Somebody had asked me about the schematics for my divergence meter project. All the design files are in the mercurial repository on Bitbucket, but here’s a high-resolution capture of the schematic for those unable or unwilling to use Eagle to view the schematic: dm-rev1.1.png. Be advised that this version of the schematic does not reflect the current design, as I have not updated it with a FET driver per my last post on this project.
On the actual project front, I haven’t been able to test the FET driver bodge yet. Maybe next weekend..
I got some time to work on the divergence meter project more, now that the new board revision is in. I assembled the boost converter portion of the circuit and plugged in a signal generator to see what sort of performance I can get out of it. The bad news: I was rather dumb in choosing a FET, so the one I have is fast, but can’t be driven fully on with my 3.3V MSP430. Good news is that with 5V PWM input to the FET, I was able to handily get 190V on the Nixie supply rail.
Looking at possible FET replacements, I discovered that my choice of part, the IRFD220, appears to be the only MOSFET that Mouser sell that’s available in a 4-pin DIP package. Since it seems incredibly wasteful to create another board revision at this point, I went ahead with designing a daughterboard to plug in where the FET currently does.
I got some ICL7667 FET driver samples from Maxim and have assembled this unit onto some perfboard, but have not yet tested it. Given I was driving the FET with a 9V square wave while testing, it’s possible that I blew out the timer output to the FET on my microcontroller while testing. Next time I get to work on this, I’ll be exercising that output to see if I blew it with high voltages, and connecting up the perfboard driver to try the high voltage supply all driven on-board.
I’ve been able to do some more work on the divergence meter now. The university’s labs made short work of the surface-mount soldering, but there were some hitches in the assembly and testing phase, in which I discovered some of the part footprints were wrong, and it was a bit of trouble getting the programmer working.
I was able to work around most of the bad footprints, but some of them were barely salvageable, since the through-holes were too small. I was able to drill them out on the drill press in the lab, but that left me with very small contact areas to solder to, so I had a few hideous solder joints.
After getting the power supply portions of the board soldered came getting the MSP430 talking to my MSP430 Launchpad, which I’m using as a programmer. Initial attempts to program the micro were met with silence (and mspdebug reporting no response from the target), but the problem turned out to be due to using cables that were too long- I had simply clipped test leads onto the relevant headers, yielding a programming cable that was around 1 meter long, while the MSP430 Hardware Tools User’s Guide (SLAU278) indicates that a programming cable should not exceed 20 cm in length. I assembled a shorter cable in response (by soldering a few wires onto the leads of a female 0.1″ socket) and all was well.
The most recent snag in assembly was the discovery that I had botched some of the MSP430’s outputs. I had connected the boost converter’s PWM input to Timer A output 0 on the micro, but I discovered while writing the code to control the boost converter that it’s impossible to output PWM on output module 0, due to the assignment of SFRs for timer control. The user’s manual for the chip even mentions this, but I simply failed to appreciate it.
I could have cut the a few traces and performed a blue wire fix, but it seemed like a very poor solution, and I was still concerned about the poor contact on the other vias I had to drill out, so I bit the bullet and created a new revision of the board with correct footprints for all the parts, and a more comprehensive ground plane (hopefully reducing inductive spiking on the optocoupler control lines). I’ve now sent revision 1.1 out to be made, so improved boards will be here in a few weeks. Until then, I’ll be working on the software a bit more, and hopefully updating this post with photographs.
One project which I’ve been working on since about October and just got around to creating a project page for is the divergence meter.
There’s not a lot to see there yet, but I’ve recorded my notes on what the design needs and the outline for the control and power supply module. I ordered the PCB in early December in the hopes that they would be available for me to work on while in Wauwatosa during the semester break. That didn’t pan out, so unfortunately the whole project won’t move until next week, when I return to Houghton and can get my boards from the mailbox.
My batch of nixie tubes arrived earlier than expected, however, and I got the components to populate the board in mid-November. All I need is the boards and some time to solder, while hoping I don’t completely botch the job of soldering a 38-TSOP package, especially since that chip (the MSP430F2272) cost me $5. Photos follow.
One I find the time to assemble the control board, the software should come together pretty quickly. Just a matter of time now..