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 Post subject: AC or DC rating for Relays
 Post Posted: Wed Jul 15, 2009 7:49 pm 
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Joined: Sat Apr 18, 2009 9:33 pm
Posts: 40
Location: Cal Poly San Luis Obispo
I am currently looking for a relay that can handle high DC voltage but only needs to handle small amounts of current (i.e., a Tyco EV200 relay is over-kill). However, I'm having a hard time finding a low-current relay with a DC contact rating, most have an AC contact rating such as this one: http://www.skycraftsurplus.com/index.asp?PageAction=VIEWPROD&ProdID=1803.

I am wondering if the DC and AC ratings for relays are typically different, such as they are for fuses. Is it safe to apply 116VDC to a relay rated for 240 VAC and no DC rating? If not, what are some suggestions for relays with contacts that can handle 116VDC at 10-20 amps?

-CPsparky


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 Post Posted: Wed Jul 15, 2009 8:42 pm 
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Joined: Tue Oct 30, 2007 6:50 pm
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Location: Illinois Tech
Try the LEV100 from Tyco. It costs about $100 and it is rated at the voltage that you need but it is still an overkill for the current rating that you are looking for.


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 Post Posted: Thu Jul 16, 2009 12:17 pm 
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Joined: Sat Apr 18, 2009 9:33 pm
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Location: Cal Poly San Luis Obispo
antoant,

That's one alternative, and I actually already know of relays that are around the specs I need, such as the POTTER & BRUMFIELD - PRD-11DH0-12: http://www.newark.com/jsp/search/productdetail.jsp?SKU=18M9286&CMP=AFC-GB100000001 However, for the sake of cost and practicality, it would be better if there was a more affordable solution. The relay and relay socket I mentioned in the first post costs a total of $7.00, this Potter-Brumfield (P&B) relay is $38.

If it is safe to substitute AC ratings for DC ratings for relays, then the 440% extra isn't worthwhile. I only want to use the P&B if necessary, and that's what I'm trying to find out from this post.

Charlie, I suppose this is a question directly for you.


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 Post Posted: Fri Jul 17, 2009 11:49 am 
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Joined: Tue Oct 30, 2007 6:50 pm
Posts: 78
Location: Illinois Tech
CPSparky check your PMs.

As far as safety goes you can be sure that AC and DC ratings cannot be substituted in most cases. The reason you can have much higher AC voltage ratings is the same as in fuses (http://relays.tycoelectronics.com/kilov ... direct.asp). An AC signal will cross zero several times a second and that makes it easier for a relay to interrupt an AC signal than a DC signal of the same magnitude. For an example check the data sheet of the relay you mentioned in your first post. It is rated at 10A 250VAC/30VDC.


Last edited by antoant on Fri Jul 17, 2009 12:20 pm, edited 1 time in total.

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 Post Posted: Fri Jul 17, 2009 11:54 am 
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Joined: Sat Apr 18, 2009 9:33 pm
Posts: 40
Location: Cal Poly San Luis Obispo
Heh, you're right. Didn't think of checking the spec sheet (kinda a no-brainier kind of thing). Thank you for your help.

It looks like the lesson here for any other teams is that you need to look carefully at the voltage ratings of relays (and fuses), and realize that the DC rating can be significantly less than the AC rating. The difference between the AC and DC rating of the relay I referenced earlier was by a factor of 8! So even if the AC rating is well above and beyond the required DC rating, it doesn't assure safe operation.

...but what if fill that relay with sulfur hexafluoride like they do for big substation-sized circuit breakers.... Just kidding :)


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