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toggle switch VA ratings, and On-On-On

A project log for today's assorted project ramble "grab-bag"

Assorted project-ideas/brainstorms/achievements, etc. Likely to contain thoughts that'd be better-organized into other project-pages

eric-hertzEric Hertz 10/25/2019 at 19:223 Comments

Have an interesting toggle-switch... pretty high quality, made in the USA, wound-up in my collection probably more than 20 years ago [I'm pretty sure I remember the day/person], and... it's still available new at regular suppliers. [This is the kinda quality design we're talking about]. 

C & K 7211.

Two odd things I never noticed about it in my previous uses, guess it didn't matter there:

A) On-On-On (DPDT, three positions, so what's with three Ons?!)

B) [Relevant to current project] "0.4VA MAX"

First-off, if VA means what I think it means, then this switch is no-go for my project. [And sure-enough, it would seem that way].

I had a bit of difficulty finding a decent explanation of this rating, so here's the best reference I found:

http://www.aeroelectric.com/articles/Switch_Ratings.pdf

Briefly: there are two common contact platings, gold and silver. Gold's used for low-power [logic], silver for high-power. 

Silver oxidizes, so arcs caused by high-power switching burns off that oxidation. It's by-design! So silver-contacts aren't so great for low-power/non-arcing switching. 

Meanwhile, such arcing would burn off the gold-plating on low-power contacts, and you'd be left with tin or copper [or something] contacts which make poorer contact than silver, and also oxidize and such.

So there you have it.

And, from the sounds of things, 0.4 VA MAX at <28V [20V AC or DC, per my datasheet] is pretty much standard for gold contacts...

And, yes, that does seem to be a measly 20mA at 20V = 0.4V*A = 0.4W. But it's a little more complicated than Watts, what with AC, RMS, peaks, etc. Or with DC and inductive loads [motors, or even long wires] and Capacitive loads [with large inrush currents, *during* switch-bounce!].

So, I guess the answer for my project is that, sure, this switch, if it had silver contacts, would handle 5A @250V [AC]... I thought it seemed pretty heavy-duty... And it'd probably do the job of switching 40VDC unlikely-max 3A [most-often a fraction] for a while even with bare copper or tin.. but probably not an ideal solution.

BTW look into that document, also, for info on derating for DC circuits [no zero-crossing = longer arcs] it's something like x Amps @ 120VAC -> x Amps @ 14VDC!

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Oh, and finally, On-On-On, that's also nicely-covered in there. Basically it's like a typical three-position DPDT switch, but the middle position puts it in a weird-to-me state where the two poles are in opposite positions [What?!].

I once used these guys to drive/reverse the motors in a wired toy-car [hahaha 0.4VA my a**]. It worked... but now I see the middle-on was actually *braking* the motor [shorting its winding via one of the power rails]. Hadn't noticed. But, could be handy.

And, finally, they can be wired as a SP3T switch... cool!

which, also, makes for Off-On-On as a possibility [e.g. Off, Slow, Fast, or Off, Dim, Bright]

Whoda-thunk? Certainly wouldn'ta thought to look for such a switch, expecting it to be somewhat common!

Discussions

Ken Yap wrote 10/25/2019 at 21:48 point

Mouser has the datasheet which has an explanation of the on on on showing which terminals are connected in each position: https://mouser.com/datasheet/2/60/7000toggle-1315363.pdf

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Eric Hertz wrote 10/25/2019 at 23:01 point

indeed! Thanks for looking that up.

Imagine an electronic component still manufactured 20+ years!

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Ken Yap wrote 10/25/2019 at 23:04 point

First page of search results. No suprise some components are in production for a long time. If there's a market for it and it addresses the requirements, why not?

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