[CQ-Contest] Optical and galvanic isolation

[tom.mcdermott4 at sbcglobal.net]

> I'm building a keyer and headphone switch box for my small contest
> station.  I have a few questions:
>
> Some keyers use optoisolators for the key and PTT lines and others
> just transistors or FETs such as the 2N7000.  Has anyone had an
> experience where the optoisolator actually helped or saved equipment?
>
> Some headphone switches use transformers for galvanic isolation.  Has
> this solved problems for anyone?  And can anyone recommend a source
> for a good transformer for this purpose?
>
> I understand that isolation and transformers "good things".  But if I
> put all good things in this box it might not fit in my small contest
> station.
>
>                              Paul, K1XM


	There's very few cases where isolation is really needed. Normally,
it's ground that we're trying to isolate. Probably the most important case
is low-level audio from a microphone (normally via a computer).
Especially when the speech-processor is turned on, even microvolt level
audio signals will be amplified into audible sound. At the microvolt level,
there are a lot of sources of interfering AC signals (such as RS-232 cables,
AC hum, etc.). Isolating the ground on the microphone input to the radio is
one place where you can sometimes make some improvements to your audio.

	Some have used optoisolators to try to prevent damaging RS-232 IC's
in the radio. While it's not a bad practice, damage would indicate that a 
more serious problem probably exists in your AC/radio/feedline wiring.
Usually
it means that the AC Power ground and the radio chassis are at different
potentials
(probably during a transient) - a pretty sure recipe for damage. Bond those
grounds together within a couple feet of the radio and the damage issue 
usually goes away (follow the National Electric Code!).

	-- Tom, N5EG