the lights settle down as the revs come up?

Tim Stevens13/08/2020 18:52:50
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1268 forum posts

A bulb? A BULB? The whole machine is devoted to LEDs instead of those desperately inefficient filament contraptions. And I need to use it at night and 8 amps max is not enough.

Cheers, Tim

not done it yet13/08/2020 18:52:52
5031 forum posts
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The simplest is maybe a large capacitor before the current limiting resistor in series with the LED?

Tim Stevens13/08/2020 18:56:48
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1268 forum posts

The rest of the lighting system comprises a range of LEDs, Headlamps sidelamps, rear lamps brake and rear fog and reversing lamps, white, amber, red, all 12v LEDs, and they all seem to last well. Just these tiny little 3mm failures.

Cheers, Tim

Edited By Tim Stevens on 13/08/2020 18:57:25

Tim Stevens13/08/2020 19:00:00
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1268 forum posts

A capacitor across the feed, you mean, (not in series with the feed) ? What capacity, roughly, and electrolytic or not?

Tim

Emgee13/08/2020 19:06:04
1728 forum posts
232 photos

Tim

Be patient, i'm sure one of the many electronic buffs on here will provide a solution, you may even have many choices of methods to try.

Emgee

Maurice Taylor13/08/2020 19:38:58
134 forum posts
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Have you measured voltage across the led when the engine is running ,it will be more than 12V .Probably the led does not like the higher voltage .Try putting a resistor in series with the led, 470 ohms will do ,see what happens,might need different value series resistor.

Ian P13/08/2020 19:41:49
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2420 forum posts
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In your original post you said you had added a '12 Volt LED'.

The 3mm LED you mentioned does not sound like one that incorporates its own limiting resistor or constant current circuit, so have you fitted a limiting resistor externally?

Ian P

roy entwistle13/08/2020 19:42:29
1258 forum posts

Are headlamp LEDs legal ?

Les Jones 113/08/2020 20:04:52
2162 forum posts
149 photos

Tim, You are going to have to provide more information on the "LED". A single actual LED (I am assuming it is a white LED as different coloured LEDs have different forward voltages.) would have a voltage of between 3.0 and 3.6 volts across it when driven with it's rated current. As you are calling it a 12 volt LED it must contain something to limit the current and it may consist of more than one actual LED. Until we know more about the "LED" we can't work out a possible reason for it failing.

Les.

SillyOldDuffer13/08/2020 20:29:29
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6335 forum posts
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Couple of possibilities, assuming the LED is the ordinary 12V type with a built in resistor:

  • The supply itself is spiky, or
  • The LED's unshielded cable runs closely parallel with wiring carrying ignition pulses, and they're coupling.

A regulator/filter should clean up a spiky power supply but it may need a bigger load than a single LED to work properly. I'd try it first.

If no luck, try shielding the indicator cable. (Pickup or microphone cable?) If that doesn't work, maybe a 12V Zener & 0.1uF in parallel with the LED at the dial end.

Dave

Dave Halford13/08/2020 20:41:45
940 forum posts
9 photos

Didn't dynamo volts bob around 16V ? 12V might be the very max the 3mm can take.

Robert Atkinson 213/08/2020 20:52:30
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Posted by SillyOldDuffer on 13/08/2020 20:29:29:

Couple of possibilities, assuming the LED is the ordinary 12V type with a built in resistor:

  • The supply itself is spiky, or
  • The LED's unshielded cable runs closely parallel with wiring carrying ignition pulses, and they're coupling.

A regulator/filter should clean up a spiky power supply but it may need a bigger load than a single LED to work properly. I'd try it first.

If no luck, try shielding the indicator cable. (Pickup or microphone cable?) If that doesn't work, maybe a 12V Zener & 0.1uF in parallel with the LED at the dial end.

Dave

 

"ordinary" LEDs do not have an internal resistor Some do but it's not standard. Can the OP provide a part number or link for the LED they are using and then we can stop guessing!

Edit,
That "regulator/filter" could be anything, even a empty box, there is no specification at all.

Edited By Robert Atkinson 2 on 13/08/2020 21:14:16

Robert Atkinson 213/08/2020 21:10:04
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772 forum posts
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Posted by roy entwistle on 13/08/2020 19:42:29:

Are headlamp LEDs legal ?

OT but LED hedlamps are legal, my car has them.

What is NOT legal is replacing the filament lamp (bulb) in any exterior car light with an LED. The only exception to this is the reversing light. The legal reason for this is car lights and replacable lamps have to meet certain standards and be tested and approved . They are "E" marked to show compliance. There is NO approved standard for LED repalcable lamps. It is possible to have a complete LED light compliant but the LEDs in it are not replaceable.

The practical reason is that LEDs have different optical characteristics to filaments so a reflector and lens desigined for a filament almost certainly won't have the same pattern and brightness with an LED fitted. It might be possible to design a LED replacement lamp that works in one, or some, lights but it is impossible to design one that will work in any because there i so much variation in light optical design. You can get legal LED replacements for sealled beam headlight units because thi is a complete light with optics, not a replacement lamp for a set of optics.

Robert G8RPI.

Tim Stevens13/08/2020 21:20:26
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1268 forum posts

I know the supply is spiky - from the flickering of the flasher LED. I cannot suggest another reason for that LED to flicker.

Now some other responses:

the white LED (from E-... like most of the others) comes fitted with wires with bulges in. I presume these include the necessary resistance. Other similar but coloured LEDs - same 3mm type - are OK so far.

The dynamo is regulated and charges at about 14.2v - the same voltage is applied to all the other bits of the system.

No I have not measured the voltage at the LED, but I have a voltmeter (see above) across the feed to the Charging socket, which is connected to the LED in question through the sidelights switch, so no reason to be higher.

The regulations for headlights specify that for some (not all) vehicles any required lights using filament bulbs must be properly marked. This has caused a huge raft of confusion as it is interpreted by users and dealers as covering LEDs. My view is that LEDs are not filament bulbs. I don't think the matter has come before the courts.

The actual regs covering my vehicle require a headlamp. No more - I have two and they both dip. I invite the court to consider which is safer, your honour ...

I will try a capacitor in the feed - as close as possible, or up to 60mm away?

As far as I am able to search, I can find no part number (etc ) for the LED which fails. But I recognise that alternative sources might be better or worse at curing this (minor) problem. So I will see what I can find. The oil gauge is in the original dashboard position, but no provision for lighting it was made back in 1928. Proper oil pressure is now more important as more modern big ends etc are fitted.

Anything else? I will try to help if I can - thanks

Tim

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