Topband: Blowing diodes on relay switches

[Tom W8JI]

> 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.

Things like this are not rare. Simple circuits can have multiple modes of 
operation that are hidden from view on a schematic. A page linked on not 
using MOV's on power lines is similarly narrow focused and tunnel visioned 
to cure one "problem" while being completely oblivious to more common issues 
and new problems created by a solution!

The common mode  issue with that system is you float the 75-ohm secondary 
for CM isolation.  Then you hang a relay coil, which can have considerable 
capacitance to the contacts, plus the capacitance of all those components, 
on the coax shield. If the coax shield has significant unwanted currents 
(and voltages), you would not want a direct connection running around on the 
board. This is especially a problem with a relay that has high capacitance 
from coil leads to contacts, which can easily be the case. I've found some 
relays in the dozens of pF, and that even have series resonances from coil 
to contacts!! Some relays integrate the electromagnet pole piece with the 
contact bar with a direct connection, so the big coil effectively has the 
armature passing right through the center..

The second issue is the electrolytics, if fed pulsing DC, have high peak 
currents and high harmonic content. All that "stuff" flows across the 
primary. This issue isn't nearly as bad if the dc is filtered, and if the 
relay is low current, and if the lowest possible capacitance filter is 
used.....but it is generally better to keep that current out of the 
transformer unless you are sure it won't be an issue.

Separate control wires are no better for CM issues.

You could be blowing the diodes from current flow between the loop system, 
through coupling to the relay coil, and through the diodes to the coax.

This is what happens when we only look at differential mode.

73 Tom