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 A.C. Relays on D.C.

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Jack



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Join date : 2010-10-13

PostSubject: A.C. Relays on D.C.   Wed Oct 13, 2010 7:15 pm

Years ago I read an article about how to convert a.c. operated relays to d.c. It was a simple process, but I don't remember it and can't find the article anywhere. I thought it was in QST, but a search of the archives here and on line at ARRL turns up nada. Does anyone here know how to do it or know the publication date of the article? Thanks.
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radiotechnician



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PostSubject: Re: A.C. Relays on D.C.   Thu Oct 14, 2010 9:19 am



The issue has to be prefaced--What Relay ?


An example--the Potter and Brumfield plug in AC relays with octal bases.

To operate a 110 volt 60Hz relay on DC you have to determine DC voltages for reliable pull-in without heating ofd coil.The relays have shading rings, solid copper top slug which may slow release time .

DC soak may cause stiction sometimes. Some long throw DC relays pull in
with fewer turns than they hold with. They have a last-to-open contact shunting
part of the coil.

Always of course, operating anything on other than its ratings-frequency and volltage
is inviting a host of problems.


Years ago, we tried to operate a large lighting contactor on DC because it was making too much noise (it was inside a sound studio) . The outcome was not stellar.



de
VE7ASO
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Reece

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PostSubject: Re: A.C. Relays on D.C.   Thu Oct 14, 2010 10:00 am

What about the other way around? I have a bunch of 24VDC "ice cube" type relays that I'd like to use on 24VAC. They just buzz if hooked up direct.
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Jack



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PostSubject: Re: A.C. Relays on D.C.   Thu Oct 14, 2010 10:04 am

The article covered relay solenoids in general, but did focus on relays that had the DowKey form factor, sort of.

I believe it also covered converting DC to AC operation as well.
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burnt fingers



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Location : New Hampshire

PostSubject: Re: A.C. Relays on D.C.   Thu Oct 14, 2010 11:00 am

Many of the smaller open frame AC relays work fine on DC at 25-35% of the voltage. Each one takes a few minutes with a variable DC supply to find the sweet spot unless you want to do it the hard way with measuring and computing.

Ive also had 24VAC 10-15A contact style P&B quit working after 12+ years of either RF or near lightning strikes. Dont know which but they were all up on the towers and used to switch phasing and selection of of various yagi stacks. They now work fine on 24VDC.

Carl
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Dean Huster



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PostSubject: Re: A.C. Relays on D.C.   Thu Oct 14, 2010 5:33 pm

DC relays can't be used on AC. While AC relays have laminated cores just like transformers, DC relays (and solenoid plungers) do not and eddy currents will be rampant.

One of the other considerations is the contact ratings. Remember that a set of contacts rated for 10A at 120vac may only be rated for 1a at 24vdc, if that. Contact arcingwith DC becomes a big problem with AC-rated contacts.
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Dave Wise



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PostSubject: Re: A.C. Relays on D.C.   Mon Oct 18, 2010 2:49 pm

Quote :
DC relays can't be used on AC

But it's a simple matter to rectify the AC and filter the result.
You'll still have to figure out the voltage.
Drop-out will be slower than before. In many applications this is not important.

Dave Wise
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Phil W

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Location : West Bloomfield, MI

PostSubject: Re: A.C. Relays on D.C.   Mon Oct 18, 2010 3:09 pm

About 30 years ago, I had an old furnace that had a 24 VAC plug-in relay. The relay died after a good 20 years of service and I happened to have a couple P&B 24VDC relays of the same style (8-pin octal) and contact rating. I opened the relay and stuck in 4 1N4004 diodes as a bridge rectifier accross the coil and got another 7 years of service out of the furnace before I replaced it. The old low voltage control circuits weren't too fussy about things like filtering.

Going the other way, using an AC relay in a DC circiuit, is something I haven't tried. I'd be worried because the design of the coil is so different.
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Alan Douglas



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PostSubject: Re: A.C. Relays on D.C.   Mon Oct 18, 2010 7:26 pm

Quote :
Quote:
DC relays can't be used on AC


But it's a simple matter to rectify the AC and filter the result.
You'll still have to figure out the voltage.
Drop-out will be slower than before. In many applications this is not important.

You can try a series diode, and another diode across the coil with reverse polarity to provide the "filtering" (it caries the coil current during the "off" half of the AC cycle).
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Dave Wise



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PostSubject: Re: A.C. Relays on D.C.   Tue Oct 19, 2010 2:32 pm

That's a neat idea, Alan. I'll try it sometime.

Dave
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