Topband: Blowing diodes on relay switches

[Tom W8JI]

> The antenna end of mine, looks very similar to PG0A/PA3FYM implementation, 
> drawn at 
> http://remco.org/wordpress/wp-content/uploads/2008/01/pg0a-k9ay-small.jpg
>

I guess he uses the LED as a rectifier? Not even remotely the best idea I 
have seen. LED's are horrible for anything with high peak currents (like 
charging capacitors).

Also, it is generally a bad idea to have high peak currents (like charging 
electrolytics) in iron or ferrite cores. You can pass some current through 
the secondary, but you better be sure the current level is a long way from 
altering the core's magnetic properties, and this flux level would include 
any flux from external signals.  This has nothing to do with your diodes 
blowing, just to say the circuit you linked does not look the least bit good 
for multiple reasons.

> Uses an isolation transformer (not an autotransformer) and provides 
> galvanic isolation between antenna, antenna ground rod, and the coax.
>
> I do not use LED's in mine, I just use regular diodes, and these are the 
> diodes that are blowing. I'm starting to believe that it's during RTTY 
> contest weekends when I'm on 10 a lot, that the diodes are blowing, and I 
> think Tom's suggestion about bypassing effective for  10M (not just 160M) 
> may be most relevant.
>


Is this a  + , - , and AC logic system?

That circuit has a terrible bypassing and isolation scheme. It has no diode 
bypassing for the common mode path from the cable through the diodes to the 
relay coils and to the loops, except for what the electrolytics might 
marginally provide at radio frequencies. That circuit can also induce 
significant common mode into the loops, exactly what the isolated secondary 
is attempting to cure! The wiring undoes the goal of the isolated secondary. 
I would give that circuit a completely failing grade.  0 out of 100. Bad 
design.

Of course this is getting more and more common these days, as we look at 
systems with tunnel vision to focus on and cure one thing.

73 Tom