Sam's TMS Coils

Version 1.00 (14-Nov-15)

Copyright © 1994-2015
Samuel M. Goldwasser
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For captions, "15T" denotes 15 turns; "2x5T denotes 10 turns total in 2 pancakes of 5 turns each sandwiched together, or equivalently, 5 layers of 2 turns each; "D" is mean diameter.

Unless otherwise noted, the following air-core coils run without problems in that the pulsar does not self desctruct. However, only Sam #13 (the last one constructed to date) produces any response, and only with a minimum of around 80 Joules of stored energy. Some or all of the other air-core coils may also work at above this energy level but have not been tested with more than around 25 J.

Sam #1, #2


Coil 1: Twin 21T, 3" D                 Coil 2: Twin 15T, 2.5" D, Pancake

Sam #2, #4, #5

Coils 3-5: 10T, 2.5" D; 6T; 2" D, 10T; 2x5T,2.5" D

Sam #6

Coil 6: 2x5T, 2.38" D

Sam #7

The one below used the yoke from some cheap electromagnet and as should have been expected did not work worth a darn at all due to eddy current losses in the central part of the yoke and most like core saturation as well. All three pulse types triggered the free-wheeling SCR and collapsed to the monophasic shape for anything but the smallest voltage.

Coil 7: 12T on Electromagnet Core

Sam #8

The one below was made using laminations from an unidentified power transformer with the center leg cut off. But it was a total disaster either due to eddy-current losses and probably core saturation. It blew the free-wheeling SCR on the first shot even though though it wasn't even being used for the monophasic pulse.


Coil 8: 12T on Laminated 2x3" C-Core

Sam #9, #10

Below is Coil #9 during three stages of construction, first pancake pair curing, then both curing, final coil with cable awaiting attachment, soldered, and insulated. Plastic sheets (~0.01" thick) are placed between the pair of pancakes, as well as top and bottom, with Epoxy holding everything together and providing additional insulation. Finally, the entire assembly is covered with electrical tape. Coil #10 is virtually identical, except with sets of 6 turns instead of 2 sets of 10 turns.


Coil 9: Twin 2x5T, 1.2" D, Steps in Construction

Sam #11, #12

Coils #11 and #12 were single coils of 2x5 and 3x5 turns, respectively.

Sam #13

Coil #13 is modeled after the Medtronic/Magventure B70 but is slightly smaller with two sets of 8 turns instead of 9 or 10 turns using #10 wire instead of #8. So, it might be called a B56. :-) The reduce size and number of turns was an attempt to more closely match the lower voltage of MP1 by reducing its inductance while maintaining the advantages of the overlapped pancake architecture. Total inductance is approximately 10.8 uH for the coil with an additional 0.2 uH for the 5 foot cable.


Coil 13: Twin 1x8T Overlapped, 2.25" D, Diagram, Before Covering, and Complete

As with the ones above, a jig is used to fix the coils during initial construction and gluing. The coils are insulated where they overlap by a piece of 0.02" Fiberglas-Epoxy PCB unplated material. They are coated first in Epoxy and then RTV Silicone (Loctite 37467 Black RTV 598) with a pair of 0.02" Fiberglas-Epoxy PCB unplated sheets providing the primary high voltage insulation front and rear. Edges have Epoxy, RTV, two layers of 0.001" Kapton/Polyamide tape. Then the entire structure is covered in at least one layer of vinyl electrical tape. The cable is 3 #14 AWG flexible insulated wires for each direction inside a rubber hose for additional insulation. The equivalent wire is between AWG #10 and 9. The resistance is approximately 0.01 ohms for the coil and 0.008 ohms for the cable.

Sam #13 is the only coil to have evoked a consistent cerebral neural response but this is most likely due to the fact that it is also the only one to have been used with the Rev 2. pulsar and its higher energy of up to more than 170 J. All the others were only used at a maximum energy of around 25 J.

Sam #14 (Future)

Coil #14 is currently in the design phase. It is anticipated to have dimensions similar to those of the Medtronic B70 using dual 10 turn overlapped pancake windings made of 1/8" copper tubing with cooling provided by a recirculating oil-to-air heat exchanger, so hopefully run time will be infinite. :) Due to the skin effect, the effective resistance of the copper tubing will be similar to the althernative using #8 AWG copper magnet wire. Insulation between turns will be via Epoxy and Kapton tape. To increase dB/dt at the maximum voltage of 1,450 V, the two windings may be wired in parallel instead of series, resulting in an inductance between 3 and 4 µH for a full biphasic cycle of 150-160 µs with the 163 µF energy storage capacitor.

Coil 14: Twin 1x10T Overlapped, 2.67" D, Diagram