Help pls - Alternator voltage too high, blowing LEDs?

Technical advice Q&A

cactusman
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#21 Re: Help pls - Alternator voltage too high, blowing LEDs?

Post by cactusman » Wed Nov 14, 2018 3:51 pm

No way of knowing without reference to the data sheet. A simple protection against brief spikes in voltage can be provided by putting a 2.2 ohm 1 watt resistor in the positive and then connecting a 18volt 5 watt zener diode to ground after the resistor. Part no for the zener is 1N5355B. It will soak up brief spikes but not permanent over voltages from an alternator. For that replace the resistor with a 200 mA slow blow fuse. Then if the supply gets to 18 volts the fuse pops. All seems a bit overkill though.....
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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64etype
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#22 Re: Help pls - Alternator voltage too high, blowing LEDs?

Post by 64etype » Wed Nov 14, 2018 4:54 pm

I couldn't find a data sheet for the dimmer, but one commenter on Amazon said he took one apart and the are three field effect transisters inside. My limited reading on Wikipedia suggests that FETs have an inherent resistance to voltage spikes. So maybe the little dimmer is a sort of blocking device. Anyway, thank you for the help. Perhaps the gents who are designing LED lighting kits and components for the E-type will contribute to the discussion.

In the mean time, I will either add the solution you describe above, or find an inexpensive (floating) voltage limiting device on line and wire it in ahead of the excellent little dimmer. Removing and disassembling the tach and speedo to replace that strip lighting is not something I care to contemplate. Thank you very much for answering my questions. Very helpful.
Eric

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neal herridge
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#23 Re: Help pls - Alternator voltage too high, blowing LEDs?

Post by neal herridge » Tue Dec 18, 2018 5:24 pm

Did anyone come up with an answer on how to protect Marek's LED reversing light.
Its a great piece of kit but I would like to keep it that way.
Its the only LED lights on the car.
Many thanks Neal.

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Jeremy
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#24 Re: Help pls - Alternator voltage too high, blowing LEDs?

Post by Jeremy » Tue Dec 18, 2018 7:35 pm

Neal, this what Marek recommended to me:
https://www.ebay.co.uk/itm/Inline-Autom ... :rk:1:pf:0
I have inserted it into the positive feed of the reversing light LEDs. So far, so good - but as it's had very little use ( I'm not taking the car out in the winter weather), I can't say yet if it's solved my problem permanently. I hope so as the dual LED upgrade is a v clever idea..
Jeremy
1967 S1 4.2 FHC

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mgcjag
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#25 Re: Help pls - Alternator voltage too high, blowing LEDs?

Post by mgcjag » Tue Dec 18, 2018 7:52 pm

If its just a Diode then all it does is allows current to flow in one direction only.....cant see how it will solve the overvolt problem....Steve
Steve
1969 S2 2+2 & Building a C type replica

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Jeremy
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#26 Re: Help pls - Alternator voltage too high, blowing LEDs?

Post by Jeremy » Tue Dec 18, 2018 10:37 pm

Steve,
I asked Marek that very question. He replied that the forward barrier voltage (voltage drop) for a silicon diode is 0.67V. Other sources I searched say the same, or "about 0.7V". (For germanium diodes the equivalent figure is 0.3V). It does vary with current, but only by very little within the normal operating range. So this diode should take the voltage in my case down to below the breakdown threshold for Marek's LEDs, and should solve the problem. :fingerscrossed:
Jeremy
1967 S1 4.2 FHC

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neal herridge
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#27 Re: Help pls - Alternator voltage too high, blowing LEDs?

Post by neal herridge » Wed Dec 19, 2018 5:23 pm

Many thanks Jeremy.
I see what you are saying in that the diode just reduces the incoming voltage by 0.7 volts what ever that voltage is.
It will not work if there is a higher spike in the voltage .
You will also need to protect the extra brake light LED's that are part of the setup.
I was hoping for something to wire in to protect the reversing light against any spike in the voltage that was over LED's limit.
It would be good to get feed back from other people who are using this set up.
Like you Jeremy because of the bad weather I have not been out much after fitting it, a boat would be better here in Devon at the moment.
Thanks Neal.

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64etype
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#28 Re: Help pls - Alternator voltage too high, blowing LEDs?

Post by 64etype » Wed Dec 26, 2018 3:47 pm

Follow up to my earlier post....

A friend who is handy with electronics believes that the little pulsed dimming device (based on Field Effect Transistors) that I'm using for the strip lights in the tach and speedometer does provide a modicum of over voltage protection as long as it is set to reduce the light intensity somewhat below maximum. While the FET passes on maximum input voltage, the pulsing effect results in a reduced average "on" time, thereby reducing the heat build up in the LED segments. For my installation, full brightness lights up the inside of the car like a space ship, so they are dialed back significantly . So the potential burnout problem seems to be addressed in a round about manner.

I will try the diode solution on my boat, which has a few home-built strip light arrays, and where the on-board charging system for the house battery bank has a float setting of 13.5 volts. I will stack two in series ahead of the LED light arrays for a 1.4 volt reduction. Not the best fix, but probably adequate and simple.
Eric

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