On Monday 06 March 2017 04:06:49 Erik Christiansen wrote: > On 04.03.17 12:29, Jon Elson wrote: > > On 03/04/2017 12:02 PM, Gene Heskett wrote: > > > Hexfets can be hard to deal with. Bear in mind they are generally > > > shipped with a fine wire wrapped around all 3 legs > > > > I have never received a transistor with wire around it. I > > have purchased roughly 3000 FET power transistors for Pico > > Systems production. > > Nor down here, closer to the penguins. But 100 off 2N7000 small > MOSFETs came last month on metallised cardboard strip, the IRLZ44s > came in anti-static plastic tubing, and years ago a solid handful of > MOSFETs came in a black conductive plastic bag with big yellow ESD > handling warning. > > > The capacitance of a power transistor's gate is so huge it greatly > > mitigates the ESD susceptibility. > > Hadn't thought of that ... sounds good vs the 30 pF of human body > capacitance ... but even at 3 nF, that's 10v/kv on 30 pF, so even 20 > kv from crossing carpet in dry air becomes 200v = dead MOSFET. I think > I'll stick with suppliers who adhere to established antistatic norms, > thanks. > > Back at Siemens, we weren't allowed into the PCB assembly area without > conductive shoe-shorting straps, and even then were confined between > painted yellow lines bracketing the aisles, out of reach of trouble. > (And that wasn't just for engineers - other visitors received the same > treatment. ;-) > > > But, I'm pretty sure the servo amps in these older machines > > were bipolar junction transistors. However, after a fault > > that blew a fuse, it is quite likely the base drive > > circuitry may also be damaged. When I get FETs blown out, I > > don't even TEST the gate driver, I just replace it every > > time. I think at least the base drive circuits should be > > checked. > > It's well worth that to avoid having to pull it apart again - after > having ruined a valuable workpiece. Apropos that, ESD damage can be > latent - the device continues to function for a while, then dies > later, just like the stressed gate driver, above. I wouldn't install > MOSFETs which had not received proper ESD-safe handling from > manufacturer to board. > > Erik
Same here. Having found a more robust type in an old computer psu, the first thing I did was tangle its legs up in some stripped 30 gauge wrapping wire before applying the hot air to remove it. And it wasn't removed until it had been planted solidly in the target board. That was 7 or 8 years ago. I was concerned that the increased gate capacitance, about double the OEM bugs, would be hard on the driver, but I surveyed the board while it was running that spindle motor in an HF micro-mill at full song and found nothing more than 5F over ambient. I also raised the fuse from 2 amps to 3 amps which got rid of the blown fuse syndrome as it was formerly blowing fuses without ever slowing the motor. My blown fuse budget is so low now that I am still running on the first box of 5 I bought. That cheap single pwm control is so stiff I had to put an ammeter in the power circuit to tell me how hard it was working, you could not tell from the sound or running speed. Someday I should put an encoder on it, but little room, and the gears are all white nylon or delrin, nothing to sense IOW. I have a belt drive about half made for it, using the 2x more powerfull motor I took out of TLM, but ran out of round tuits. Something with guts enough to turn a 10-32 tap, so low gear would be slower than the 1250 it runs now. More like 250 maybe? > (Who experienced the abrupt suicidal tendencies of NMOS devices on > mild ESD exposure in the early '70s. Thank the pixies that MOS is > somewhat more robust now.) > Somewhat, but not 100% bulletproof yet. And definitely not in an analog circuit. BTDT in the old ntsc transmitter. Dismal failure over time frames just long enough to bore one. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
