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LEDs



Subject:   LEDs
  Date:    Tue, 6 May 97 09:29:47 EDT
  From:    pierson-at-ggone.ENET.dec-dot-com
    To: 
       
mail11:;;;;;;-at-us4rmc.pko.dec-dot-com-at-us4rmc.pko.dec-dot-com-at-unknown.domain.pupman-dot-com;;;;;;;;;;;;;;;;;;;
(-at-teslatech)
    CC: 
        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