Topband: Blowing diodes on relay switches

[Shoppa, Tim]

Is there a better way to use an isolation transformer to couple relay control voltages over the coax? Or is it as simple as, Bias-T's are always best, and there's no good way to do it with a transformer winding? Or just skip all these methods and go to separate control wires?

Tim.

-----Original Message-----
From: Tom W8JI [mailto:w8ji at w8ji.com] 
Sent: Friday, November 15, 2013 10:14 AM
To: Shoppa, Tim; topband at contesting.com
Subject: Re: Topband: Blowing diodes on relay switches

> 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