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
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Topband Reflector
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