# DC through 120V AC contactor

Discussion in 'General Electronics Chat' started by Neil Dsouza, Sep 6, 2017.

1. ### Neil Dsouza Thread Starter New Member

Sep 6, 2017
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I am working on a project in which I want to use DC to actuate a 120V AC contactor. I found out that I can not go over 24VDC, to avoid overheating the coil. I have a 12VDC power supply, but that fails to activate the coil of the contactor. Based on some threads, I found that a 14VDC can be used to actuate a 120VAC contactor.
I need help in figuring out the Voltage and Amperes of DC that is required to actuate a 120VAC contactor.
the contactor I am using is a 'JARD 17322' 2Pole 120V AC
Any help would be deeply appreciated

2. ### strantor AAC Fanatic!

Oct 3, 2010
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Sounds like you need a DC contactor

panic mode likes this.
3. ### crutschow Expert

Mar 14, 2008
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To determine what DC voltage it requires, you need to measure the coil resistance with an ohmmeter, and the coil current with an AC ammeter (multimeter) when energized by 120Vac.
Then you multiply this coil current times the coil resistance to get the required nominal DC voltage (which will give the same RMS current).

4. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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Would it not be the average current that is important here?
I don't know but strongly suspect that the magnetic pull would go by average current rather than RMS.

5. ### LesJones Well-Known Member

Jan 8, 2017
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Hi crutschow,
The problem with that method is that you will be measuring the current with the contactor closed. When it is closed there is no gap in the magnetic path so it's inductance will be higher than when it is open. So the equivelent DC current may not be enough to pull it in. If you increase the DC current so that it pulls in then it may overheat as the current will not be reduced when it is closed as it is with AC. I did do a test to prove this theory when a similar question came up on another forum thread. I suppose you could get round this problem by connecting a resistor in parallel with a capacitor in series with the coil so the charging current of the capacitor boosted the initial current when power was apllied.

Les.

Last edited: Sep 6, 2017
6. ### crutschow Expert

Mar 14, 2008
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That's likely true.
I ignored that because it's a small difference and the error causes the calculation of a slightly higher current than needed.

But to account for that, he could reduce the calculated DC voltage by the ratio of RMS to average or about 10%.

Last edited: Sep 6, 2017
7. ### crutschow Expert

Mar 14, 2008
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So what was the result?

8. ### LesJones Well-Known Member

Jan 8, 2017
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There was quite a significant reduction in current when the contactor was closed. I put something into the gap to prevent the contactor closing so I could measure the current when it was open. I can't remember the ratio between the open and closed currents but Ithink it was about a 30% reduction..

Les.

Feb 20, 2016
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One option would be to get a DC coil for the contactor. Most contractors are available with coil options.
Another is to have a large capacitor charged up to supply the closing current and then limit the holding current via a series resistor from the power supply to the cap.
Years ago, I made a simple circuit using a 2 input NAND Schmitt trigger that when operated, applied full power to the coils, then after a short delay, switched to PWM to limit the coil current. This was to give fast operation and no smoke

10. ### crutschow Expert

Mar 14, 2008
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I would think there is no difference in the contactor between an AC and DC coil other than the impedances.
And DC coil contactors don't require a two stage voltage application to energize.
So I would think there is a DC voltage that would properly energize an AC coil contactor without overheating if left energized with that voltage.
Is there a flaw in that reasoning?

Jul 18, 2013
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And contactors too.
But seriously, if you use a popular contactor supplier such as Telemecanique, you can obtain any AC or DC coil you need as a replacement option.
That is a Definate Purpose Contactor, so whatever replacement you get should be the same specifications.
What is the application you are using it in?
Max.

Jul 18, 2013
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Never seen a AC contactor coil that required two stages to energize?
The AC variety has a shading ring of course, which the DC does not need, or use, if fitted..
The OP apparently found out that 12vdc was the highest he could go due to the very low resistance of the AC coil.
Max.

Feb 20, 2016
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Aha! The iPad auto complete strikes again !
Contractors may have coil options too

14. ### tcmtech Distinguished Member

Nov 4, 2013
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Unfortunately they don't work so well on the DC without a bit of a cheat being the energizing voltage that will be high enough to achieve a reliable pull in is just bit too high to keep the coil from overheating in continuous use.

As you saw they have a ~30% difference in energizing VS closed position on AC and you need to do a similar effect with DC as well. The cheat I have used is to feed the coil through a resistor that is in parallel with a large enough capacitor to give it the needed kick to close the contacts then drop the voltage being fed to the coil through the resistor after that so that the coil doesn't overheat.

Unfortunately there is no real universal one ratio fits all for the size of capacitor or resistor or DC voltage any given AC relay or contactor will work with so with every different brand and model of device there is a bit of test and tuning work to find what does the job the best.

15. ### crutschow Expert

Mar 14, 2008
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How is that different from the same contactor that has a coil spec'd for DC.

Jul 18, 2013
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That appears to be a bit of an El-Cheapo contactor for a Special Purpose spec. also they are available with 24vac coil at not that high a cost.
Max.

17. ### ebeowulf17 Distinguished Member

Aug 12, 2014
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I'm probably going to embarrass myself demonstrating my lack of knowledge about coils, but couldn't a coil manufacturer make a DC coil stronger without requiring more current by changing the number of turns... or is it strictly a function of current?

Or, taking another approach, if the DC coil requires enough current to be risky, maybe you design the geometry and the housing of the coil such that it dissipates heat better? So, an AC coil gets a simple build because its current level will automatically reduce when the contactor closes, but the DC coil gets a different design specifically engineered to shed heat fast enough for the amount of sustained current that will run through it.

I don't know (obviously.) I'm just thinking out loud, but it certainly seems like there could be ways to design a coil differently to make it work well with DC current, even when that same amount of sustained current through a typical AC coil would be problematic.

18. ### tcmtech Distinguished Member

Nov 4, 2013
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Of all of the relays and contactors I have ever dissected the differences were most often seen in having slightly different core and coil sizing and designs between the two types.

The AC type have a small single turn shading coil on the center pole of their core (buzz prevention) plus, in many, a different core size and design.

All of the AC type I have ever taken apart used laminated cores while I have found some DC units to either have solid cores or cores with thicker laminations than their AC coil counter parts.

On some of the larger DC contactors they use a dual winding coil where one coil is the continuous duty one that draws a small amount of current to keep the contactor pulled in while heavier second coil does the momentary higher powered pull in work then is taken out of the circuit via a thermistor or similar cutout.
Those tend to be a more difficult design to troubleshoot being they will show continuity and will hold a contactor in if manually closed but won't pull in on their own if the cut out for the higher power momentary pull in coil set goes bad.

Last edited: Sep 7, 2017
19. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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I am no expert on contactor coils but it seems to me there is a difference in the way the current is restricted and heat generated. With a coil designed for AC, the principal restriction on current flow is the inductive impedance which dissipates no power and the resistance of the winding will contribute relatively less. A coil designed for DC uses only the resistance of the winding to restrict the current which does dissipate power.