wetting current of standard household wall switch - Page 1
Forum section: electronics / projects

author: Clear as mud
author_rank: Regular Contributor
timestamp: November 05, 2020, 01:17:42 am
content: I've been searching for a while, and haven't found an answer.Does the standard household wall switch (AC-rated "quiet" switch) have a specified wetting current? I'm assuming it might not work reliably on 5VDC and minimal current, as an input to a lighting control system. What if I want to use such switches as inputs? Should I switch something higher-current, like a relay coil? What if I didn't want the power waste associated with having the relay coil always on?The switch would be something like this:Leviton 2653-2I. A 3-way switch (SPDT) so that one side or the other would always be on (except when the toggle is in between states).

author: wraper
author_rank: Unknown
timestamp: November 05, 2020, 01:31:49 am
content: For switches with such intended usage wetting current is not even needed to be considered. So I doubt you will find such specs. If it's only a few mA @5V YMMV. Most likely you'll be fine if in your application increased contact resistance does not matter much.

author: james_s
author_rank: Super Contributor
timestamp: November 05, 2020, 01:40:46 am
content: I've never seen that spec published. I'd probably just try it and see what happens, worst case it gets flaky after a while.They used to make mercury switches, the first silent types worked that way, both of my grandparents had them on the bathroom lights and I have a couple of the switches around that I found at junk shops. I don't know how easy it is to find them these days.

author: jbb
author_rank: Super Contributor
timestamp: November 05, 2020, 02:10:59 am
content: I don’t know what sort of ball park you might end up in with a wall switch.However, you might be able to drive a good wetting current without wrecking your average current consumption. One option would be to have a fairly low value pull-up resistor (feeding 10mA or more) which is pulsed; only turn it on when you need it.

author: wraper
author_rank: Unknown
timestamp: November 05, 2020, 02:18:38 am
content: Quote from: jbb on November 05, 2020, 02:10:59 am10mA is not much for such switch. 10 mA is even less than a minimum wetting current for microswitches with silver contacts like general-load models of Omron D2F (min 16.6 mA rating @ 30V and min 100mA @5V).

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 05, 2020, 03:44:52 am
content: Add a capacitor across the switch. You'll end up with a pretty large current spike when the switch closes.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: November 05, 2020, 04:25:01 am
content: Be aware that not all wall switches are created equal. The fireplace here is controlled by a wall switch... which is fine except that it's switching a thermopile signal. That's around 350mV, low current, and as classic an example of "low level switching" as you could hope for, calling for Au/Pd/etc switch contacts.Standard wall switches do not have those, so it was no surprise when a regular old household switch (actually commercial rated "specification grade", so not bottom of the barrel) failed in this application in maybe six months. Not wanting to replace it with another switch from the same box, I grabbed whatever else was lying around: a 30A-rated industrial wall switch. I did not have high expectations that its burly probably-silver-alloy contacts would do the job, but there was nothing to lose: the only consequence of failure would be that I have to swap it again.Several years later, it still works just fine. Go figure!

author: Clear as mud
author_rank: Regular Contributor
timestamp: November 05, 2020, 12:40:19 pm
content: The other complication is that these quiet switches are not rated for DC at all; they're supposed to be rated for standard mains AC voltage only. In the early days of wall toggle switches, before they had decided to standardize on AC, there were snap switches that were rated for either AC or DC. The quiet switch is known as such because it doesn't make a loud snap noise like those old ones. And, I was also aware of the mercury silent switches, but as james already said, they most likely are not made with mercury any more, and old ones would be harder to find.One option is to use mains voltage AC at the switches: then they probably would reliably switch with just a few mA. After all, many newer LED light bulbs draw less than 100 mA and I have never heard of a switch failing on those because of lack of current. But, using the AC voltage to switch an input to a microcontroller would be more difficult, and there is then no easy way to draw just a single pulse of current when switching.The other option is to use DC. Although the switches are not rated for DC, I am thinking that is really a problem only if switching large voltages and currents, like if you were directly switching light bulbs supplied by 120V DC. The internal arcing would be a problem then. But, for small values of voltage and current, the arcing should be similar or less than what the switch has during normal use on AC.So, I think I will try what NiHaoMike suggested, and take the 5V DC directly to my switches, and add a capacitor so the initial current draw will be something like 50 mA, but drop off as the capacitor charges. It will make the design a lot easier, and seems to have a reasonable chance of the switches remaining reliable.

author: Gregg
author_rank: Super Contributor
timestamp: November 05, 2020, 04:52:47 pm
content: You could always buy several brands and quality of switches and take them apart in true EEVblog fashion and determine which are more suitable for your purposes from the contact action; if the contacts make with a wiping action, you are probably good to go. Bob may even be your uncle.Another approach may be to find a local Habitat for Humanity Re-store (my favorite place to shop for household repair items) near you and possibly find some old stock or even mercury switches.When I rewired my cabin, I found that Lowes had the best prices for spec-grade receptacles and switches.

author: james_s
author_rank: Super Contributor
timestamp: November 05, 2020, 09:30:58 pm
content: Another option that may be viable is to modify the switches yourself. Depending on the design you could glue a mercury tilt switch to the inside of the toggle mechanism, or glue on a magnet and put a reed switch inside the body. Spec grade switches are usually more bulky and likely a better candidate for internal modifications than the cheap builder grade ones.

author: floobydust
author_rank: Super Contributor
timestamp: November 05, 2020, 11:15:52 pm
content: I'm not sure how much wiping action there is.

author: Gregg
author_rank: Super Contributor
timestamp: November 06, 2020, 07:27:40 pm
content: You might be able to modify an existing wall switch to use a reed switch and a small magnet.How many of these switches do you need? I may have a few NOS Hg switches I could send you.

author: wizard69
author_rank: Super Contributor
timestamp: November 07, 2020, 05:30:31 pm
content: I'm not sure wetting current is the right spec for low voltage switching. What you need is a high enough voltage to punch through any oxides that build up on the contacts. The other thing to consider is that input currents to many devices with 5VD inputs are very low even if the device optically isolates those inputs. What use to be a 20 ma current is often half that or even less these days for isolated inputs.I once had this very problem with industrial push buttons trying to interface to a 5VDC input on a device. The standard contact blocks were not reliable at all, we switched to low voltage rated reed contacts and never had an issue again. Given this and how cheap common household lighting switches are I would not consider them for any low voltage use. Back in the day we had problems with relays used in low voltage applications and had to switch to "dry contact" relays. These days you have options for isolation between circuits so you can choose between solid state or relay type isolators.For human interface devices (pushbuttons or toggles) you are better off making sure the switch is rated for low voltage use. You have to be careful because it is easy to have two switches that look the same on the outside that will behave differently at low voltages.Quote from: Clear as mud on November 05, 2020, 01:17:42 am

author: Gyro
author_rank: Super Contributor
timestamp: November 07, 2020, 10:17:39 pm
content: Quote from: NiHaoMike on November 05, 2020, 03:44:52 amI 100% agree on the capacitor, with Silver alloy contacts, voltage isn't necessary enough at very low currents - unless it is significant (ie. mains).I remember converting my Weller TCP soldering iron to use an opto-triac switch to extend the life of the expensive Magnastat switch contacts. As part of the conversion, I removed the (Rifa!) contact suppression capacitor. Even though the contacts were still switching 24V AC, at the reduced 20mA effectively resistive load, contact closure quickly became intermittent, requiring tapping on the bench. Restoring the capacitor across the switch contacts immediately cured the problem.The current spike from a capacitor, through the closing contacts is sufficient to burn off oxides and impurities which otherwise build up to the point that their breakdown voltage exceeds what is available. Hopefully this would still be effective down at 5V too.

author: Nusa
author_rank: Super Contributor
timestamp: November 07, 2020, 10:43:37 pm
content: Leviton does make a 24V AC/DC wall switch:https://www.leviton.com/en/products/12021-2i