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Old 8th April 2009, 10:17 PM   #1
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Default Contact Rating: AC and DC

So, this is a pretty basic question. I notice on switches and relays I often see contact rating in terms of AC voltage and current. I'm wanting to control a B+ 400VDC-450VDC @ 100mA rail with a relay to allow for controlled heater warm-up. Is a 220VAC 20A mechanical relay suitable for this?
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Old 9th April 2009, 12:33 PM   #2
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Short answer, no.

The main problem here is not the current you want to put through the relay as a 20A parts should easily be able to handle 100mA DC, the problem is the fact that you'd be running a relay rated at 220V AC (310V peak) in a circuit which has voltages way in excess of this rated value. So, I'd say that a relay with a higer voltage rating would be the first thing to look for such as teh part below (although to be honest this may be slight overkill for what you had in mind)

http://search.digikey.com/scripts/Dk...me=306-1010-ND
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Old 9th April 2009, 01:06 PM   #3
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The problem with switching DC is the arc produced when opening the contacts isn't quenched.
All switches arc when opened, but with AC the arc dies as the AC cycle goes through zero volts, and the arc never re-establishes itself as the contacts are by then open.
With DC the arc is sustained.

The DC/AC voltage difference for ordinary relays is very marked - even a 440V AC 'air contacts' relay can only handle 48V DC

You either need a special contact with arc quenching capabilities, or use a solid state switch (MOSFET).

(If it's only to give a switch-on delay, you could switch the DC with a triac (+ opto-isolater), then it would require no continuous control signal - just one 'on' pulse. It would then hold itself 'on' until the power supply was completely turned off.)
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Old 9th April 2009, 02:14 PM   #4
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A cap across the relay will make an AC relay survive DC for a while.


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Old 9th April 2009, 02:19 PM   #5
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Question:

Why don't you just switch the AC side, before the rectifier? That's what I usually do when dealing with DC. Switching DC is a PITA.


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Old 9th April 2009, 04:04 PM   #6
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Magura:

Well, I'd like to have a controlled warm-up for tube heaters and figured this would be one way to do this. I suppose switching the AC side between the toroid and rectifiers would work as well. Here I have 350 VAC, so I'm back to finding a relay that works here, as most I see are rated to 220 VAC. Ideally, I'd also have some way to deal with inrush from the toroid and capacitors after the rectifiers as well, as that's proving to be annoying.

With regard to the DC switching that Steerpike suggested, what about this device:

http://www.clare.com/home/pdfs.nsf/www/CPC1786.pdf/$file/CPC1786.pdf
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Old 9th April 2009, 04:17 PM   #7
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There are plenty of 400VAC relays out there. To deal with the inrush, a thermistor comes to mind.


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Old 9th April 2009, 04:21 PM   #8
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Omron G2R-1T takes 380VAC.


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Old 9th April 2009, 04:29 PM   #9
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Quote:
Originally posted by Magura
There are plenty of 400VAC relays out there. To deal with the inrush, a thermistor comes to mind.


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thanks, I suppose these are one option, assuming I don't have to worry about too much inrush current here... probably > 4A is a good idea, huh..

http://www.teledynerelays.com/pdf/industrial/as4.pdf
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Old 9th April 2009, 07:09 PM   #10
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Quote:
Originally posted by luvdunhill


thanks, I suppose these are one option, assuming I don't have to worry about too much inrush current here... probably > 4A is a good idea, huh..

http://www.teledynerelays.com/pdf/industrial/as4.pdf

Those would be fine.



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