LED indicator keeps failing

I'd suspect reverse spikes.

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JF

Wes,

Assuming 2.0v LEDs, with a resistor of 820 ohms you are putting around

27mA through the LED. A lot of common LEDs are rated for 20mA constant so it sounds like you are overdriving the LEDs just a bit, which could account for the differences in how quickly they fail.

You might try something a little closer to 1400 ohms to get ~15mA through the LED.

I tried to find a data sheet for that part to find out the actual rating for the LEDs but couldn't find one easily. The numbers above are just safe(ish) guesses.

Jason

What is the failure mode? Have you put a line voltage monitor on the system to look for spikes?

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The led's already have a limiting resistor in the package. I added an additional one.

Thanks,

Wes

I don't have a line voltage monitor. I do have a fluke 12 that I put in min-max mode to monitor voltage. It didn't show any spikes but I doubt it is fast enough.

WEs

I was thinking of adding a diode to protect against that. There are some power control relays that get switched in and out > 5000 times a day. I need to look to see if snubbing diodes were installed across the coils.

Wes

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Good idea.  Try this too: (View in Courier)

    +--[--+---[GRN>]----+
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    +--[--+---[WHT>]----+
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    +--[--+---[YEL>]----+
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3>----------------+

If you can't see/detect the fault causing condition, you have to guess at what it might be and try 'fixes' to deal with it. It's down to too much forward voltage / current or too much reverse voltage or too high a temperature.

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I don't think John was talking about line spike protection, but more switched-inductor spikes.

Reverse-parallel diodes sound like a very good idea.

Cheers! Rich

Newark says those things are $92 per each, apiece!!!

For that price you should get a personal visit from the manufacturer to find the problem.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
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One would think. I recently sent an email inquiry to P+F concerning the problem. I thought someone engineering had checked but you never know. The Lean manufacturing stuff.... More like cut to the bone...

There are about 10 of these things on the line. My adding limiting resistors to each extended their life a bit but has not solved the problem.

I'm going to check my toolbox and see if I have the 1N4002's and put them in whenever the line goes down.

I will also look for any contactors that might not have snubbers across the coils.

I sure appreciate everyone's comments. I work maintenance and would like to fix it for good rather than swap in parts.

Wes

Replying to myself. I took a look at all the control panels, the prints show diodes across the coils of all contactors with 24VDC coils. I didn't find a single one actually in the control panels.

I think I have just had my first practical example of why those diodes are so important.

Thanks all,

Wes

If an 820 ohm resistor drops 9 volts, then the current is about 11 mA. If an internal-resistor LED has 15 volts across it and is conducting 11 mA, it will be dissipating about .165 watt - which sounds a little high to me for an induicator LED.

Meanwhile, I would suspect spikes, maybe reverse ones.

Which colors are failing? If red is holding up, what shade of green is the green?

Most RYG indicators have older tech LEDs (the green is yellowish) but the superbright greens from the late 1990's and later that are pure green, whitish green or bluish green are often static sensitive.

- Don Klipstein ( snipped-for-privacy@misty.com)

These are 'high powered' led's. The green (really is green) go first, yellow second, reds seem to last. See recent post where I found all diodes that were supposed to be across contactors missing.

(this is a datasheet from p+f)

Thanks,

Wes