Friday, September 14, 2012

A little test jig work

This evening I didn't get home until after 9pm, so I only had a couple of hours. I mounted additional FDV301s on Surfboard adapters to bring the count to 40. I thought about using some of the other MOSFETs I'd tried, like the DMN26 and BSS138 types, but decided it would be smarter to have them all the same. Here I'm taking advantage of the FDV301's protection diode from Source to Gate as additional protection against the Source going dangerously high and damaging the FPGA.

Because of the way the FETs are lined up, connecting all the gates to the Vcc bus (3.3V) on the expansion board was just a matter of running a heavy wire down the column hitting all the middle pins, and a little solder work. I got them all soldered onto the test jig board, but only had time to wire through and test 4 of the 40 FPGA I/Os. Completing the rest of the wiring shouldn't take any longer than it would if I'd used resistors.

Here is a photo of the top of the board:


If it's not totally obvious, each of the little cards is a Surfboard adapter card carrying one FDV301 MOSFET. Four columns of ten is forty FETs, each protecting one of the 40 FPGA I/O lines that show up at the pin header to the left of the prototyping area. Sorry if the photo isn't great.

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