Friday, 28 August 2015

Magstim 200: HV Charging & Control Boards

Following the teardown of the Magstim 200 TMS device i featured on my youtube channel a few weeks ago i have been slowly de-potting the two (what i call) bricks.

The two devices seemed to control the charging and discharging of the main HV capacitor in the device. Although i could infer their purpose i did want to explore further.

The first brick to get some treatment was what appeared to be the charging brick, firstly i used a hot air gun, this seemed to work well initially by softening the potting compound but the effect only went a few mm deep but it did reveal some components.

Thanks to a couple of comments about how to better remove the potting compound i ordered a litre of Dichloromethane, removed as much of the external packaging as much as i could and immersed them.

After a just hours the first brick softened and i was able to remove enough of the compound to discover it's operation.

The connections across the top are 240v AC IN from the supply mains, which is switched through two Solid State Relays operated by the main control board. The output is fed to the primary of the HV transformer.

The secondary of the HV transformer runs through a set of external power resistors and back into the HV AC IN of the charging brick where there is a bridge rectifier consisting of 8 discrete diodes. The output of the rectifier runs through 4 power resistors to supply rectified HV to the remaining connector on the power brick (HV DC OUT).

The other brick which i see as the charging & triggering brick is more complex with 6 connections, it was potted with a different material and took much longer to come off, i wasn't able to remove all of it as i ran out of dichloromethane but it was enough to see what each of the connections did. The High Voltage DC from the charging brick connects to two connections and also pass through to the main HV capacitor. The positive is internally connected through two 30MOhm resistors to what i suspect will be voltage monitoring feeding back to the main control board. The negative side of the High Voltage DC seems to connect internally to the SCR switching outputs. The final connection is clearly a low voltage input control from the main control board. It took over three weeks in Dichloromethane to reveal this.