Wetting current through relay/switch contacts AC vs DC? - Page 1
Forum section: electronics / projects

author: Curi0sity
author_rank: Newbie
timestamp: June 17, 2014, 12:34:09 pm
content: Here's a question for you experts out there regarding wetting current (minimum current) through relay/switch contactsI am designing an EPO (emergency power off) system which is fail safe and the circuit operates a relay coil through about 10 normally closed switch contacts. (push the switch, break the connection which shuts down the connected system) The circuit operates at 24VDC and the relay coil consumes about 30mA of current at this voltage which is below the minimum wetting current that I could find. Also I should mention that the switch contacts are silver alloy and it would be difficult to find gold contacts for this switch.I believe this means that the EPO circuit will work... until the switch contacts get crummy and then it won't work anymore. So this leads me to a question. If I went with 24VAC over the switch contacts instead of 24VDC would it make any difference at all with the wetting current required? I could find no information on low voltage AC vs DC and switch contacts online, and I have heard from an unverifiable source that using 24VAC may solve the problem.The solution I see here (other than potentially using AC) is to simply increase the current demands of the circuit slightly by adding a status light or a capacitor to the circuit.Thanks for reading!

author: eetech00
author_rank: Regular Contributor
timestamp: June 17, 2014, 03:32:30 pm
content: What about using a relay designed for a lower coil voltage? It will energize faster, increase contact pressure, and draw more current at a higher operate voltage. I used to do this with traffic control systems years ago.Why are you using so many contacts to energize a relay?You mentioned "failsafe", so some things to consider..Is the relay designed to be failsafe?Can the contacts weld together thru some malfunction causing them not to open?Does the controlling circuit hold the relay normally energized?eT

author: Stray Electron
author_rank: Super Contributor
timestamp: June 17, 2014, 04:26:12 pm
content: Cutler Hammer (IIRC!) makes a series of switches and relays that are designed for digital use and thus can handle low wetting current. I learned this the hard way years ago when I needed some LOW current switches for a similar project and the digital rated ones were about 3x the costs of standard ones. Being a cheap SOB, I tried to get away with using standard switches but as you say, the contacts closed but didn't make contact (ie stayed open) so I had to go buy the more expensive CH digital switches. After installing them, everything worked as intended.

author: Curi0sity
author_rank: Newbie
timestamp: June 17, 2014, 05:35:16 pm
content: @eetech00The EPO works by wiring 5 to 10 normally closed switches in series with a relay. Push one of the switches, the relay opens and causes some stuff to happen downstream to shut the system down.The relay is held normally energized. I do not believe there would be enough power going through this relay to weld the contacts shut (about 250mA). And this relay is not specifically a fail-safe relay (I wasn't aware such a thing existed) but it is a high quality industrial relay.@free electronI'll do some more research and see if there are any switches that fit this bill that are rated for low current. I'd like to avoid changing the switch I had already planned on using though as it would involve a re-design.Does anyone know if low voltage AC is any different than low voltage DC for reliability through crummy contacts? This seems to be an 'old wives tale' and I'm trying to find out if there is any validity to this.Thanks again!

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2014, 05:55:30 pm
content: Low voltage DC with a humid atmosphere will corrode faster. Either use a 0.1uF capacitor across the switches to create a small arc on closing that cleans the faces or increase the current to 100mA.

author: eetech00
author_rank: Regular Contributor
timestamp: June 17, 2014, 10:01:06 pm
content: Quote from: Curi0sity on June 17, 2014, 05:35:16 pmYes...they exist. They're used quite a bit in the railroad industry. Among other things, the relays have carbon contacts to prevent the contacts from welding together in the event of a surge or overload.eT