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Join Date: Dec 2006
Location: Greenville Texas
Posts: 7

AC relays versus DC relays

05/19/2007 9:54 PM

I asked before but no one responded, so I will ask again. How does a DC relay work? DC sees a coil as a short so how come it doesn't in a DC relay? Can AC and DC relays be interchanged? If they can't be interchanged, why not? I understand Ohm's Law very well so I can follow explanations.

Thanks,

Roy

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Roy Daniel
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Anonymous Poster
#1

Re: AC relays versus DC relays

05/19/2007 11:04 PM

The coil in a DC relay is wound with wire that has some self-resistance, so there isn't a short circuit presented to the supply. Only sufficient current flows so as to create the ampere-turns that generate sufficient magnetic field to pull the armature into the 'energised' position.

Often, a DC relay will have a 'flywheel rectifier' [FR] placed reverse-biased across the input terminals, so as to prevent a high back-EMF as the magnetic field collapses upon de-energising. Mischief in schools can centre around an unsuspecting person being asked to touch a relay without an FR as the supply to it is disconnected!

Plugging a DC relay with a FR into an AC circuit may cause an overload as half the AC cycle could be passing through the FR as a short-circuit. Incorrectly connecting the DC supply will cause a similar effect. Sometimes a second rectifier is installed in the casing of the relay so as to overcome the problem of reverse polarity.

An AC relay will be wound with different wire as the inductance of the wire will offer some limitation to the current that the relay draws when energised. Instead of an FR it may be possible to place a small capacitor or a neon indicator across the terminals of the relay to minimise any back-EMF effects as the magnetic field collapses, should the need arise, if indeed the relay does not already have these components fitted inside its casing.

So, in summary, they are two different animals. While one may get away with interchanging DC and AC relays on occasions, it is a practice that involves risk to the relays themselves and to the equipment being controlled by their contacts, so it is a practice best avoided.

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Member

Join Date: May 2007
Posts: 5
#2

Re: AC relays versus DC relays

05/19/2007 11:57 PM

Hey Roy,

Forgive my somewhat sarcastic reply here.

It's a Saturday Night!

I'm starting to see a pattern of how the lack of old catalogs

and sales literature is leading to having to relearn a lot of stuff

all over again.

And these goofy "Street Driven" forums create

a kind of "duh" as well. (A few out there/here will likely

sense what I'm alluding to.)

The fact that old sales literature, catalogs, etc.

had lots of really useful information in them that came directly

from the manufacturers of the products themselves,

a question like this would have been easily answered if a person

had access to it, and merely knew how to read.

Your question is kind of jagged, in that you repeated the term

"D.C." within your question.

It's been quite a while since I've looked at relay specifications.

A Direct Current Relay typically has more turns of wire

than one rated for Alternating Current.

An Alternating Current Relay will have an additional

loop of copper nestled within its construction called

a "shading coil".

Find a source of "Elements of Electricity",

or some other title flying around out there to understand

the differences between A.C. and D.C.

A.C. and D.C. relays can be interchanged.

Depending on the voltage applied however, you may ruin

one, or the other.

Use an A.C. relay rated for the application.

Use a D.C. relay rated for the application.

And their life will be just fine.

Whew!

John

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Anonymous Poster
#3

Re: AC relays versus DC relays

05/20/2007 6:36 AM

There is a wise saying that states "You can't teach OLD dogs new tricks" or you can't tell them that does not want to know! "Doh Bart get my pills"

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Guru
Engineering Fields - Electrical Engineering - New Member

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#4

Re: AC relays versus DC relays

05/20/2007 5:03 PM

I'll let you google your own answer to this question. Just wanted to say that I was born in Greenville - home of "the Blackest Earth, the Whitest People".

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Power-User

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#5

Re: AC relays versus DC relays

05/20/2007 6:58 PM

"I understand Ohm's Law very well " Actually you should read Maxwell and Faraday's laws. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

A DC coil does not burn up because the electrical energy is converted to magnetic energy. The magnetic energy is being used to do work. The work being done is basically the force required to hold the relay contacts closed against the spring, or whatever mechanical device is used to return the contacts to their de-energized state. If you pry the contacts open while the coil is energized the coil amps will rise causing the coil to burn.

This explanation oversimplifies the process but there is no magic; just conversion and conservation of energy.

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#6
In reply to #5

Re: AC relays versus DC relays

05/21/2007 2:09 AM

For a DC coil the current will not increase when the contacts are prised open, but it will for an AC coil. This is why enonomy resistors and other current reduction schemes are often necessary for DC contactors and not so for AC, without modification a DC coil draws constant current (ignoring the effect of heat on resistance). The reduction in reluctance of the magnetic circuit when the AC contactor closes increases the back EMF reducing the AC current. This cannot happen with DC as there is no changing flux to generate the back EMF, resistance is then the only limit to current

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Power-User

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#8
In reply to #6

Re: AC relays versus DC relays

05/21/2007 8:19 AM

You are right regarding DC coils... I should have been clearer, my mistake. I have seen wood blocks (wedged into contacts) used to disable dc contactors. Not quite equal to today's standard of "lock out tag out"

Thanks for the correction.

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Anonymous Poster
#14
In reply to #5

Re: AC relays versus DC relays

11/19/2009 11:54 AM

Actually, a DC relay does not use inductance to regulate coil current. It is the resistance of the coil wire that dictates coil current. Therefore, opening the contacts of an energized DC relay will have no effect on the coil current and the coil will not burn up.

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#7

Re: AC relays versus DC relays

05/21/2007 3:26 AM

If I understand you right, you want to know which is better to use.

It really depends on you application, AC current is less expensive to come by that Dc current, because you need a rectifying circuit for DC, the simplest rectifier is of course e bridge-rectifying which cost you peanuts, but even peanuts aren't free.

If you doing a car application:
Then you would go for DC because the car already has a DC circuit.

If your doing a household application:
Then you would go for AC, because the house hold circuit is already AC.

If your doing an industrial circuit:
Then 24 VDC is the standard.

As said it depends on your application.

Hope this brought you some clarity.

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#9
In reply to #7

Re: AC relays versus DC relays

05/21/2007 9:37 AM

Bring back Burstein-Applebee

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#10

Re: AC relays versus DC relays

05/21/2007 9:47 AM

In DC relays the resistance of the coil windings limits the current so it will not be a short circuit providing it is used with correct voltage rating.

In AC relays the impedance of the coil limits the current, if AC relay is supplied with a DC voltage of the same value as the stated AC voltage they will probably burn out.

Some AC relays can be used with DC but the voltage must be lowered to avoid burnout.

DC relays are not suitable for use with AC as they lack the copper loops which stabilise the moving parts, to use with AC fit a suitable bridge rectifier and de rate the voltage.

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Anonymous Poster
#11

Re: AC relays versus DC relays

05/21/2007 5:31 PM

If you understand Ohm, good.

Now go on to Farady and Henry,

and stop asking us to do your homework.

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Member

Join Date: Dec 2006
Location: Greenville Texas
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#12
In reply to #11

Re: AC relays versus DC relays

05/22/2007 7:50 PM

Thanks for your really mind numbing answer. If you don't know the answer there is no shame in saying so. By the way, if questions were not asked then there would be no need for you to be here or indeed, this entire site.

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Anonymous Poster
#13

Re: AC relays versus DC relays

07/10/2007 9:59 PM

Why are so many of your responses so demeaning? I had this question too, and thanks to this forum, i found it. But i DID look on GOOGLE and i couldnt find somethinng that could bring the peices together like this did so be nice!

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Anonymous Poster
#15

Re: AC relays versus DC relays

07/22/2010 8:14 AM

I had d same doubt n i got screwed by my frnd. he explained me but i googled n came across dis thread. It gives g8 explaination n i understood but still i will go in depth.

Thanks all

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Anonymous Poster
#16

Re: AC relays versus DC relays

12/01/2010 6:27 PM

Dc does not see the coil as a short, since there is a designed resistance buit into the DC coil.

You can operate an AC coil with a DC source, but you need to drop the voltage by 25%. You could be able to operate a DC coil with AC source, but it may not pickup.

Hope, I have answered your questions adequatly.

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Guru

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#17

Re: AC relays versus DC relays

06/25/2015 9:21 PM

This was an old thread, and it did had a more than usual bit of nastiness to it. I found the thread because I had a similar question just today.

I was handed a pair of 120Vac input relay which we are using just to block a signal input. We wanted the signal to appear only when two ac lines were active.

I didn't have some loose 120V ac wires to wire up and clip on, so I wondered if my desktop dc supply would suffice. It only goes up to about 18V and 5 amps or so. I was looking to see if that would be enough to just click across the coils to test the relays. (apparently it wasn't). But my question was the same as here: would it work, and would it hurt.

Of course, in a real application, you use the right tool for the right job wired in the correct manner. There's a reason they have different parts with different specs.

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