On 2012-02-25, Michael A. Terrell wrote:
"DoN. Nichols" wrote:
wrote:
"DoN. Nichols" wrote:
Two LEDs, in antiparallel work OK. That's where we get Bicolor LEDS
with two leads. Now they make four lead RGB LEDs.
Depends. If the current limiting resistor is outside the LED
package, yes. Each LED prevents much reverse voltage from being applied
to the other. However, if the LEDs have the resistor (or other current
limiter) packaged inside the lamp assembly, then it gets the full
reverse voltage (unless they also included either a series diode to
block reverse voltage, or a parallel diode to shunt the reverse
voltage.)
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
And the forward current per diode is 100 mA at about 2.2 V,
which is a maximum time of about 50 uS.
As for the 327 base LEDs, I've re-found and I see no bi-color
lamps, and no discussion of maximum allowed current -- just a voltage
rating of 24 to 28 V, which says that the current limiting feature is
built into the base of the LED lamp. And for the DC only ones (either
polarity) I can't find a spec to tell me whether it will accept the same
voltage in reverse.
Enjoy,
DoN.
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