Product information needed on electromagnets and solenoids

1

Please forgive my ignorance - I am not an electronics enthusiast and I know almost nothing about solid state electronics, so I hope any answer I get will not be too full of jargon; I just want to know which product will best suit my purpose and if I absolutely need any additional equipment to use it. Also, this is my first time on this forum.

Today I ordered from Core Electronics: (1) this 5v electromagnet, (SKU: ADA3872), (220 mA, 2.5 Kg holding force). However, to suspend a load from it, I would need a lightweight hook or eye bolt with a flat, circular, ferrous base, with the hook positioned such that when a load is hung on it, the centre of gravity will be at the centre of the base, whose diameter should be similar to that of the electromagnet. (A magnetic hook is not appropriate for this purpose because you do not put a permanent magnet in close proximity to an electromagnet.)

Because I am not confident of finding such an item, I also ordered: (2) this small push-pull solenoid (SKU: ADA412). After placing the order, I noticed: (3) this small solenoid (SKU: ROB-11015).

The order is due to be shipped tomorrow (Wednesday) afternoon, and I’d like to know what the difference is between product (2) and (3) so that I can vary the order before it is dispatched if it turns out that (3) would better suit my purpose, which is described here.

Whichever one I end up using, I am proposing to power it by hooking it up to this 600 mA, switchable power-supply from Jaycar, which has a 5v setting. I need the elecromagnet to be on the whole time (possibly for 16 hours), supporting the load, until the circuit is broken by the switch, at which time the load should fall. If I use the solenoid, I will suspend the weight (about 15-20 g) by a thread looped over the rod, which would need to be extended the whole time until the circuit is broken, at which time it should retract, thus dropping the load suspended from the rod.

I also need to know if, for this very simple purpose, I can operate the electromagnet or solenoid with just that 600 mA power supply, and no additional electronics in the circuit. The website mentions a power transistor and a diode. Do I really need them, and if so, is there a ready-made (plug-n-play-style) product that I can put into the circuit?

2

Maybe check out these hooks on ebay: https://www.ebay.com.au/itm/15Pc-Strong-Magnetic-Hook-12LB-5-5KG-Rare-Earth-Neodymium-Magnet-Hanger-Holder/163246215919?hash=item26023b92ef:g:NAUAAOSw1SJbk16U

You can definitely use a permanent magnet with an electro magnet - not sure where you got the idea that you can’t.

That said, maybe you’d be better off with a set up that doesn’t need to be powered all the time - ie. it’s powered off, and powering on your mouse trap flicks the switch.

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3

Thanks for the suggestion, Oliver, but you must have missed the part in my question where I wrote, “A magnetic hook is not appropriate for this purpose because you do not put a permanent magnet in close proximity to an electromagnet.” (That goes doubly so for powerful neodymium magnets.) I have been aware for years that that is bad for for an electromagnet. More recently I learned why from an impeccable scientific source: The permanent magnet affects the magnetic particles in the iron core of the electromagnet so that they increasingly tend to retain their magnetic alignment with the magnetic field of the electromagnet even after it is switched off. This increasingly diminishes the usefulness of the electromagnet, because switching off the current does not entirely kill its magnetic force.

4

Also, it wouldn’t work for another reason: When the circuit is broken, the hook is meant to fall. With a magnet in its base, it won’t.

5

The product ADA412 is a push-pull type. It has a rod that extends beyond the body on both sides, allowing one side to push, and the other to pull.

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ROB-11015 is just a pull-type, it will pull the rod into the solenoid with power applied. The spring will push the rod back out if power is removed.

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Often those types of power supplies are fine to handle light inductive loads. To be safe and sure, you can add a flyback (kickback) diode. Schotty diodes are popular as they are fast. You would wire one up in reverse polarity as shown below.

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Keep in mind you’ll need to turn the PSU off at the wall to de-energise the solenoid. The transistor comment is a tip to control the device from a microcontroller or other low-power device. Which doesn’t seem to be relevant in your case.

Much like a DC motor, the coil has electrical resistance and will ensure the device will only draw what it needs. Caution though, don’t go beyond the rated voltages (which that PSU is able to), as it will lead to additional current draw on the coils and will permanently damage them.

1 Like
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@Graham: Thank you for your reply. It seems the push-pull type of solenoid like the ADA412 that you described may not do what I want it it to do. I need a solenoid like the one depicted here, from which a load can be suspended, which is held up as long as the current is flowing, and drops the load when the circuit is broken. Can any of the ones sold by Core Electronics be used in that way?

7

We don’t have a solenoid where the rod will fall-out when unenergised by default. Though each model could likely be hacked to do that, by removing the mechanical part that prevents the rod from pulling out. It may lead to damage depending how you go about it. Uncharted teritory.

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Re the above three devices: SKU: ADA3872, ADA412 and ROB-11015: Presumably they could also be powered from a laptop USB port. Is there anything wrong with doing that?

And if that’s OK, do you know of any Windows software that can monitor the power draw from the USB port and will sound an alarm when the circuit to the solenoid is broken and the device stops drawing power from the USB port?

9

While USB is 5 volts, it would be worth having a flyback diode as the solenoid loads are inductive. Solenoids will send a possibly hazardous reverse voltage spike into the USB port.

Not that I am aware of, however the USB HID specification does have power management built-in. That would require a HID-compliant device such as a PIC16F145x or PIC18FxxK50 microcontroller, though the time needed to develop such a product might outwieigh the benifit.

Perhaps an easier option would be to use a phone battery bank such as a Cygnett model from JBHifi which will charge and power at the same time. They can then act as an uninterruptable power supply which would have hours (days?) of energy should power be removed.

With that said, it would be best to test that the power bank can charge and power at the same time, as some models don’t. They are never labelled for this feature, some purchasing experimentation might be needed.

10

I have a new question about SKU: ROB-11015, small 5v solenoid (pull type). Both of its leads have black insulation. Is polarity significant on that item?

Also, here is an piece of user-feedback regarding that product: The leads terminate in a female connector for a circuit-board and they are are only 5 cm long. For my use case, I intend to connect it directly to a power adapter, not a circuit board, but I didn’t want to risk snipping off the terminal block and stripping the ends of the leads to put a different connector on it, because 5 cm does not leave much room for error or change of mind. Luckily, I found an old computer part (a CPU fan) that had a pair of 15 cm leads on it terminating in a matching male connector. I cannibalised it for the leads and the connector. Not every customer will be so lucky to find such an extender for the leads. It would be better if that solenoid’s leads were a little longer. than 5 cm.

11

I believe the answer to my question above (post 9) is that polarity is not significant for that product. Please correct me if I am wrong. But I have another concern about the same product (ROB-11015). I tried it out today powered by this switchable power adapter from Jaycar, set to 5 v. After being on for only a couple of minutes, it gets very hot (enough to scald my fingers when I pick it up by its metal housing). Is this to be expected or is it a cause for concern? Also, I would need to leave it powered on for many hours at a time. Is there a limit to how hot it gets or will its temperature keep rising as long as current is flowing through it? If so, will that damage it or is it built to withstand that kind of heat?

12

Hi Mottel,

Sorry for the delay! We’ve been a little short staffed this week, and very busy to boot!

Thanks for the feedback re the lead length, we’ll pass it on to the manufacturer. FYI though it is a JST-PH connector. If you needed some more cable you could use a pig-tail like this with a male JST-PH connector already attached:

As you’ve determined, no it is not polarity sensitive. It works equally well in both directions.

Regarding the heating, yes it is to be expected. Without motion a solenoid is basically a short circuit. I’ve taken one off the shelf and measured the current using a similar supply to the one you used and it is 1A. This means your solenoid basically becomes a 5W heater while powered.

According to the datasheet, it is designed to be operated at 25% duty cycle at 0.25Hz (On for 1s, off for 3s) (at an ambient temp of 20°C), with a max allowable temp rise of 65°C - ie. It’s only good for about 1.25W continuously.

I noticed an outdated datasheet was linked on the product page, so I’ve just updated that reference.

As such, it sounds like you will need a different solenoid for your application.

Regards,
Oliver
Support | Core Electronics

13

To the Core Electronics staff: At the end of the armature of your Small Push-Pull solenoid, SKU: ROB-11015, (the side with the spring), there is a small hole about 3 mm in diameter. It is the opening of a threaded shaft bored into the armature parallel to its length, to a depth of about 10 mm, so that the first 10 mm of the armature is hollow at that end. I would like to know the specifications of that shaft (i.e. its exact length, diameter and thread type) so that I can get a matching bolt for it. If you don’t have the information to hand, could you please obtain it from the manufacturer? (I would suggest that you also put that information on your website.)

14

Hi Mottel,

Okay, unfortunately, I don’t have one in local stock that I can measure up and the datasheet from our supplier (only partially in English) doesn’t appear to list it. I’m quite certain that I’ve identified the point you’re talking about as I’ve listed in the image below, it’s an Adafruit part so I’d say that the standards on all the other parts which attach to shafts with that same M3 connection will be maintained, but it doesn’t appear to be listed anywhere I could find. If you would like to sign up for when the part comes into local stock and then reply to this post the team here can measure it up for you to confirm the specs exactly as soon as it arrives. If there’s anything else that we can help you with please let us know!

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Bryce
Core Electronics | Support

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Bryce, you said that if it followed the same standard as the other Adafruit solenoids, it would be an “M3 connection,” but you gave no details of what that is. Can you elaborate please, and, more to the point, where can bolt that match that connection be obtained?

16

Good afternoon Mottel,

Below is the Adafruit site on this part. We don’t have local stock at the moment, so I can’t check what the exact details are by measuring it. The information that our supplier (Adafruit) has for this part in the datasheet I’ve linked above also isn’t in English (it doesn’t appear to have the exact specs you’d need) however, this will almost certainly be the same metric standard as these bolts, but I can’t verify that right now with stock. Please let me know if there’s anything else that you need!

Bryce
Core Electronics | Support

17

Hey Mottel

You’ll want to do a bit of googling on this. ISO standard threads are identified by M# where Mstands for metric and # is the nominal diameter in millimeters. There’s a lot of different thread standards in the world for all sorts of applications.

Definitely try typing something into google search if you don’t know it as it’s often the fastest and easiest way to get an answer