On 2012-02-26, Michael A. Terrell wrote:

"DoN. Nichols" wrote:

[ ... ]

? The opposite polarity diode will keep the voltage low enough to
? protect the one not in use. In some applications they feed an AC signal
? to the LED for a third color.
?
? http://www.vishay.com/leds/bi-color/
?
? http://www.vishay.com/docs/83056/tluv5300.pdf

You're assuming two things:

1) The two LEDs are connected in parallel (with one reversed).

2) The current limiting element is external to the package.

I do not believe that (2) is true with the LEDs in the 327 lamp
package which I found earlier, and note from your PDF the following for
the bare LED:

Absolute maximum ratings:

Reverse Voltage Per Diode VsubR 6 V

So -- two of those connected as described, with individual current
limiting resistors (to operate at the 28 VDC of the 327 lamp) would
successively subject each to over four times the max Vr


You want a single resistor, not two.

But -- the limiting resistor (or perhaps a FET wired as a
constant current source) is *already* present inside the 327 format LED,
since it is designed to run from a 27V supply. This is what I have been
trying to point out this whole time.

If you got some designed with no limiting resistor -- that would
be different -- but would be asking for trouble in a system with a lot
of 28V rated 327 style LED assemblies, and a few of the color reversing
unlimited ones. What are the chances of someone swapping them without
knowing the difference?

[ ... ]

You would need to wire the lamp sockets for bicolor operation, so an
external resistor per pair of LEDs is no big deal.

*If* the LEDs did not already have built-in limiting resistors
or alternate circuits.

If there was enough space in the base you could use an 0402 SMD
resistor inside the base for a bicolor, for the 6 volt lamp. It sounds
like that lamp is intended for TTL logic drive of under 5 V and the AC
version has a higher resistance for the 6..3 typical voltage. There
could be a problem if they were fed from a higher voltage, with a
dropping resistor.

You are assuming two colors of LED in a single 327 style lamp
base? But this maker does not offer such, so we are stuck u sing the
ones which they already make -- or contracting for a special run.

Enjoy,
DoN.

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