I had a very challenging time last night. I have been working on producing a decent prototype of a machine I described in a previous blog.
The construction had been proceeding very well until last night as I neared completion and initial testing.
To power the LED Scanner I had decided to use the 2N7000 400mA logic level MOSFETs. These, eleven in all, had all been soldered into the board. and all the necessary connections had been made to the reverse side of the board. It was all neat and pretty except for one thing. It didn't work properly. It looked like most of the MOSFETs were showing a partial short from D to S.
A quick check with the meter showed that this was the case. 10 of the 11 MOSFET had from 10 to several K ohms of short between their drains and sources. My initial thought was that I had been careless with handling them and that static had gotten them. I hated what I had to do but I began to peal back the wiring from their contacts and with all the care and grounding straps I could find I replaced them one by one. At last reassembled I once again applied power and voila! Same problem, same appearance of partially shorted MOSFETS. So we dove back in for another go at it. This time I monitored the status of the MOSFETS as I went along. To my surprise they were being killed by me as they were being installed. At this point there was no way that static was to blame. Too many precautions and ground wires. It dawned on me at this point I was perhaps heat damaging them as I soldered them in. Because of space considerations the MOSFETs had been pulled down fairly tight to the board leaving about 3 mm of lead before entering the board. My solder iron does not have temperature control and runs at about 325 C. The data sheet for the 2N7000 says that it can tolerate 300 C on a 1/16 inch lead for 10 seconds. I think, from my experience, this is a bit optimistic. My usual technique is to minimize time on the contact but obviously something was damaging these parts. I got out the heat sink clips for leads and carefully began to replaced the remaining bad FETs. I continued to monitor them as I installed them. Since I was tacking wires and resistors to their solder pads It was necessary to make extra certain that time on the pad was minimized. I also sprayed coolant on the heat sink clips. Despite these precautions I still managed to kill a couple more. Finally after 6 hours and lots of frustration I had the machine up and running properly.
In all I killed 19 of the 2N7000 MOSFETS
Fortunately for the pocket book they are not expensive and while I did not have any left to spare after this debacle I did have exactly enough to complete the job.
Here is a picture of the completed project:
Here is a short video showing how the lights scan back and forth while hand held tactile devices buzz periodically in the patient's hands and a tone is emitted in the patient's ear as the light on the bar hits each end. I personally question the therapeutic value of this device but then what do I know, I am a technician who can't even seem to install eleven MOSFETs without Killing 19 of them.