Sam Goldwasser wrote in
:

bz writes:

"Mortrek" wrote in
news:1170923228.704325.259980 @j27g2000cwj.googlegroups.com:

no schematic but basically, it's a shot clock with multiple
independent LED drivers (one per "line", with 7 "lines" per digit).
Each "line"/driver has a 5ohm resistor in series, along with 6 LEDs
in series, and a total of 12V cumulative across the series circuit (I
measured 2V per LED). I have no information on the specs/tolerances
of the LEDs that came with the shot clock (the manufacturer refuses
to disclose this) but my replacements are wp1503id available at
mouser.com, data sheet at
http://www.us.kingbright.com/images/catalog/ SPEC/WP1503ID.pdf


If you replace one OEM LED in a string, you should probably replace the
whole string because you have no idea of the ratings of the originals.

Also, you might try paralleling each LED with a fixed resistor (all the
same value for that string of LEDs. You might aim for 10 ma through the
resistor string, that would be 2 volts/10 ma or 44.4 ohms for each
resistor. It WOULD increase the load on the LED drivers by about 11%,
which might be a problem.) to 'equalize' the voltage drops across each
diode in the string of LEDs.

Why? LEDs are current driven. Their exact voltage drop is rather
irrelevant as long as it isn't far off.

Agreed, As long as it isn't far off. I do know that strings of diodes often
need equalizing resistors but that may be to make sure that the reverse
voltage doesn't 'pile up across one diode and blow it'. I also seem to
recall that there are times when zeners in series require parallel
resistors.

Maybe he needs some capacitors rather than resistors.
Perhaps transients are killing the LEDs. Something is.

The fact that it works fine in the shop is puzzling.

With some such problems I use a variac to run the voltage up to 125 or 130
to 'stress test' it.


How close to 2 volts are you getting? If, for example you have one at
2.5 and another at 1.5, differences might allow one LED to overheat and
go down in resistance and efficiency, over heating more and overloading
other LEDs in the same string until one or more fail.

Sorry, if they were that different, one of the LEDs is bad.

No need to be sorry.

LEDs of a
given color from a given manufacturer will be quite close in voltage at
the same current.

Hence my suggestion that he replace the whole string when one goes bad.





--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

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