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LEDs
Subject: LEDs
Date: Tue, 6 May 97 09:29:47 EDT
From: pierson-at-ggone.ENET.dec-dot-com
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mail11:;;;;;;-at-us4rmc.pko.dec-dot-com-at-us4rmc.pko.dec-dot-com-at-unknown.domain.pupman-dot-com;;;;;;;;;;;;;;;;;;;
(-at-teslatech)
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pierson-at-ggone.ENET.dec-dot-com
[again, i am hand formatting to 70 characters, a message that
arrived, mangled, apparently, by the mailer]
>From: "William Noble" <William_B_Noble-at-msn-dot-com>
>Your comment about 2 volts is suspicious - LED's are diodes which emit
>when
[sample as received...]
[70 coll corrections below.]
>forward biased
Yes.
>- depending on the LED, there will be one or more "standard" diode
>drops
0.7 is standard for a Silcon Diode.
LEDs are are summat else, and have a different forward drop.
Material varies with color.
>in the forward direction - most red LEDs are a single diode,
Yes.
>so 0.7 volts will cause them to turn on.
JUST barely. Full on is 1.2 to 1.4 v.
>I have some green LEDs here that take 1.4 volts (e.g. they have
>2 diodes in series).
The 0.7 is chracteristic of silicon. LEDs are different. The
green, as the red, is a single diode. In general, this can be
seen by looking at the assembly, if need be, with a magnifier.
>You can easily determine what you have by putting some current
>through the LED (with a suitable dropping resistor) and then measuring
>the voltage across the LED.
Just so. Make the measurement at specced current (or briteness,
but current is easier to measure) and red will run 1.2 to 1.4.
I admit never measuring a green, but they are nearly the same.
regards
dwp