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Anonymous Poster

# Would This Relay Work in a DC Circuit?

12/13/2009 7:28 PM

Hey everyone,

Basically i need 6 dpdt relays in a 12VDC circuit. They are controlling Bilge pump motors that pull about 4-5 amps each. In New Zealand it seems that these are really expensive but this one here thats fairly cheap:

And i think this is the data sheet for it?

http://parts.digikey.com/1/parts/indexd12994.html

anyway i remember reading somewhere that AC relays work with DC current, as long as the DC voltage is about a fifth as much as the AC rating. Is this true? would that relay work in my circuit? thanks heaps.

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Guru

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#1

### Re: Would This Relay work in a DC circuit?

12/13/2009 8:48 PM

Yes in most applications an AC rated relay can carry it same amps ratings but at around 1/5 the voltage.

Typical relay ratings you may see on a single relay can be like this.

10 amps @120 VAC, 5 amps @240VAC, 10 amps @24 VDC. Non inductive load.

If you have a simple snubber diode across the load and a simple snubber capacitor across the relay contacts they will most likely work fine in a DC circuit up to their rated amps and volts limits.

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#2

### Re: Would This Relay work in a DC circuit?

12/13/2009 9:17 PM

When a contactor (e.g., relay or motor starter) breaks the circuit, current may continue to jump as an arc between the separating contacts. In the case of AC, the voltage transits through zero within 1/2 cycle, extinguishing the arc. In the case of DC, the full voltage continues to be applied; thus it typically takes faster/farther separation to extinguish the arc. (Think of DC arc welding, in which an arc can be struck and then extended to a sizable fraction of an inch or more.) As a result, DC contact designs and ratings differ from AC.

The 1/5 voltage idea seems to be a common rule of thumb. When you test your circuit, the main thing to watch will be that the contacts actually open the circuit successfully. Look for excessive or prolonged sparking, for instance.

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Anonymous Poster
#3

### Re: Would This Relay work in a DC circuit?

12/13/2009 9:42 PM

Ok, guess I'll try these. Thanks for your help Guys.

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#4

### Re: Would This Relay work in a DC circuit?

12/13/2009 11:40 PM

Why would you need a double throw double pole relay to control Bilge pump motors? When you say using ac relays, does that mean you will be using dc for the operating coils? Are the coils rated for 12vdc?

If it's double throw and ac, it must have two coils in there, one for each throw. The relays you are looking at don't come with the bases, mounting and wiring will not be easy.

It's just a bilge pump. Use a horn relay from a car.

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mike k
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#5

### Re: Would This Relay work in a DC circuit?

12/14/2009 12:01 AM

check here Allelectronics. Kest #KRLY-2012

Another Allalectronics. 12v DPDT 10A

or from Galco. You might want to consider socket mounted relays w/clips and Options like Check Button and Indicator Light.

To answer your question on de-rating ac relays; a low voltage high current dc can arc across the contacts of a relay where in ac as the voltage swings through the zero crossing it self-ignite the arc. As to contact points size,gap spacing, speed of opening (return springs) and other specs. can determine if a relay would be suitable for use.

Now with what your controlling bilge pumps on a boat, do you really want the cheapest relay? The environment has a lot of vibrations,damp(?), no quick stop at the store for a replacement when your on the water. Also for the life of the relay I would consider over-sizing/rating of the current rating of the load being controlled.

Last make sure the relay coil voltage is appropriate 12vdc, 120vac etc. etc.because there are low voltage controls relays that switch high voltages and high voltages controls that switch low voltages.

They also make SSR.s Solid State Relays Omron G3NA-D210B-DC5-24. Newark. Allied electronics, Digikey I know they are expensive and these are spst but if you could use something like this they have vibration and shock ratings that good and being electronic versus mechanical they should withstand the environment better. But not knowing how your system is wired I can not say it would be worth the effort or expense to rewire too be able to use them. That you would have to figure out.

Charles

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#6

### Re: Would This Relay work in a DC circuit?

12/14/2009 3:56 AM

Is there nothing else at the marine chandlery retail outlet that would do the job?

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#7

### Re: Would This Relay Work in a DC Circuit?

12/15/2009 6:29 AM

Well i have designed a cct having AC and DC.In the cct,i energised the DC relay with 12v DC voltage and passed AC current thru the contacts of the relay and the cct perform very well and could also be done vice-versa.The cct is still working till date.

Only makesure,yr relay contacts is able to withstand the current passing thru it.Use a quality rugged relay.At times u need to do a simple test and apply a load and see the performance.

Patrick Whowha

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Patrick Whowha
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#8

### Re: Would This Relay Work in a DC Circuit?

12/15/2009 7:14 AM

Regards.

AC relays [Coil wise not contacts rating wise] cannot work on DC as the design of both have quite different considerations; like Shading of coil-lamination, air-space between pole & operating-reed & more... Read from web about it.

As far as the contacts are concerned it is clearly marked on Relays & in data sheets.

DC ratings are far less than AC ratings due to continuity of arc after the break of contact.

Have a fine day.

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#9

### Re: Would This Relay Work in a DC Circuit?

12/15/2009 8:09 AM

Regards.

I lways refer to the links given at the end f page:

You might be interested in: Electromechanical Relays, Safety Relays and Control Modules, Reed Relays Almost these links have quite a good info of the subject under discussion. Have a look

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#10

### Re: Would This Relay Work in a DC Circuit?

12/15/2009 10:21 AM

If this relay is to be used in a 'reistered' or 'licensed' marine vessel, the New Zealand authorities (like U.S. Coast Guard) might REQUIRE a 'certified, or 'approved' device. Not all relays are suitable for marine environments. Cheap relays are often not manufactured for the high reliability and long operational life required for commercial service.

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#11

### Re: Would This Relay Work in a DC Circuit?

12/15/2009 1:13 PM

As you are using 12Vdc we can reasonably assume that the boat you are using these on is less than 50ft (15m). The relay you have linked to is a 240V coil. This cannot be use with a dc control voltage and you should not consider any "tricks" to get them to work. As Keith has said, you are on the water and, for this application especially, you need reliability.

The best choice is a relay normally used for car headlights etc from Bosch or Hella.

Look at the Hella iso-mini. They are cheap, robust, and very readily available. Just about every manufacturer who makes relays for the auto trade makes an equivalent.

Since you have asked this question you are clearly not working in this field. there is a real safety issue here. I suggest you get a marine electrician to wire this for you. If you can't afford that then you need to invest in a book (Nigel Calders is good) and some time to learn the info therein.

Regards

Chas

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#12

### Re: Would This Relay Work in a DC Circuit?

12/19/2009 3:46 PM

I am michele moscati

ramos srl

www.ramos.it

specialize in relays

do you use many relays

contact me at mmoscati@iol.it

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#13

### Re: Would This Relay Work in a DC Circuit?

12/19/2009 4:35 PM

Please see the difference of AC DC ratings of relay contacts above Green markings

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