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I have run into the same problem described here and here, when I added diode to electromagnet. Basically, the magnet still holds with about 20kg force several minutes after removing power. The mechanical solution of leaving air gap or adding a "peel-off" spring is not plausible due to design constraints. So, I need a snubber circuit that will protect relay contacts why still de-energize the magnet rapidly.

While looking for the solution I've stumbled upon this answer, listing many possible methods. Not having any experience with powerful electromagnets I can't decide which one would be the best for this particular setup (24V 14W magnet, 30V 10A SRD-05VDC-SL-C relay).

At this moment I am leaning towards either rectifier + zener or RC + MOV combination. Any advice, please?

UPDATE

I've confirmed that the problem is residual magnetization, not back EMF, just as @DKNguyen and others suggested. After disconnecting power I also disconnected diode and the magnet was still holding. Apparently the initial experiment without diode was done with heavy (~200kg) load that was enough to hide the magnetization.

So, I am going with RC snubber, hoping that oscillations will provide sufficient demagnetizing effect. Maybe combined with MOV if initial amplitude is too high.

Maple
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  • The Amazon links don't really work. Please link to manufacturer datasheets directly. – Justme Apr 30 '22 at 15:45
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    No, the problem is problem is residual magnetism, nor snubbing. The collapse of the current in the electromagnet actually happens fastest with NO snubber. You need to temporarily reverse the current through the electromagnet to a lesser amount than what you used to magnetize. Not too much though) to reset the magnetic domains. – DKNguyen Apr 30 '22 at 15:46
  • The SRD-05VDC is a small contact relay, not an electromagnet. Something I didnt catch here. Are you trying to drive a big electromagnet with an SRD relay? is that the idea? – Fred Cailloux Apr 30 '22 at 15:47
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    Unless the problem you really mean to say is that your relay is sticking when you try to open it so the electromagnet remains energized which is a different problem altogether from residual magnetism. But if that was the problem I would have expected you to word your question differently. – DKNguyen Apr 30 '22 at 15:49
  • @Justme I've replaced amazon link to relay with link to datasheet. The link to RC snubber module was only for illustration. There is no schematics on the web but it uses 220Ohm 0.1uF RC and [10D471K](https://www.bourns.com/docs/Product-Datasheets/MOV10D.pdf) varistor – Maple Apr 30 '22 at 15:51
  • @FredCailloux yes, that is exactly what I am doing. – Maple Apr 30 '22 at 15:54
  • 1. Please phrase your question that it can be understood without following any links. Add diagrams as necessary. 2. Back-EMF diodes will delay the field collapse by some milliseconds, not seconds. If it's a long term problem then it's an iron magnetization problem. This is also solvable but you need to ask a different question. – tomnexus Apr 30 '22 at 15:56
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    I don't know how to word the question differently. I did say that I added diode to electromagnet to protect relay contacts. So, there are just three components in discussion - relay switching the power to electromagnet and a diode to protect relay contacts from arcing. – Maple Apr 30 '22 at 15:57
  • Electromagnet data sheet quote: *When turned off, the magnet will still have some residual magnetism.* – Andy aka Apr 30 '22 at 15:59
  • If you've ever taken a look at an electromagnetic chuck in a machine shop which is basically just a flat steel table that is an electromagnet so you can stick your steel workpiece to it without clamping jaws, they have three positions for power: Energize-Off-Demagnetize, specifically because of residual magnetism if you just turn the chuck off and try to remove the workpiece. This is even an issue on non-electromagnets where you release the magnet by rotating it so the poles align with a steel loop so the flux lines don't enter the workpiece. It sticks even in this case. Not a snubber issue. – DKNguyen Apr 30 '22 at 16:01
  • The comments by DKNguyen and tomnexus imply the problem is with residual magnetism, not back EMF. And I would agree with this in a second, if not for the small fact - when we did ad-hoc testing of the setup by connecting electromagnet to the battery directly it would release the load immediately when power disconnected. The sparks would fly, of course, hence the diode addition. – Maple Apr 30 '22 at 16:02
  • @Andyaka yes, I saw that small print in the description and was really concerned. Is that common for all electromagnets or this one is especially bad quality? – Maple Apr 30 '22 at 16:04
  • @Maple So you are saying that your electromagnet remains energized even when you try and open the relay? Because that's a blatant thing that was never actually mentioned in your post. But instead in your post you mentioned "airgaps" and "peel-off springs" which is definitely *not* the solution to an electromagnet that remains energized but are solutions to residual magnetism. Mixed signals are being sent. Measure current when you turn the electromagnet off to verify sticking contacts. It is strange that a current measurement with a supposedly open relay was not done before the battery test – DKNguyen Apr 30 '22 at 16:11
  • @DKNguyen that thing is actually mentioned in the second sentence of the question. I will try to measure current when I am back to work on Monday and post it right away. – Maple Apr 30 '22 at 16:14
  • Thank you for the clarification. If you are referring to "*Basically, the magnet still holds with about 20kg force several minutes after removing power.*" This sentence does not actually say that the magnet still remains *energized* (i.e. having current flowing, being powered). It actually says the opposite: that the magnet still attracts *after you have cut off power*. It does not actually say that the magnet is still powered even after you try removing power. If so, that is the mixed signal along with the mention of airgaps and springs. – DKNguyen Apr 30 '22 at 16:16
  • Also, if you encounter a sticking relay, tap the relay lightly with a stick or rubber mallet to see if the sticking contacts get jolted open. If the magnet releases, that is another way to verify sticking contacts other than current measurement. – DKNguyen Apr 30 '22 at 16:22
  • @DKNguyen I've mentioned air gaps and springs because those were proposed solutions to the [same problem](https://electronics.stackexchange.com/questions/259407/how-to-stop-an-electro-magnet-from-sticking) the link to which I also posted. Now, I frankly admitted that __I have no idea what I am dealing with here__, residual magnetism or back EMF. All I know is that with direct connection to battery the magnet will release immediately with a lot of sparks. With diode added across the coil there are no sparks but it will stick for minutes, requiring really strong force to peel the plate off it. – Maple Apr 30 '22 at 16:23
  • @Maple Fair enough if you have no idea. Just know that "removing power" in this context is probably going to be interpreted as the relay primary contacts opening rather than just removing power from the relay coil. Note that if the problem is relay contacts sticking, that particular individual relay is probably forever prone to sticking and that simply adding a snubber may not improve the situation without changing the relay for a fresh one. The sticking is caused by arc corrosion make the contacts very high friction which remains even after you add a snubber. – DKNguyen Apr 30 '22 at 16:25
  • @DKNguyen I can confirm right away that relay does not stick. That was my first guess too, so I simply disconnected magnet powering battery from relay contacts and nothing happened, the magnet was still holding. I guess I should have tried disconnecting the magnet from diode too, but I did not at that time. I will do it on Monday – Maple Apr 30 '22 at 17:25

2 Answers2

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This is not a simple Inductor with 100 Henries and 40 Ohms DCR to yield 24V^2/40ohm =Pd=~14 W because that (assuming huge L ) has a time constant of L/R= 2.5 seconds which does not account for your time measurements of several minutes after removing power.

The diode resistance is negligible compared to the DCR which limits power in the coil during Power On or time constant switch release.

So what causes the time delay?

Core Remanence will be huge leaving it almost permanently magnetized until the charge decays. i.e a semi-permanent lossy electromagnet.

How can you cancel Remanence?

Apply and opposite energy or equal power x time of 14W for stored energy after L/R seconds stored in E=0.5LI^2 ( L is unknown ).

What voltage and duration is that? (Hypothetical answer)

Without L value and remanence values, it will be trial and error but could be -24V for a few seconds on the order of L/R =Tau seconds, or perhaps a bit less time than it takes to activate the electromagnet to full current.

Tip: Measuring the magnetic force with some sensor helps to determine this and expect thermal variations in results.

How can you realize this?

With a timed "full-bridge" and bipolar diode snubbers.

schematic

simulate this circuit – Schematic created using CircuitLab

How do you design this more elegantly?

That's up to you. A one-shot timer switch springs to mind. Small RC snubbers can assist if the switch is slower than the ionization-time of an arc in microseconds)

Tony Stewart EE75
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Snubbing is not a problem at all.

The core is magnetized. To turn such an electromagnet fully off, you need to apply a demagnetizing waveform.

Typically, you’d attach a capacitor in parallel with the coil, and then disconnect the power source, eg. using a small relay, vs. a mosfet or a transistor that would go into reverse conduction.

The current will decay as a sinusoidal waveform with exponential envelope. It will demagnetize the electromagnet. That’s how it’s typically done, and it’s very simple.

Kuba hasn't forgotten Monica
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    exactly. But please expand the cap selection because you can easily blow up the cap. It has to store the full inductive energy. and it shouldn't be too large in C in order to have reasonable high Q (actually oscillate a.few times) I.e. a smallish cap with a high voltage rating – tobalt May 01 '22 at 05:48
  • I have mentioned RC + MOV combination as one of the options I was considering. I hoped that LC will oscillate and demagnetize itself, while being much simpler than externally supplying reverse power as @Tony suggested in his answer. Will the RC+MOV combination behave similar to the capacitor alone? My understanding is that it will have a) limited voltage swing due to MOV and b) faster decay due to R, but otherwise it should oscillate just the same as capacitor in your suggestion. – Maple May 01 '22 at 16:57
  • I'm not sure what sort of decay times you had in mind, but we're talking seconds, and the MOV is not necessary at all. You may have a resistance in series with the capacitor to lower the quality of the tank and make it decay faster. Too fast will not demagnetize as well as it could. Design for oscillation frequency of a kHz or so, and then the decay time constant can be 0.1 to 1s. That's plenty fast for this application I'm sure. Another option is to use an electromagnet with a core that has no remanence. – Kuba hasn't forgotten Monica May 02 '22 at 11:35