Operating a 24VAC coil on a valve or contactor (relay) with a DC coil voltage.

** The design of the iron core is special - to prevent mechanical chatter with 50/60Hz AC current.

What is the downside of operating them on D.C.?

** They may get too hot or not release due to the iorn core magnetising.

** Reads like a pile of crap to me.

YOU need to know if YOUR solenoid will operate with DC.

Only way is to try it out, with a variable voltage PSU.

... Phil

Are you familiar with shaded pole motors? There's a copper clip around part of the core, which attenuates and phase shifts the AC. If the contactor armature were free to rotate (and made of more copper), it would spin.

It's been so long since I wired a panel, I don't remember what the typical specs of those sorts of things are. As Phil said, check the datasheet.

Tim

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Its been ages since I did this. ISTR some wanting less volts dc for the right current, some not, but may be remembering wrong.

NT

Yeah, that article basically says to figure the current needed on AC and then determine the DC voltage where the same current is being drawn. I did that and the coil operates fine, but I don't know what any other implications may be.

It's not a big deal for me to operate them on 24VAC so I will probably do that. I will just use a 24VAC/40VA power supply and generate my 5V and 3.3V with a switcher and an LDO respectively.

A 24 volt AC relay or solenoid will probably fry if run on 24 DC. AC relays draw a lot of current when they are first energized, when the magnetic loop is open. After they close and the mag loop is closed, the inductance increases and the current drops. AC relays are desiged with this change of current in mind. Coil current won't drop with DC drive.

You could use it if you arrange to have full voltage initially, but reduce it after it pulls in. There are several ways to do that.

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These are, I take it, pre-existing in the project and you are doing a new controller. In which case, if you can supply the AC, do so.

Square D is now part of the Schneider Group and I was unable to find he details of that product on their web-site. So, it looks like it is discontinued now. Many of their contactor offering have a selection of coils that can be fitted (230VAC/120VAC/24VAC/24VDC etc).

Others have adequately answered that question and I have nothing to add to their comments. If you decide on DC energisation I would conduct testing on reduced voltage (and hence current) after energising to maintain the ability for it to release when de-energised. This complicates the drive circuitry somewhat though. If you really want just a simple DC drive to the solenoid and contactor then seek out alternative DC operated components.

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There is nothing pre-existing. The valve and contactor (2 versions of the product) are being selected by me. I'd rather use super-standard

24VAC contactors and valves and operate them on 24VAC than spend even $5 more for ones with DC coils.

For devices with AC coils the power supply will be something like this: which will simplify regulatory approval of the system, though this does raise the component cost slightly. A switching regulator with up to a

36VDC input is very cheap, an and LDO will give me 3.3V from that.

That sounds pretty much correct. You want to match the RMS current it needs to the DC current you will supply. Make them the same and the temperature rise of the coil will be within specifications. If it works, you are good to go. However, you might need to estimate the contact force in both situations. I don't know how to tell you to do that because I've never gone that far with my experiments. Good luck.

This is something that just isn't worth it. I wanted a 5VDC/2.1A power supply, since these are only a few dollars (since they are used for tablet charging), but there are two many unknowns to risk it. Also, these coils take a lot of current so a boost regulator to get sufficient voltage to operate the coil would add some cost and complexity.

The 24VAC supply is only around $6 in small quantities and I'm sure that if I went to the manufacturer in China it would be $2-3, if the quantities I need ever reach that sort of volume.

Won't work! The scheme is that when the armature is out of the coil, the inductance is low, high current flows, pulls the armature into the coil, then the inductance is raised a LOT, and the current draw drops greatly. The current is enough to hold the armature in place. If you run it on 24 V DC, or run it on AC without the armature being able to pull in (coil removed from valve, armature jammed) the coil will burn up. It may take a couple hours, but it will happen for sure. Don't ask how I know this.

The onyl way to do this on DC is to have a circuit that reduces the current once the armature has pulled in. There's no way to do this with a simple circuit that guarantees the plunger pulls in first, but some experimentation ought to get you there with reasonable repliability. What you need is a BIG electrolytic capacitor and a series resistor. Put this in series with the coil. When the cap is discharged, and the circuit connected to 24 V DC, the capacitor applies 24 V DC to the coil for a moment. As the capacitor charges, the coil voltage (and thus, current) is reduced. Adjust the capacitor value until the the armature is solidly pulled into the coil. adjust the resistor value until the holding current is about twice the minimum needed to hold the valve open. That should work with pretty good reliability.

Jon

Oh, on a contactor with a NC contact, you can rig it so the coil gets full voltage until the armature pulls in, then the contact opens to cut in a dropping resistor in series. In the worst case, the thing will start cycling, but if the values are right, it ought to work. the R-C trick is for solenoid valves where you have no contact to detect the pulled-in state.

Jon

Just saw that I screwed up the contactor part number, it's a Square D

8910DP32V14 . This is sufficient for the electric version of the product I'm designing.

Linear motors may or may not have a conversion to DC. Conversion only works if the motor locks into position rather than needing adaptive strength based on inductance provided by the core.

You can take a look at old pinball machine schematics for a zillion ways to use a linear motor. The flippers use a parallel RC pulse circuit because their cores strongly magnetically bind, metal to metal, with the coil's case. Large solenoids that pull with no specific stopping point use AC so that the current drawn is proportional to the core's position.

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I have done this in a test set I designed, which draws high current surges from the mains, and thus an AC supply derived from it can sag enough for the AC contactors to drop out, which can have bad consequences for the contacts when they start to open at maximum current. Actually, you need a capacitor and resistor in parallel, feeding the coil. The time constant of the capacitor and the DC resistance of the coil is chosen to exceed the operating time of the relay, typically 30 mSec, and the resistor such that it supplies the rated holding current.

Recently I redesigned the boards to use a MOSFET driving the coil, and controlled by a PIC12F1822. It applies full voltage for about 100 mSec and then uses PWM to reduce the current. This works pretty well, but I had some problems with the MOSFETs shorting out after a few operations. I used a self-protected MOSFET designed for inductive loads and that worked better, plus I added a capacitor across the coil and a TVS diode across the MOSFET.

It's also possible that the shorted turn on the coil to keep it from buzzing on AC, may cause a very high current when the PWM (2-10 kHz) is applied. It was difficult to get good scope readings because of noise spikes, which may indicate high current surges, but the circuit seems to work reliably.

A similar PWM circuit appears to be used in some recent ABB contactors, so they will work on DC without drawing excess current and not requiring an "economizer resistor" as some need. This is attached across a pair of special delayed opening NC contacts which do not open until the armature has pulled most of the way into the coil. You can't use ordinary NC contacts because they open too quickly.

Paul