Using AC relays with DC power

You'll have problems at that voltage/current, especially with an inductive load. What's the nature of the load?

Best regards, Spehro Pefhany

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The load we were using was resistive. The test setup consisted of applying the load at 72Vdc and then opening the relay contacts. Upon turning off the coil voltage, I heard a sustained "sizzling" sound (i'm sure it was the arc) then puffs of smoke coming from the relay. We tried a snubber circuit-- 1uF electrolytic cap (rated at 100Vdc) ins eries with a 4 ohm resistor placed across the contacts and got the same results.

[snip]

Any significant inductance in the DC supply?

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The model you are using has a contact-to-contact breakdown of only

1,000 VAC. Here is perhaps a more suitable type of relay- one that has curves showing the maximum switching capicty (Fig 1)

Note that this '15A' relay has a switching capacity of between 1A and

2A for inductive *or* resistive loads at ~70VDC. There is always some inductance in the wiring, of course.

If you don't need isolation, a MOSFET would be the obvious way to handle this. For example, an IRFB61N15D would not need much of a heat sink.

Best regards, Spehro Pefhany

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That's kind of irrelevant, since you don't need inductance to make a contact arc when switching DC. When the gap goes from zero to not-zero, it starts out small enough that the vacuum between the atoms breaks down. When the contacts are one air molecule apart, they're close enough to break down the air, forming plasma, which continues to conduct until either the gap gets too big to sustain the arc, or something else interrupts the current.

Cheers! Rich

That same one, and some others:

http://205.243.100.155/frames/longarc.htm

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Is there a way to save that clip?

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Or until the voltage across the arc falls below about 22V (the arc sustaining voltage at ntp)?

I have a huge contactor, rated to break 600Vdc at 15A. There is nothing fancy about it, like magnetic blowout. It merely consists of about 2 dozen contacts in series, arranged in a circle, with all contacts simultaneously switched by a central rotary actuator.

I have always assumed that 600/24 is about 25V per contact and the simultaneous switching means that the first contact to open (and arc) only has to withstand the arc for a very short time, perhaps something less than a mS or so.

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That effect is the same for either an AC or DC circuit, with or without series inductance. Its the inductance that provides additional e.m.f. to sustain the arc's duration beyond the point at which a theoretical purely resistive circuit arc would be extinguished.

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You don't _have_ to save it. It is on another URL, called

along with a couple other spectacular arcs!.