[TowerTalk] Wetting Your Relay Contacts

[Leeson]

Along with your tower and antennas, you have relays and connectors, 
perhaps with a rotator, a directional antenna switch or even a remote 
station. There’s a common but difficult-to-diagnose problem you may 
experience with these.

Many years ago, Dave Pruett, K8CC (SK) was working for Chrysler-Jeep, 
and he mentioned to me that auto manufacturers specified a minimum 
current to maintain the contacts in every switch, relay or connector, 
typically 10 mA. Without this current, there is potential for surface 
oxide or sulfide to develop on the contacts, resulting in a failure and 
safety risk. Sadly, Dave became SK in 2020, see 
https://www.qrz.com/db/k8cc and http://hamgallery.com/Tribute/K8CC/

He told me those with a maintaining current level were termed “wet 
contacts” while those with too little (or no) current were called “dry 
contacts.” This came back to mind as I’ve recently experienced a spate 
of relay and connector failures, all of which were situations without a 
“wetting current” (also termed “fritting current”).

I refurbished my old IC-730 for my grandson who recently upgraded, and 
both the T/R relay and the preamp relay weren’t working on receive. 
ICOM’s bulletin suggests adding resistors to provide a wetting current 
for the preamp relay, and I made a similar fix for the T/R relay. Then I 
had a failure in my remote antenna rotating system, which I finally 
traced to a dry contact failure in the RJ45 connector on the cable that 
included a wire to the solid-state relay that controls the rotator 
primary power. With the dominance today of CMOS circuits that don’t need 
input current, it seems this problem has become more common with control 
circuits, as well.

Cleaning the contacts with contact cleaner or a mild abrasive can 
temporarily fix things, but the problem can return without a real 
wetting current. In addition to providing this small current through 
resistors, there is a suggestion to discharge a capacitor through relay 
contacts on closing. Some relays have wiping contacts, which can help 
remove surface contamination. Apparently, even 1 mA is enough to do the 
job. I hope this alert can be useful.

Online references include these:
“Wetting current,” https://en.wikipedia.org/wiki/Wetting_current;
P-T de Boer, PA3FWM, “Relay contacts and ‘fritting’"
https://www.pa3fwm.nl/technotes/tn17a.html;
“Wetting Current,” 
https://resources.pcb.cadence.com/blog/2023-wetting-current;
D. McCarty, “Wetting or Sealing Current,” 
https://www.isemag.com/columnist/article/14266765/wetting-or-sealing-current.

For the ICOM upgrades, see A. Copland, GM1SXX, “IC-730_relay_fix.txt, “ 
www.f6hoy.com/wp-content/uploads/2022/01/IC-730_relay_fix.pdf and 
"Preamp Relay Circuit Modification for IC-730" on mods.dk.

Dave, W6NL/HC8L