Topband: Topband relays

Richard (Rick) Karlquist richard at
Wed Dec 11 10:34:16 EST 2019

On 12/11/2019 5:13 AM, Tim Shoppa wrote:

> Not relevant to my topband setup, but For the high-impedance (ladder line) side of my switching, I use open-frame relays with the contacts pried a little further apart than stock. Example part is the McMaster Carr 7384K14. N6RK outlines the process of widening the contacts in the May 2009 QST Hints and Kinks.
> Tim N3QE

Postscript to my QST article from 10 years ago:

Modifying relays this way increases the pull-in voltage.
Depending on how much design margin the relay has,
and depending on how much the contacts have been
opened up, it may be necessary to boost the coil voltage
momentarily.  To implement this, you can repurpose
a T/R relay "speed up" circuit, such as this one
designed by Tony, K1KP and described by Bob, K6XX:

The nice thing about this circuit is that it
doesn't require an auxiliary power supply voltage
that is higher than the normal relay coil
drive voltage.  Extremely elegant IMHO.

Will this drive circuit, you have the luxury of
overkilling the contact spacing so there is no
concern about arcing.  It is hard to say just how
much contact spacing is "enough".

Not mentioned in the write-up is that contact bounce
will establish the ultimate limit as to how fast the
relay can close, regardless of how much boost is
applied to the relay coil.

In any event, once the modified relay does pull in,
the holding force is the same as it was before the
relay was modified.

Rick N6RK

More information about the Topband mailing list