Proton Precession Magnetometer - Part 3. Polarisation Coil Pulse Controller

 I've made reasonably good progress on the pulse controller over the last two months. As often happens, some things I thought would be easy turned out to be hard and some I thought would be hard, not so much.

The power supply and controller is now all set up and finally working. I'd located a nice roomy case that fitted the power supply (150 W Meanwell 12V), installed a fused main switch and done the appropriate connections. I've done my best to cover any mains level exposed connections with tape since I'm likely to be poking around a bit while it's powered.

Also shown is the Raspberry Pi Zero W that I'll use to control the whole thing and analyse the data. The direct control of the pulse is done by a Arduino Pro Nano which hopefully will also collect the data and therefore as well as including the coil activation circuit (basically an opto-isolater controlling the gate of the mosfet bank) the pcb also includes an SPI controlled SRAM and an ADC. Brief tests of each suggest that they are fundamentally working as I'm able to read and write to/from the SRAM and when I feed a ca. 2000 Hz signal into the ADC I can sample it, pass the data by Serial to the Pi and perform an FFT, recovering the frequency within 0.1%. Still this is very much on border of how fast the Arduino can sample so I do regret simply going with a device I had to hand and not a faster variant. More detailed analysis will be needed to establish if the sampling rate is fast and accurate enough. Also on the PCB are connections to the front panel for a RGB LED to show the status of the device (ready, energised, collecting, cooling down etc) and a push button to trigger the coil activation and data collection cycle which is useful for testing.

Unfortunately when I hooked up coil and pressed the button, I soon realised (or smelled) there was a problem. Something was wrong with the MOSFETs used to switch on and off the coil. In fact as far as I could tell they were not turning off and so rapidly heating. I removed them and tested them, finding one of them had gone short. Thinking it might have been just a bad MOSFET I replaced it but the the same problem soon occurred. At this point I decided to rethink the MOSFET system. I had followed the book's suggestion and mounted them on perfboard with wires soldered between them and a bar of aluminium as a heat sink between them and the case:

This seemed a bit mechanically dubious as it required bending the pins of the MOSFETs and some marginal soldering. Instead I decided to design a PCB, using as thick traces as I could and 2oz copper (I did a calculation here to estimate how thick the traces needed to be). The result was much cleaner. 

Still there was an issue which I think might have also been the initial problem. With these IRF6215 MOSFETs the tab with the screw whole is also connected to the drain. Although I had used some thin silicone insulating pads they were difficult to hold in place and clearly not doing the job. I purchased some slightly thicker adhesive TO-220 insluating pads as well as redrilling the holes in the aluminium heatsink and the case more carefully. I progressively tested it with my bench power supply and it seemed fine. Then, fairly confident, I hooked up the coil and pressed the button. Nothing bad happened. I found I could run it fairly regularly (the coil is only on for 6 seconds) and the MOSFETS hardly got warm to the touch. So it seemed to be working. But how to be sure? Well one test is to use a compass to see if there's a disruption to the local magnetic field:

Finally - I was curious about how quickly the coil can be turned on and off. With my oscilloscope I estimated that it's about 7uS to turn it on:

 And 3.7uS to turn it off:

The next step is the signal amplifier - I've got all the parts and the PCB so it should come together fairly quickly.