Coping with voltage spikes

Robert,

Sincere apologies for my stupid mis-reading of the Mad Scientist’s table

I confess that I was looking for 100 Ohms [to match the ebay photo] and saw what I wanted to see

mea culpa

MichaelG.

Never mind the table Michael, very informative page of reading, well worth a read, thanks for the link.

Emgee

Hi Michael,

Try reading the resistor in the ebay photo from the other end - looks like green,blue,black,black,brown which would be 560 Ohm at 1% tolerance.

Keith

Good point Keith

Oops !

MichaelG.

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Reminder to self: 

1.Don’t assume that things are ‘written’ left-to-right !!

2. Digikey has a nifty little calculator:

https://www.digikey.co.uk/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-5-band

Edited By Michael Gilligan on 20/08/2020 22:51:23

Hi,

Just done a test with green led from junk box in series with 270 ohm resistor ,to test the effect of negative spikes,

Equipment used 12volt power supply,picoscope and 12volt car ht coil. I was able to produce an 80volt negative pulse ,did this a few times.Led and picoscope both connected across the coil.

Disconnected led ,connected it to 12volt supply still works.

All comments welcome good or bad

Maurice

Maurice

Do tell us more about your picoscope, please

Tim

I love experiments, and this one motivates me to have another go! I failed to kill a red LED with -30V, and Maurice failed to kill his green one with -80V. Seems LEDs are as tough as old boots despite what my books say about them being sensitive to negative spikes.

Tim's problem is proving remarkably difficult. At the moment I suspect bad joints due to faulty manufacture of the LED and its built-in resistor rather than the LED being blasted by spikey electrics. If I understood correctly Tim said one of the failed LEDs lights up when connected directly to a power supply: it's not been killed by a spike.

Dave

After my faux pas, yesterday ... I don’t feel worthy to pass any comment except:

Thanks, Maurice

An interesting exercise, thanks for documenting it.

MichaelG.

Maurice,
Interesting experiment, but again producing very narrow spikes.

Tim,
Picoscope is a trade name of Pico Technology (picotech, www.picotech.com ) for a range of oscilloscopes that use a personal computer for a display and processing. As well as "ordinary" 'scopes thay do an automotive range, www.picoauto.com that include very comprehensive vehicle diagnostics. For example you can check compression just by clipping a current sensor on the battery lead and crankng with igntion off.
They are great units, I have several, including some their early ones, and a full 4 channel automotive kit. There are cheaper similar units but Pico are probably the best.

Dave,
Did you see my earlier explanation of how partial junction damage could produce the effect seen by Tim in the failed LED directly connected to a 6V supply. I've blown up LEDs in the past when pushing the limits under pulse operation and seen this kind of effect. Under a microscope you could see the damaged area and a curve tracer showed a distinct change in the forward voltage characteristic.

Robert G8RPI.

Hi ,Robert

If you’ve got all this equipment ,why haven’t you done any experiments to show us all what spikes do to an led?

Maurice

REPORT:

I have now converted my holder to 5mm, and tested the 5mm LEDs kindly sent by Robert and Duncan (see above). They both worked exactly as they should when tested to 15v and neither showed any trace of flicker etc when starting the engine or switching other stuff on and off. It is not fit for a drive here (persisting down) so I will report further on extended use when the sun next shines. But only at first for the slightly better focussed* of the two, from Duncan.

*Note: For readers who are less familiar with LEDs: the focussing is part of the manufacture and depends on the quality of the plastic dome and the size and depth of the emitting surface.

Having now looked very carefully at one of the remaining supply from which the failed LEDs were taken, it seems that the failures may have been due to the feeble assembly in manufacture of the included resistors. The wiring had to be curved to fit in the holder and this may have started a failure which allowed the circuit to break as the door was closed or the first road bump was crossed. Both the LEDs I have been sent have the resistor further from the diode and not affected by any bending.

Regards, Tim

Hi Tim ,did you do any of the tests I suggested about 3 days ago regarding connecting your leds across battery?

Robert has already answered question about Picoscope. Go to ‘picotech.com’ for info .Software is free and runs in demo mode .Prices at around £100.

Maurice

Maurice - I'm sorry, but if you can point me to the message I will try.

I have been without my PC over the last few days and I may not have a full set of messages.

Tim

Hi Tim,

The above was what I posted ,if you still have one of your Leds connect it across the battery and see if it works as you won’t need to bend it.

Maurice

Hi Robert,

No I didn't see it! Very strange, I thought I was paying attention to this thread, but missed it.

Very interesting. Another divesrion from amusements and duty galore, but it would be fun to spend time investigating LEDs. Not just blowing them up! For example, I notice they behave like a noisy zener and wonder if that's useful as a noise source. I don't need a noise source, but...

Look's like Tim has cracked the problem. Hurrah!

Dave

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Aha - that message. I did try to explain that there were no leads to the oil gauge as it was completely non-electrical.

Secondly, there is no fan belt to remove, as the dynamo is gear driven, and I commented that fan belts came in about 1930.

And yes, if I connect one of the original batch of LEDs to the battery, it does light.

But having said all that, I'm sorry, but I'm not sure how much further it gets us.

Did I misunderstand you somewhere?

Tim

Hi Maurice,

Unfortunatly I'm pretty busy with the day job (mostly from home) and the workshop is full of stuff due to some re-rranging and decorating for SWMBO. so no time. Additionally I can't replacae a 1929 car.

However Tim's recent posts indicate it might have been mechanical failure of connections. I've not seen the car or LEDs and I guess we may never know for sure. I'm not convinced by the broken connection theory though.

Tim,
The LED I sent is a wide angle type. It's not a quality issue, they make them with different viewing angles. The one I sent was left over from a backlight modification on a digital meter.

Robert G8RPI.

Dave,

LEDs do make fairly good low frequency noise sources. .I've seen them used for that in the past.

Robert G8RPI.

Hi Robert ,thanks for your reply ,faults like this usually are bad connections ,that’s what I tried to say early in the thread. I’ve had cars for 50 years and most electric faults have been bad connections etc. I appreciate your a busy man ,and don’t have time.

Maurice

Edited By Maurice Taylor on 21/08/2020 21:02:40

Duplicate ,deleted

Edited By Maurice Taylor on 21/08/2020 21:01:51