Helmholtz coil controller - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 18, 2017, 01:57:11 pm
content: Hi, I'm slowly working on a home brew permanent magnet NMR machine, and I want a set of hula hoop sized Helmholtz coils for field homogeneity. Problem is I can't find any, ANY info on the coil drivers.How would one go about designing a 3 - axis Helmholtz controller? What current/power to put through them? Do they need to be balanced to each other? Should they have a current reg. Knob and indicator?Has anyone done this before? Well, I mean on this forum, I've seen them for sale, bruker or Varian branded I believe, and very pricy.Any info would be greatly appreciated!

author: janekm
author_rank: Unknown
timestamp: January 18, 2017, 02:02:29 pm
content: I am curious too, always wondered how they actually work. I wonder if they use some feedback mechanism?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 18, 2017, 10:31:22 pm
content: Quote from: janekm on January 18, 2017, 02:02:29 pmThat's another point, how well regulated should the coil current be? I'd imagine if your field homogenizing system is drifting, you could just as well leave it off, but I don't know.Of course, calculating what current/power would be needed for a specific field strength isn't that tough, but I'm unsure what a useful range would be, for my kind of application.

author: poorchava
author_rank: Super Contributor
timestamp: January 18, 2017, 10:52:18 pm
content: I've used a diy 3-axis Helmholtz coil at work for calibrating some magnetic field sensors.We used to simply connect the coils across an output from a Tek AFG generating a sine. It worked well enough for our purpose.Sent from my HTC One M8s

author: qwaarjet
author_rank: Regular Contributor
timestamp: February 06, 2017, 04:20:09 am
content: There is nothing special about driving a helmholtz coil any constant current supply will do*. Current control is important because B field changes with current. If you use a voltage controlled supply, as the coil warms the current will changes with it resistance, affecting the B field. The lab I worked in would use either Sorsenson DLMs and SFA supplies for DC fields or Kepco Bipolar supplies for AC (all running in CC mode) to drive our various coils . A hypothetical setup for you would involve 3 separate constant current supplies (the two identical coils of the Helmholtz wired in series in each axis) i would just use bench supplies but I don't know your budget here. To support you better we would need to know the current, power, noise, and stability requirement for your project. Also if you want this to be manual control or have a some complex control system possibly running with magnetometers that dynamical tune your setup.

author: KE5FX
author_rank: Super Contributor
timestamp: February 06, 2017, 05:05:39 am
content: Sounds similar to the problem of driving YIG-tuned oscillators and filters. Google for "YIG drivers" and see what comes up. The most critical component will be the sense resistor. You don't need a fancy four-wire part, but you should still use a nice metal-finned Dale power resistor.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: February 19, 2017, 12:12:06 pm
content: You might find these interesting/useful:https://hackaday.io/project/7409-highly-configurable-3d-printed-helmholtz-coilhttps://hackaday.io/project/11865-3d-magnetic-field-scanner- See the past project log entries going over Helmholtz vs. Maxwell coils and the shapes/sizes of the homogeneous regions of each type, as well as the 3D Helmholtz coil design