At 20:31 03-08-99 -0700, punkymcd@pacbell.net wrote:
>
>Hi,
>Can anyone give me a good reason for the diode in series with the LED in
>this circuit? It looks like a diode in series with another diode (LED)
>to me.
>
>73, Don, W6OA
Hello Don,
The circuit on my web site has the diode in series to protect the LED
when the RS-232 level goes to the opposite state. The -12v when the keying
is turned off is way over the PIV rating for the opto-coupler LED.
I've tried the circuit with a number of current limiting resistor values,
and it keys reliably with 1 ma or more forward current. Ones I've built
with 1k and 2k resistors have been in use since 1992 with no failures.
73 -- Glenn
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