Coping with voltage spikes

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... or perhaps more strictly 14V

See my post of 14th

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Tim,

I tried your ebay search phrase and found a seller whose ‘description’ reads thus:

”Lens Type : Water Clear Lead Length : 20cm Optimum Supply Voltage : 9v - 12v for highest brightness Minimum Supply Voltage : 5v with much reduced brightness Maximum Supply Voltage : 14v Current : 20mA @ 12v Viewing Angle : 20~25 Deg”

IF those are the same LEDs then 14v looks a risky Maximum

MichaelG.

Edited By Michael Gilligan on 14/08/2020 00:01:52

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MichaelG.

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Edit: 

What voltage will I measure at the battery using the DVR3/DVR4

Nominally 7.2 V in a six volt set-up and 14.2 V for twelve volts. More is definitely not better here. For example 14.5 V as commonly stated as 'correct' will mean higher trickle charge leading to more topping up and shorter life for your battery.

Ref. http://edsltd.ddns.net/tech_talk_regs.html#what_voltage

Edited By Michael Gilligan on 16/08/2020 16:14:48

Rod - if you really think that I am going to drive a 92 year old car while holding an oil lamp by my right ear, then you do not understand the complications of a crash gearbox, or the subtleties of double declutching.

Tim

Hi Tim, it was just an idea!

The experts seem to be strugging to assist you to find a workable solution to the expiring LED problem. I can't remember a thread as long as this about one little lamp to read a gauge by.

I hope you do get it sorted and can then enjoy driving your mother in law around.

Regards

Rod

Just some out of the box thinking -

Maybe decouple the LED's power source from the car entirely. A pound shop book reading LED lamp with a switch and small battery could be deconstructed and the components arranged near the gauge to be read. The wires and bits could be camouflaged behind panels etc. to keep original appearances. This way you would have a trouble free light for use at night to read your gauge, with its' own power supply - no fancy circuit or phD in electronics required.

Forgive me if this approach is too basic or crude, but I could probably adapt the pound shop book lamp in less time than it took to write this. A cheap LED penlight type flashlight would likely work too.

Offered purely as food for thought.

I've not been keeping up to date with this thread and there have been some misunderstandings. Andrew, the 180R nd 12 V zener was my suggestion. SOD's diagram was a little simplistic in not including the resistor (unknown value) that is part of the "LED" and had a polarity error. My suggestion was based on a presumed "LED" specification of 12 V and 20 mA. An additional 180 ohms will drop 3.6 V at 20 mA keeping the "LED" within it's 12 V rating even at 14.4 V (typical maximum charging volte for a 6 cell lead acid battery.The zener will keep the voltage at the "LED" terminals at 12 V (give or take a bit) even if the LED does not draw it's full current. It will also provide suppression of positive spikes. I did suggest in a later post that a 10 or 11 V zener would be even better. The resstor / zener combination provides very good protection against negative spikes (clamping to less than 1 V most LEDs will withstand 3-4 V). I think the most likely cause of the failures is negative spikes but unless an oscilloscope is put on the car we are just making assumptions, some based on experience, some guessing.

I'm happy to put my money on my suggestion. I'll send the OP a 180 ohm resistor, 12 V zenner to try with one of his "LEDs" on the car. If the LED fails I'll make a donation to a charity of his choice. @ Tim Stevens send me a personal message with you address and I'll post them ready wired.

To use a LM317T (or LM317L) regulator its input voltage must be at least 2 V higher than the required output (datasheet says 2.5 V). So would have to be set to 10V output to work with the battery off charge. Even worse, the LM317 is also susceptible to spkes, especially negative, on the supply, To use it safely in a car it needs additional components. As a minium it is a LM317, 0.1uF capacitior. diode and two resistors. A lot more complication for little, if any, advantage of one resistor and one zener.

This stuff is part of my day job - on aircraft.

Robert G8RPI

The point about the LM317 solution is that it would supply a simple LED (not a 12V component of unknown content) with a constant current (constant light output) irrespective of applied voltage. Though primarily intended as a voltage regulator, it is equally suitable for this application.

For a white LED, say the forward voltage at the required light output is 4V, allow 4V minimum drop across the LM317 and the associated resistor, and the supply can be anywhere between 8V and 35V. If there are spikes greater than that, agreed additional clipping components might be necessary.

The version of the LM317 needs to be chosen for the dissipation that results from dropping the excess volts. However, provided that at normal supply voltage the power is within the working limit, the device does contain thermal shut down that would allow it to weather occasional spikes. Note also that the 0.1 micro-F input capacitor is recommended to swamp the inductance of the wiring, which might otherwise cause instability.

F

Edited By Macolm on 16/08/2020 21:07:27

But a LM317 in constant current mode won't work with the LED + Resistor lights that the OP is using.

Quite so, but he has had limited success with that.

We need to be sensible here. There are dozens of ways to run an LED off a car supply, with different pros and cons. No disrespect to the OP but if his electronics knowledge is such that h is buying "12V LEDs" with resistor already fitted he is not going to be working out a LM317 constant current design.

Just to reassure folk - the main reason I am using an LED with already attached resistor is simple. I already had a bunch of them for a different project. Not only that, the lamp is on the door, and needs a neat but flexible visible connection to the wiring behind the dashboard. There is nothing stopping me from using a different arrangement, as long as it is not bulky at the LED end. And it would be almost as easy to mount a different LED alongside the steering column and aim the light at about the same angle but from the left instead of the right. I await developments (and the postman.)

Regards, Tim (OP)

I concur! It can only be a guess, but negative spikes fit the bill.

Summarising the evidence:

• 92 year old car, 12v electrics from a dynamo but the regulator is modern.
• Lighting system is LED throughout.
• LEDs appear to be automotive types apart from the failing 3mm type which is an ordinary panel LED with built in dropper resistor.
• Failing LED is switched on & off with the sidelights and glovebox.
• LED fails when the engine is running, works OK off the battery. Other LEDs on the same circuit are reliable.
• Failing LED is in the door with the oil-pressure gauge? Presumably the gauge and LED wiring run in parallel for some distance, and the gauge wires connect to a sensor on the engine near the ignition. Possibility of inductive or capacitive pick up
• Flasher indicator LED flickers at low rpm despite being off. Suggests inductive or capacitive pick up of ignition pulses.
• Maurice's test shows LEDs aren't overly sensitive to positive spikes; his similar 12V 3mm example survived 32V.

Provided the current doesn't last long enough to overheat it, LEDs can be deliberately pulsed to increase peak power output substantially, perhaps to increase the range of a TV remote. I think LEDs are relatively immune to positive spikes. The same can't be said about negatives! Although failure at -4V or -5V is typical, some LEDs avalanche at -1V, while others can stick -15V. Not sure what happens if the negative current is limited by a resistor; but I suspect LEDs are much more vulnerable to negative spikes than positive. Perhaps an experiment is in order!

Car electronics are beefed up to cope with grubby automotive power supplies. I think Tim's working automotive LEDs are protected against spikes, while his 3mm indicator LED has no built in protection. As Robert's Zener circuit protects against positive and negative spikes, there's a good chance it will do the necessary.

Love to put an oscilloscope on Tim's car. I guess the trace with the engine running would show an average 13.8 vdc regularly modulated by short positive and negative going spikes at ignition frequency, which is what Tim suspected at the outset. But without an oscilloscope or peak voltage detector it's guesswork.

Dave

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So perhaps try inserting a bridge rectifier between supply and lamp

MichaelG.

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I suspect that Maurice was fortunate in his choice of LED

The ebay description that I found quoted a maximum of 14V

MichaelG.

"Never before in the history of Model Engineer Forum has such a small item attracted such intrest"

Hi Tim,

Bit OT but it has come up - legality of LED bulbs. This does NOT apply to your 1929 Vehicle, but the regulations are clear, all required exterior lights on a modern car (post 1985 at least, some regs apply earlier) must be approved and "E" marked. The "E" mark confirms that the light meets the required standards. These standards include that REPLACEABLE lamps (bulbs / filaments) must also meet certain standards (ECE) and be E marked. There are currently NO ECE standards for replacable LED lamps or LED alternatives to filament lamps. If it is not E marked you can't use it for exterior lighting on a modern car. The odd exception is the reversing light.
So How come some new cars have LED lights? This is because the whole light assembly has been tested, approved and E marked and there are no replacable lamps, bulbs or LEDs. You can for instance buy an LED replacement for a sealled beam uit because it is a complete light. LED DRLs are also available.

Back to spikes,

A resistor zener suppressor has been posted to Tim today

180 ohm resistor and 12 V 1 W zener (found a 1 W before a 500 mW) with a bit of Kapton tape insulaion so you can see how it's wired. I also included a 5 mm "12 V 20 mA" LED that made up this has a 470 ohm series resistor and draws 19 mA at 12V. I noted tha while the catalog listed the LED as 20 mA, the data sheet says 20 mA is absolute maximum. So this is righ on the edge. Conneced to the suppressor circuit it survived 28V input (19 mA through LED) though the resistor and zener run hot at that. It also survived 12V negative for short duration (resistor is overloaded by 20 % at this).

I'm confident this will fix the issue.

Robert G8RPI.

Hi,

I have just tested my clear Led with 820 ohm resistor in series on my 40 year old coil and distributer ignition Fiesta.

1 Connected Led to battery, Led lit.

2 Started and revved engine ,left running 5 minutes ,still lit.

3 Turned engine off ,still lit.

4 Connected Led between points side of coil and ground.

5 Started engine ,Led lit ,revved as before,left running, still lit.

6 Stopped engine ,Led not lit, restarted engine ,Led lit

7 Connected Picoscope to check for spikes

The above is battery voltage with engine running.

The above is across the coil connected to Led leads

Hope this helps regarding spikes etc, negative spikes 20v positive approx 130 v

This is why I think there is a wiring fault in the car

The spikes do not affect the Led

Maurice

Edited By Maurice Taylor on 17/08/2020 14:19:15

Apart from coronavirus, which is much smaller - only 125 nanometres in diameter!

Dave

SoD's summary is almost right. The oil pressure gauge is totally non-electrical, so is not related to the failure. And the LED in questions is on the door and shines like a torch onto the oil gauge in the dashboard (there being no easy way to put a lamp into the body of the gauge).

Has anyone handy to Ludlow got an oscillator they are not using ... ?

Cheers, Tim

While this is an interesting demonstration of the magnitude of spikes I note your LED had a 820 ohm series resistor. This is a lot higher than the 470 ohm in the OP's. This gives a much more reasonable 11mA current at 12V (assuming 3 V Vf for LED).

I can't see what wiring fault would cause the LED to fail.
As I stated earlier the OP's LEDS seem to be running close t absolute maximum curret rating at 12V.

Robert G8RPI

P.S whats the box of electronics next to the coil?

Edited By Robert Atkinson 2 on 17/08/2020 15:23:50

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With 51 posts to your name ... have you been lurking for a long while, or do you just read very quickly ?

MichaelG.