Resistor for DC circuit driving homemade electromagnet

Thread Starter

jaym203

Joined May 16, 2020
3
So I hope this is the right place for this.

I have a laptop that does not support Wake-On-Lan. I want that functionality. The laptop wakes up with the lid opening. A reed switch controls this. There is a magnet at the bottom of the screen that activates the switch.

So I fashioned a simple electromagnet (wire around nail) and I am driving it using a DC power adapter connected to a smart switch I can voice control. Joyfully, it works.

My question is what sort of resistor do I need in this circuit? and is there some household object I can use to be my resistor. An old cell phone a computer fan etc. I did not think I needed a resistor because the circuit only has to be on for 2 second at a time, maybe once a day. But damn thing got stuck and took out the adapter.

Thank you for your time
 

Reloadron

Joined Jan 15, 2015
5,456
The problem becomes with DC applied eventually the nail becomes magnetized. Then your switch will remain on. This is why transformer cores are not solid like a nail but rather use lamination. You can't use AC because the switch will open and close at the AC frequency and yes, most reed switches are that fast. Additionally making an electro magnet by wrapping wire turns around a nail you have no clue how much current it will draw which is likely why your magnet ate the power source. This really over simplifies things but you should get the idea. Like they say, nice try. Really it was.

If the actual reed switch inside is easily accessible on both ends there are likely other solutions.

Ron
 

Thread Starter

jaym203

Joined May 16, 2020
3
Thanks, ugh, seems like this might not work. The reed switch is not easily accessible. as it is tucked behind part of the motherboard from what I can tell. I might just leave it as is and walk over to the computer (like a neanderthal!) when I need to.

I was aware of the magnetization problem... but I thought, given it is on for 2 seconds a day that would not be a problem...

I have other DC power adapters, so just to kill this completely dead, adding a resistor of some sort into the circuit won't protect the power source.
 

Thread Starter

jaym203

Joined May 16, 2020
3
Thanks! that is a clever thought... I've looked around me and cannot find a relay I can repurpose... I just threw out an old tape deck... darn.... any ideas in common household items?

I could order one but at that point might as well buy this electromagnet I found on amazon.
 

MisterBill2

Joined Jan 23, 2018
6,112
The problem becomes with DC applied eventually the nail becomes magnetized. Then your switch will remain on. This is why transformer cores are not solid like a nail but rather use lamination. You can't use AC because the switch will open and close at the AC frequency and yes, most reed switches are that fast. Additionally making an electro magnet by wrapping wire turns around a nail you have no clue how much current it will draw which is likely why your magnet ate the power source. This really over simplifies things but you should get the idea. Like they say, nice try. Really it was.

If the actual reed switch inside is easily accessible on both ends there are likely other solutions.

Ron
The cheap trick to avoid the magnetizing problem is simply to have a permanent magnet just strong enough to operate that reed switch, and then a coil around that magnet with the polarity set to cancel the magnetic field and release the reed switch. Or even smarter, have the magnet strong enough to hold the switch on but not strong enough to trigger it. Then a pulse through the coil of one polarity switches on and a pulse of the opposite polarity switches off. The process would be tedious but the concept is simple and the hardware not much.
 

Reloadron

Joined Jan 15, 2015
5,456
The cheap trick to avoid the magnetizing problem is simply to have a permanent magnet just strong enough to operate that reed switch, and then a coil around that magnet with the polarity set to cancel the magnetic field and release the reed switch. Or even smarter, have the magnet strong enough to hold the switch on but not strong enough to trigger it. Then a pulse through the coil of one polarity switches on and a pulse of the opposite polarity switches off. The process would be tedious but the concept is simple and the hardware not much.
You can give that a try.

Ron
 
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