# Solenoid constant power supply

Discussion in 'Power Electronics' started by Diegodc, Apr 25, 2020.

1. ### Diegodc

6
0
Apr 25, 2020
Hi everyone!
I had a small project out of curiosity to build a safe small solenoid (without a core) that would be continuously powered and maintain the inner magnetic field for long periods of time
So far I have built a coil around a plastic frame with the following characteristics:
-18 awg enamelled copper wirrer
- 46 turns per layer
- 4 layers
- 1.9cm inner diameter
- 3cm outer diameter (approx)
- 5.3cm height

What would be the best way to power it? I have a 12v , 2.3Ah battery available at the moment.

I am more of a biologist, but am starting to learn the basics and it still quite complicated to me, so your help would be greatly appreciated!

2. ### duke37

5,364
769
Jan 9, 2011
If my calcs are correct,
Diam about 24mm
Wire length 13.9 m
Resistance 20mohm/m = 0.28 ohm
With a resistance of about a third of an ohm, I would think that 1V would be enough. This would give a dissipation of about 10W so would get quite warm.

3. ### Diegodc

6
0
Apr 25, 2020
Thank you duke37 for your fast answer.

Would you be able to take me through your calculations please? This would help me get a better understanding of the theory.

Does it mean that using a 1V battery would provide optimal conditions given the solenoid characteristics?
Many thanks, DC

4. ### duke37

5,364
769
Jan 9, 2011
The final calc was wrong.
The coil is 19mm i.d. 30mm o.d. Say 24mm average
Number of tirns 46X4
Wire length = 46X4X24Xpi/1000 = 13.9 m.
18 awg has 20mΩ/m
So the resistance is 0.28Ω, say a third.

power = VxV/R or IxV or IxIxR

1V will give about 3A so power will be 3W not 10W as suggested before.

12V will dissipate 12X12/0.3 = 480W.
Your plastic former will be melted in a few seconds and your little battery could boil.

I have no idea of the solenoid characteristics.

5. ### Bluejets

4,226
897
Oct 5, 2014
Probably best to use something that is already established like for example, a solenoid from an automotive central locking system.
Would be cheap-as from a car wreckers and pretty much guaranteed to work.
I'd tend to stay away from being continuously powered as there are very few solenoids that would not self destruct from heat build up.
Rather, perhaps think along the lines of latch, unlatch.

6. ### Diegodc

6
0
Apr 25, 2020
Thank you for the update and the formulas.
It makes more sense to me, I will take a look at watts table, I don't have a scale in mind that would give me a reference of what is too high.

7. ### Diegodc

6
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Apr 25, 2020
Thank you, that probably raises my question of: how do you then define a solenoid that would be suitable for such an application? Or is it something too complicated to do in the first place?

8. ### Harald KappModeratorModerator

10,386
2,270
Nov 17, 2011
Start by defining the application.
We have no idea what you want to do with your coil. You could power it from a low voltage DC source to create a small constant magnetic field. You could also power it from a high voltaeg AC source to create a strong varying magnetic field. Or anything else.
There are no "rules" about how to power a magnetic coil.

9. ### Diegodc

6
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Apr 25, 2020
Hi Harald,
My application is more a technical proof of concept than an actual application, start from a small scale solenoid generating a constant magnetic field to a "bigger" one generating a much stronger magnetic field (than the smaller one) that still has to be constant.
Using the small solenoid I currently have, I could generate a small constant magnetic field following duke37 indications, calculate the strength of it. Then go for a scale up and define a bigger solenoid with its own power conditions that would be a good optimisation.

When you say they are no "rules" about how to power a magnetic coil, it indicates that there are some guidances or experimental knowledge than can be used which is what interests me at the same time. Would you be able to provide some information about it if you have any?

I hope it makes sense, it seems I'm asking lot of questions, sorry about that.

Thanks

10. ### Harald KappModeratorModerator

10,386
2,270
Nov 17, 2011
The mathematical equations describing the magnetic filed from an electromagnetic coil can be found e.g. here.

duke37 likes this.
11. ### Diegodc

6
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Apr 25, 2020
So, given that the small scale coil resistance should be 0.28 ohm (from duke37), the current I should be 3.57A for a 1V voltage, I find a field of 0.251 Tesla.
Am I applying it the right way?

As there is no core, I've used the air permeability at 1.257x10^-6 Henry per meter.
I will try to use that small scale and find a way to measure that field to compare it to the calculated value.
Many thanks

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