Sensible DC rating of AC light switch? - Page 1
Forum section: electronics / beginners

author: arcfault
author_rank: Contributor
timestamp: September 08, 2023, 01:13:48 am
content: What would a sensible de-rated DC voltage/current look like for a AC rated light switch?One like this:https://www.amazon.com/Leviton-54521-2W-Single-Pole-Commercial-Grounding/dp/B000U3I11CSome people say 3-5% for current, but not sure if that's overly paranoid. I have a 12V circuit, so it seems like arcing will be minimal, but not sure what the gap is like internally.Thank you!

author: coppercone2
author_rank: Super Contributor
timestamp: September 08, 2023, 01:46:58 am
content: Generally I see something like a 120/250V AC 15A switch being good for 15A @ 12VDC.to me it looks like the precise answers for higher voltages are very dependent on switch construction, its like a plasma problem.The best way might be to cut a hole in it and see how the spark looks when different voltage loads are switched, perhaps with a plexi glass fill in. If the spark starts looking worse then the AC spark then you know you got a problem. Its like thermal damage that occurs.It is delivering heat into the switch by the spark. The lenght of the spark and the voltage drop across it is like a resistor. in welding if you make a longer arc, it heats more with a constant current (makes sense, constant current + bigger resistor = more heat).The amount of heat delivered by a opening switch (before its extinguished) with 15A 120VAC vs 15A 12VDC might be similar. If its like making a similar noise (spark) then I think you are good generally. Its when you start hearing extended spark noises. I wonder if a microphone recording could tell you anything about this.120V is alot. But with AC you have a zero crossing every 8.3ms. That means that you might have a 120V spark, but it will be extinguished ideally in 8.3ms. The switch travels some distance during this time. How far can the switch travel before the spark extinguishes on 12VDC?then your heat is voltage drop * current for the time it takes for the spark to break. If you have the same current, I would suspect that if you can break your spark in 8.3ms, then you basically have the same thing. But migration of metal (like plasma deposition) will occur in only one direction. With AC, you might have matter shifting back and forth between the switch terminals (whatever this amount is) randomly, statistically around 50%. With DC its going in one direction. that probobly means there is gonna be more pitting. Maybe on some level a AC switch fills in the hole that it had from the previous cycle, sometimes. like two bakers having a food fight through dough at each other. the amount of dough will be roughly the same per baker if they on average end up throwing dough. With DC its like a baker is throwing dough at customers, so it runs out quicker. but its also like there is dough hitting a ceiling fan and getting slapped on the walls, so it won't be going back and forth forever

author: EPAIII
author_rank: Super Contributor
timestamp: September 10, 2023, 06:54:37 am
content: I think it will vary from switch to switch with no real way to tell in advance of actual testing. Different switches will have different internal construction and they are made for AC.

author: Nusa
author_rank: Super Contributor
timestamp: September 10, 2023, 07:21:30 am
content: The best advice here is to buy a switch that already has a DC rating.But if I were to try to ballpark a rule, it would likely be 10% the AC voltage rating, and 50% of the AC current rating.From the safety viewpoint, the question to ask is: "How dangerous is it if a switch with fused contacts cannot be turned off?" If it's a light, no biggie. If it's a power tool, could be serious.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 10, 2023, 07:57:02 am
content: See where it says "ac only"? It may need to wait for the voltage to cross zero before it switches off.

author: CaptDon
author_rank: Super Contributor
timestamp: September 10, 2023, 02:10:31 pm
content: That 'Quiet Switch' may contain a mercury wetted contact capsule and would be totally unsuitable for any D.C. operation. It would try to become a mercury arc lamp.

author: themadhippy
author_rank: Super Contributor
timestamp: September 10, 2023, 03:57:55 pm
content: QuoteAnd how does 2 bits of metal and a spring know when the sine wave is at the zero point?

author: mikerj
author_rank: Super Contributor
timestamp: September 10, 2023, 06:29:19 pm
content: Quote from: JustMeHere on September 10, 2023, 07:57:02 amThat just means the ratings are for AC only. The fact that it doesn't have a DC rating doesn't mean it's inherently unusable with any level of DC, it just means the manufacturer hasn't given it a rating since it's sold for for one specific purpose.

author: coppercone2
author_rank: Super Contributor
timestamp: September 10, 2023, 07:09:12 pm
content: also I was thinking maybe to add to my post, welding machines have a BIG difference in DCEP and DCEN (electrode pos/neg) ratings.Depending on how the switch is built, how much thermal mass is available on each contact, you might have switches that VERY MUCH prefer to have a certain polarity.With a welding machine (tig), the difference is something like 20 amps vs 120 amps with DCEP and DCEN. Plug the polarity in backwards and your tungsten yells "smoke bomb!" and then disappears! But its exactly what you want for stick welding (electrode). But for AC there is no real problem, so long its really AC.https://weldingpros.net/dcen-and-dcep-in-tig-welding/

author: rstofer
author_rank: Super Contributor
timestamp: September 10, 2023, 07:55:17 pm
content: There are DC rated switches and Google can find them. Like:https://www.leviton.com/en/products/56080-2wThat particular switch is rated for 3A at 24VDC and I imagine there are many others. You gave voltage (12V) but you didn't give current.It may be easier to find a suitable relay with an AC coil and then just control the relay with an AC toggle switch. You'll have to use Google a lot...Here is a relay rated 10A at 28VDC but it has a 24VDC coil. There are others, search around.https://www.grainger.com/product/DAYTON-General-Purpose-Relay-Socket-1A485

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 10, 2023, 08:05:38 pm
content: I think the old really clicky ones were AC/DC rated (to full voltage) to accommodate legacy DC installations, IDK if the switches actually changed or just dropped the marking off the heavy duty industrial/commercial switches that still have a similar feel.

author: Clear as mud
author_rank: Regular Contributor
timestamp: September 10, 2023, 10:08:16 pm
content: Quote from: CaptDon on September 10, 2023, 02:10:31 pm(The below all applies to household light switches in common use in the US and Canada during the last 70 years or so.)There were some sold as "silent" switches. They contained mercury, but I doubt they're sold anymore because there are more restrictions on the use of mercury nowadays. They had a totally different feel - a smooth motion all the way between on and off without any detent or preference for being in a particular position.The "quiet" switches are the ones we regularly use now. They don't contain any mercury, and they're normally rated for AC only.As someone else mentioned, there were others traditionally used that were AC and DC rated at the same voltage and current, but those were not quiet or silent. They were known as "snap" switches. I have four of them installed in the house I live in, which was built in 1978. The builder probably used them because they were on clearance at the time.Here's what I want to know:What is theminimumDC current level that can be used with a standard "quiet" household switch, for reliable operation? I asked this here before, but I don't know that I got a definitive answer.