AC and DC switches are sometimes different!

AC rated switches sometimes do not have enough separation to interrupt the arc formed from a DC load. As contacts separate, an arc is formed until the dielectric strength of the air increases to a point that extinguishes it. With AC, the current passes through zero 120 times per second, which aids in extinguishing the arc quickly. With DC, it never passes through zero, so only the distance is used to extinguish it. If the distance is designed around switching of AC only, it may have almost no capacity to switch DC of any appreciable amount of current. As an example, I have used for AC contactors rated for 500A at 600VAC, but they are rated for only 0.4A at 24VDC. There is no guarantee that the switch you are using will function for very long.

What I was wondering is if an AC switch could be safely used for DC if you put a cap across the terminals, hardware debounce style?

It would be a DIY use the parts you got work-around. And I do have some nice big rocker switches...

If your rocker switch is big, then it probably has enough separation to stop a DC arc.

I don't think that a cap would help. When the arc is maintained it should appear as a resistance instead of an open circuit, but that resistance must appear low enough for the arc current to be maintained.

Depends what you are switching on the DC side. If it is a regular load (ICs and things like that or resistive) then there should be no problem. Where you can run into big problems is with inductive loads (think arc welder - they typically have an inductor in series with the electrode that helps sustain the arc. Good for welding, not so good for other things!! )

I have used for AC contactors rated for 500A at 600VAC, but they are rated for only 0.4A at 24VDC.

That’s shocking!!! Are you sure 24VDC/400mA isn’t the coil rating?

DVDdoug:
That's shocking!!! Are you sure 24VDC/400mA isn't the coil rating?

That has to be the coil rating. Anything rated for 500A at 600VAC can certainly handle a significant DC component.

Oh wow...yeah, good catch.

While it's common to find relays rated for 240VAC/24VDC (like this one), the difference in current is never 1000:1.

I've been doing some research on this relating to relay contacts and I'm quite sure switch contacts would have similar issues. I don't think a capacitor would help in the long run with contact life expectancy. Especially since the capacitor would be charged the same way every time the contacts closed ... this would create a spike and crater condition on the contacts over time. Contact ARC Phenomenon

I suppose if you could alternate the charge on the capacitor + then - each time the switch is operated, then the material transfer would be each way, therefore minimizing any spike and crater condition (just a guess).

Standard RC snubber circuit in fact! R and C in series across the switch. Reduces the voltage that
can produce an arc on opening, limits the current from the cap on closing. Component values always a
compromise since small R large C is good for opening, large R small C for closing (and safety).
Inductance of the load also affects choice of values.

I had first made a guess based on how wide a gap that voltage would jump. My stupid there.
I had forgotten that once you have an arc started the ionized air will support a much longer spark.

The person in question who does his own checking (smart guy) got me that quote and uhhh, duh me.

So the switch may work 100 times or more before it doesn’t seems pretty possible to me but without testing a pile of switches I’m still going on that quote and being told that all switches on his GA plane have to be DC switches by regulation.

But for mundane nothing critical DIY, I’ll try the R-C ‘snubber’ just to keep my 3-way rockers.

I don’t think I’m ever going to become a EE at this rate… have to settle for something simpler like heart surgeon or nuclear physicist.

Electric arcs for fun and learning.

Yeah, you really don't want to create something like This nice arc at a substation :o :o

gpsmikey:
Yeah, you really don't want to create something like This nice arc at a substation :o :o

In the early 1960s I visited Battersea Power Station in the UK. Hanging on the wall were wooden poles with large leather flaps at one end. They were use for extinguishing arcs on knife switches! No health and safety rules then.

Russell.