Circuit design for research project

Vivamusdumvivamus

Joined Jan 10, 2024
8
Hi, all. I am needing some assistance designing a circuit for a research project observing the affect of magnetic fields on plant tissue. The circuit will be driving an electromagnetic coil to create the magnetic field needed. I found a project with a similar scope here. The goals for the circuit in my project are (1.) use high amperage DC current to drive a magnetic copper coil (2.) in a PWM fashion (3.) with the ability to vary the frequency, amplitude, polarity, and duty cycle of the PWM (4.) to create magnetic fields that pulse at varying frequencies and intensities.

I am having trouble figuring out how to achieve this with the amperage needed. Based on some calculations on the coil size I am wanting to use, I need to drive it with around 100A to get the required magnetic field strength.

My initial thought was to use a PWM signal generator like this, to drive a mosfet array with the square wave, however it seems in the above project that I linked that they used a DC buck down converter in their circuit. Though, their requirements for magnetic field strength were significantly lower than mine.

Any recommendations would be greatly appreciated!

k1ng 1337

Joined Sep 11, 2020
960
Hi, it might be a good idea to draw a diagram of your desired setup. Include the size of the specimen, the distance between the specimen and the electromagnetic and the desired field strength at a specific location.

If this is known, it will be easier to design a coil and a circuit. You mentioned 100A but this is a lot of current and will require a large conductor. How did you calculate this value?

You may be able to optimize the coil with ideal placement of the specimen because field strength is greatest where lines of force are most dense.

Vivamusdumvivamus

Joined Jan 10, 2024
8
This was my initial thought on the actual setup. A Petri dish with a plant tissue culture siting on a table, surrounded by the copper coil. For better exposure to specifically North or South polarity, I considered elevating the petri dish off the table above the coil by a couple inches. Does that make sense?

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Vivamusdumvivamus

Joined Jan 10, 2024
8
I came up with the 100 amps by utilizing this calculator here.

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MisterBill2

Joined Jan 23, 2018
18,535
Please consider that the current proposed requires a whole lot of power. Unfortunately the calculation sheet does not show the required magnetic flux level, nor the area involved, nor the air gap.
So there is no way to suggest a different arrangement that could provide the desired field intensity for the desired area.
And unfortunately I am not willing to spend time on guesses.
So we will need to see the desired flux and the area penetrated by that flux.
It is quite likely that a higher voltage with less current but more turns will be a better solution.

In addition, using PWM will result in a fluctuating magnetic field and that may alter the effects on the plant life being investigated.

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drjohsmith

Joined Dec 13, 2021
852
a thought
if you need say a flux of x, achieved by 10 turns and 100 Amps,
would you get the same flux with 1000 turns and 1 amp ?
what would happen if you used a soft iron core ?

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MisterBill2

Joined Jan 23, 2018
18,535
OK, after reading the research paper I see this is indeed a student engineering project After seeing the description of the coil, 180 feet of #18 wire. in a coil worn in a belt, I am a bit puzzled as to where the 100 amps figure came from.
The disparaging of current technology does set a red flags. And magnetic treatment of back pains is an "interesting" topic.

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WBahn

Joined Mar 31, 2012
30,063
Using PWM on an coil with 100 A is going to be an interesting feat. It's not going to take too much inductance in that coil before you will require enormous voltages to get it change current quickly enough.

Vivamusdumvivamus

Joined Jan 10, 2024
8
Please consider that the current proposed requires a whole lot of power. Unfortunately the calculation sheet does not show the required magnetic flux level, nor the area involved, nor the air gap.
So there is no way to suggest a different arrangement that could provide the desired field intensity for the desired area.
And unfortunately I am not willing to spend time on guesses.
So we will need to see the desired flux and the area penetrated by that flux.
It is quite likely that a higher voltage with less current but more turns will be a better solution.

In addition, using PWM will result in a fluctuating magnetic field and that may alter the effects on the plant life being investigated.
Thanks for your reply, I see I missed a key bit of information there.
I intend for the field to fluctuate or “pulse” and the pwm trigger is to give the ability to adjust that frequency. The desired magnetic flux is in the neighborhood of 0.5 tesla, or 5,000 gauss. The aim is for the plant tissue to be subjected to to that strength of magnetic field, for calculations I have assumed the tissue itself to be inside a Petri dish and estimate tissue to be 3cm from the center of the coil.

Using a coil of a wide diameter could work for the petri dish arrangement detailed above. The calculator I linked above shows that a field strength of 5,000 gauss may be achieved by using a coil diameter of 6”, height (thickness) of 1cm, 1000 turns of 18ga, driven at 100a. Utilizing an iron core could makes this immensely more approachable, though the current plan is for an air core.

Vivamusdumvivamus

Joined Jan 10, 2024
8
a thought
if you need say a flux of x, achieved by 10 turns and 100 Amps,
would you get the same flux with 1000 turns and 1 amp ?
what would happen if you used a soft iron core ?
Without factoring for additional resistance in the coil, I believe that is correct. Now how that additional resistance factors in I am still wrapping my head around.

Vivamusdumvivamus

Joined Jan 10, 2024
8
OK, after reading the research paper I see this is indeed a student engineering project After seeing the description of the coil, 180 feet of #18 wire. in a coil worn in a belt, I am a bit puzzled as to where the 100 amps figure came from.
The disparaging of current technology does set a red flags. And magnetic treatment of back pains is an "interesting" topic.
I referenced their student project as an example of concept. The design utilized seemed fairly straightforward, and in similar vein to the goals of mine. Whether there is potential for scalability without major modifications is a bit outside of my wheel house, as I think you can all tell! That’s why I am here for help.

In regards to their purpose for the device, the research papers that exist on the use of PEMF are rather interesting. Devices have been utilized in hospital settings to accelerate rates of bone growth and cell proliferation with purported success.

Using PWM on an coil with 100 A is going to be an interesting feat. It's not going to take too much inductance in that coil before you will require enormous voltages to get it change current quickly enough.
And that is where my understanding of electronics starts to lack. Could you break that down for me?

WBahn

Joined Mar 31, 2012
30,063
The voltage across an inductor is proportional to the rate at which the current in the inductor is changing.

v(t) = L di(t)/dt

What are you looking for as far as how fast you need the magnet to go from 0 A to 100 A (and vis versa)? What is the fasted PWM frequency and smallest duty cycle you are trying to achieve?

Danko

Joined Nov 22, 2017
1,835
intend for the field to fluctuate or “pulse” and the pwm trigger is to give the ability to adjust that frequency. The desired magnetic flux is in the neighborhood of 0.5 tesla, or 5,000 gauss. The aim is for the plant tissue to be subjected to to that strength of magnetic field, for calculations I have assumed the tissue itself to be inside a Petri dish and estimate tissue to be 3cm from the center of the coil.

Using a coil of a wide diameter could work for the petri dish arrangement detailed above. The calculator I linked above shows that a field strength of 5,000 gauss may be achieved by using a coil diameter of 6”, height (thickness) of 1cm, 1000 turns of 18ga, driven at 100a. Utilizing an iron core could makes this immensely more approachable, though the current plan is for an air core.
Inner diameter: 100 mm.
Outer diameter: 338 mm.
Winding length: 10 mm.
Number of turns of the coil: 1000.
Wire diameter: 1.02362 mm.
Wire diameter with insulation: 1.09474 mm.
Number of layers: 125.
Length of wire without leads: 687 m.
Weight of wire: 5 kg.
Inductance: 171.5 mH.
DC resistance of the coil: 14.4 Ohm.

So, for 100 A voltage on coil should be 1440 V.
At 100 A coil will dissipate 144 kW of power.
With such big power 5 kg of copper will melted and vaporized in seconds.
Pay attention at this 6000 Gauss coil: https://woodruffeng.com/datasheet?part_number=WE-PN-110025-01-00&access_code=0f634624a7303c5f

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MisterBill2

Joined Jan 23, 2018
18,535
Long before the whole coil melts, that #18 wire will fuse at some point and there will be a rather hot arc passing through the cloud of copper vapor. I once had a small cloud of copper vapor condense on my finger and it was rather unpleasant.
So I suggest that using an iron core will greatly increase the odds of the users surviving to create the report on the results.

drjohsmith

Joined Dec 13, 2021
852
Wonder what would happen to a person wearing say a gold ring, or metal wrist band / watch if they approached that strong a pulsed field?
This is sounding very dangerous !!
for reference, OK this is against a head, but dont look to closely at this thing your making,
https://www.nice.org.uk/guidance/ipg542

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loose_electron

Joined Jan 16, 2024
19
that coil outside the Petri dish won't give a consistent magnetic field. Consider a ferrite toroid with a gap cut through the torid. (think magnetic recording head) and the petri dish gets slipped into the gap. That will get a more consistent magnetic field across the petri dish. From there, you got to go do the math on needed magnetic field, and then get your voltages and currents to something a lot more realistic.

MisterBill2

Joined Jan 23, 2018
18,535
The field is mostly between the poles of the electromagnet and so the par some distance away is very small. This is much less poweer than an MRI machine.

loose_electron

Joined Jan 16, 2024
19
The field is mostly between the poles of the electromagnet and so the par some distance away is very small. This is much less poweer than an MRI machine.
"poles of the electromagnet" ???

The original poster has suggested a flat planar coil with a Petri dish in the middle of the coil.

If you use the petri dish in the gap of a toroidal electromagnet (see my earlier comment) you might be able to get significant magnetic field in the gap. Pushing the number of wire wraps on the toroid up would help bring the current down to something more realistic.

Switching current in the toroid electromagnet may not be practical (as others have pointed out) - That said, instead of switching current in an electromagnet, it may be better to re-direct the magnetic field through-around the petri dish instead. That might be viable.

MisterBill2

Joined Jan 23, 2018
18,535
Certainly electromagnets have poles. That is where the flux exits and enters.