In trying to get back into building circuits/electronics (it's been around 30 years since I did my electronics classes) I'm trying to get an arduino to control an array of electromagnets, still the obvious fact persists: I'm essentially a NOOB to doing this again. In particular I'm trying to get a single 12v sucking electromagnet to stay on for at least an hour without getting hot. The electromagnet is similar to this:

http://www.icstation.com/solenoid-e...tion-magnetics-industrial-control-p-9483.html

I'm driving the magnet with a 298n hbridge and controlling it from an arduino mega. I'm running the magnet off of one of the output ports of the hbridge and directly to ground. I was thinking a pull down would work but that seems to be too big of a drop (I've tried 10W power resistors with no joy), I've gone the other route of using a 220 ohm as well as a 10K, again no dice.

The current across the magnet is 250mA. The resistance of the is 50 ohms. If my math is right that would make it 12w of power. I've tried driving it from one of the pwm outputs of the arduino (connected to the hbridge) but the same symptom persists. I've attempted giving 20ms bursts to hopefully let things cool down.

Am I missing something obvious here or is this a faulty design?

 

I also meant to say that was a 10W 5ohm resistor

 

Is it a continuous duty electromagnet? With it dissipating 12W of power, why wouldn't you think that it would get hot?

 

250mA * 12V = 3W.
There are no fins on it, no heat sink. I would expect it to get hot. Is it rated for continuous duty? How hot does it actually get?

 

What you could do (since powering it via arduino should be easy) is give it the full 12V initially to give good pull-in power, then taper the power off to a minimum with PWM since holding requires less power than pulling in.

 

What you could do (since powering it via arduino should be easy) is give it the full 12V initially to give good pull-in power, then taper the power off to a minimum with PWM since holding requires less power than pulling in.

That would make sense, however it doesn't seem to work. I tried connecting the hbridge to 12v and then ran the output to the magnet, I control this with pin 3 of the arduino. I took the 3.3v as well as the 5v to the annode of an led (I don't currently have any diodes) and placed this in parallel with the output of the hbridge/magnet. I also tied the led/hbridge/magnet connection to a 220ohm pulldown and fed it to ground. I've got the other lead of the magnet running to common ground.

After this I pulse for 10ms 12 v and see a short spike in magnetism but it's linked to the 12V pulses. Is it me or does this not seem like a sucking magnet?

 

That would make sense, however it doesn't seem to work. I tried connecting the hbridge to 12v and then ran the output to the magnet, I control this with pin 3 of the arduino. I took the 3.3v as well as the 5v to the annode of an led (I don't currently have any diodes) and placed this in parallel with the output of the hbridge/magnet. I also tied the led/hbridge/magnet connection to a 220ohm pulldown and fed it to ground. I've got the other lead of the magnet running to common ground.

After this I pulse for 10ms 12 v and see a short spike in magnetism but it's linked to the 12V pulses. Is it me or does this not seem like a sucking magnet?

I'm having trouble following you. Can you post a schematic of what you've done?

 

... Something to try out maybe, if you are willing to sacrifice one of your magnets:
Drill out the metal center piece, shown in the photo of the magnet, and replace it with a longer soft iron insert. The insert would serve to focus or concentrate the magnetic field and also as a heat conductor, away from the wire coil. Use some sort of heat conductive substance to fill the gap between the core and the wire windings.

 

I've done two different solenoid drivers for two different reasons. One was a rotary solenoid for a shutter. They get hot quick and I had airflow available, so it was easy to drop the voltage across a transistor after a "kick".

The other driver was one to activate another shutter that was close to a 1mm thick $1000.00 USD lens. I used a commercial PEM driver with "slow start" built in.

A third design was a tension controller. It was a clutch and not a solenoid. At very low tensions, you actually have to go negative a bit, but we did not need the tension controller and I did not incorporate that into the design.

Fancy version: http://www.allegromicro.com/en/Desi...Output-When-Using-Hall-Effect-Sensor-ICs.aspx

Google "PWM to voltage" (remember no connection to solenoid).

You have to "kick it" and then reduce the voltage to lower the power dissipation.

It's possible your PWM isn't working. I don;t know why you used one of the outputs. and ground. The output of an H-bridge would not include ground. I'd suggest playing with no magnet and a RC filter to filter the PWM to an average voltage. I might start wit RC being equal to 5x the PWM period and observe this on a voltmeter. You might need a slight load too.

 

... Something to try out maybe, if you are willing to sacrifice one of your magnets:
Drill out the metal center piece, shown in the photo of the magnet, and replace it with a longer soft iron insert. The insert would serve to focus or concentrate the magnetic field and also as a heat conductor, away from the wire coil. Use some sort of heat conductive substance to fill the gap between the core and the wire windings.

What substance would you suggest, that is more heat conductive than the metal which is already there?

 

The hole in the back of the magnet appears to be threaded.
So you could attach it to an aluminum plate or heatsink with a bolt to help dissipate the heat.
Apply a thin layer of thermal grease at the interface to improve heat conduction.

 

Why H bridge? I assume because you want a demag feature?
Many of this type of magnet when lifting small objects have the problem of retained field and have to have a reverse shot to release light items.
I would think with only 3w for that mass and it is heating up there is something else going on here we are not seeing.
Max.

 

..Also if powering it for ~1 hour, is this lifting an object at this time? If so is the weight sufficient to lower the current as already suggested.
And why driving it with PWM?
Max.

 

What substance would you suggest, that is more heat conductive than the metal which is already there?

In order to achieve any significant heat dissipation, it is necessary to create additional surface area.

The way the magnet is now constructed, all the heat is confined and has no place to go ... thus the temperature rise.

.... find a small finned heat sink and bolt it to the threaded hole ... use heat sink compound.

... failed to see previous heat sink suggestion.

 

Many of these lifting magnets are intermittent use.
If requiring this high amount of time on that is causing a problem, another option to explore is a dial indicator base using P.M. magnet, you would need to come up with a method of rotating the release, but this could be done with some kind of double acting solenoid or other means.
They are usually exceptionally strong 60Kg.
Zero current when on.
They only cost around $10.00 on ebay.
Max.

 

Try using a TI DRV102 PWM SOLENOID/VALVE DRIVER to control your magnet.
The data sheet is here "http://www.ti.com/lit/ds/symlink/drv102.pdf"
I have used this on long holding solenoids to reduce current draw and heat buildup.
This may solved all your problems.