Alan Stummer
Research Lab Technologist

Magnetic Field Gradient Generator

I am curious who uses what.  Are these webpages a waste of time, or are they any help to others?  Are the circuits, software and utilities appearing in other labs?  Please send your comments or suggestions or what you have used (or not) or schematics of your version or pictures or anything!   Email me, or be creative and send a postcard! I want to hear from the vacuum! Links
  • Overview
  • How It Works

NOTICE: This webpage and associated files is provided for reference only.  This is not a kit site!  It is a collection of my work here at the University of Toronto in the Physics department. If you are considering using any schematics, designs, or anything else from here then be warned that you had better know something of what you are about to do.  No design is guaranteed in any way, including workable schematic, board layout, HDL code, embedded software, user software, component selection, documentation, webpages, or anything.

All that said, if it says here it works then for me it worked. To make the project work may have involved undocumented additions, changes, deletions, tweaks, tunings, alterations, modifications, adjustments, waving of a wand while wearing a pointy black hat, appeals to electron deities and just plain doing whatever it takes to make the project work.


Started Nov 2010 for Julian Schmidt (visiting from University of Freiburg) in Aephraim's group.  A pair of anti-Helmholtz coils create a magnetic gradient for the MOT experiment.  Each coil can run at up to 10A (originally 3A) for up to 10mS, independantly controlled by a 0V to -10V control signal.  Their fields will offset the MOT, exerting a force on it.

How It Works

The two channels driving the coils are identical and independant, except for sharing the +5V supply.  An opamp sums the negative control voltage to a current sense resistor.  The opamp input common mode voltage includes the negative rail.  A N-FET on its output drives the coil in common source configuration with a low side current sense resistor.  Because the coils are low duty cycle and only pulsed, no heatsinking is required for the FETs.  The shared power supply is a 5V 0.5A wall wart.  That supply charges a 2.5F ultracap to 5V.  The supercap handles the pulse current, only losing a few tens of mV per pulse.

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