davidw@lpi.UUCP (04/07/87)
I'm replacing the RF amp MOSFET in a TV and want to doublecheck the procedure for soldering these things. Besides the obvious heatsinking, I'm curious about the static problem. I know about using a grounded soldering iron, but should I also ground the TV chassis? Could there could be a static potential between "earth" ground and TV ground??? Should I also be standing on a static-shorting mat? etc?? Thanks. DW
cgs@umd5.UUCP (04/08/87)
In article <11576@teknowledge-vaxc.ARPA> dplatt@teknowledge-vaxc.ARPA (Dave Platt) writes: > ... Electronics assemblers frequently wear a conductive wrist-strap >connected by wire to a nearby ground point... >you could probably fake up something >similar by taking 10' of thin (22-gauge) insulated copper wire, >stripping and grounding one end, and stripping about 8" at the other >end and tying it loosely around your wrist. ... and doing so is VERY dangerous, and probably a violation of OSHA rules and regs... The ground strap you speak of have a 1 Megohm resistor in series between personnel and the earth ground. This is to limit the discharge current from personnel, and to provide for only a extremely poor circuit THROUGH personnel in case of inadvertent contact with powered circuits. --- Safety is Job 1 --- -- --==---==---==-- .. did gyre and gymble in the wabe: .. ARPA: cgs@umd5.UMD.EDU BITNET: cgs%umd5@umd2 UUCP: ..!seismo!umd5.umd.edu!cgs
dplatt@teknowledge-vaxc.UUCP (04/08/87)
In article <1521.umd5.umd.edu>, Chris Sylvain writes: > ... and doing so is VERY dangerous, and probably a violation of OSHA > rules and regs... The ground strap you speak of have a 1 Megohm > resistor in series between personnel and the earth ground. This is to > limit the discharge current from personnel, and to provide for only a > extremely poor circuit THROUGH personnel in case of inadvertent > contact with powered circuits. > --- Safety is Job 1 --- (this in response to my suggestion about a method of grounding oneself to keep from frying a MOSFET) AIEE! Good (excellent!) point! I hereby retract my suggestion for a simple wired body-ground... especially as any equipment that has a CRT tube probably has some VERY dangerous voltages in it. I forgot the rule-of-thumb that says "when working around voltage, stand clear of all possible grounds and keep one hand in your rear pants pocket". Thanks, Chris, for pointing this out! Once again I seemed to have proven the adage that a little knowledge is a dangerous thing. dave platt
straka@ihlpf.UUCP (04/09/87)
In article <270@lpi.UUCP>, davidw@lpi.UUCP (David Weisman) writes: > I'm replacing the RF amp MOSFET in a TV and want to doublecheck the procedure > for soldering these things. Besides the obvious heatsinking, I'm curious > about the static problem. I know about using a grounded soldering iron, but > should I also ground the TV chassis? Could there could be a static > potential between "earth" ground and TV ground??? > Should I also be standing on a static-shorting mat? etc?? The idea here is to remove ANY possibility of putting a static potential across the leads of the MOSFET or between any of the leads and the case. That is why they are shipped in foil or otherwise conductive packaging if they are even possibly sensitive. My suggestion is to MAINTAIN a conductive path between each of the leads and the case until the device is securly soldered into place with all of the other circuitry. This can be done by attaching clips or something conductive to the leads and case while the device is STILL in contact with its packaging. When handling the device, try to make sure that you do not build up any static potential relative to the device. When you finish the soldering, you can then remove the shorting equipment. -- Rich Straka ihnp4!ihlpf!straka
wtm@neoucom.UUCP (Bill Mayhew) (04/13/87)
Just a few comments: 1. Don't ever connect yourself directly to ground by putting a wire around your wrist and twisitng the opposite end around a grounded object. If you came in contact with line voltage by accident, your execellent ground could prove leathal. Commercial straps have large value resistors in series. Your work area should also be guarded by a Ground Fault Interrupter (GFI) type circuit breaker if you are using some sort of device such as the wrist strap that enhances your danger of electrocution. 2. Actually, grounding yourself isn't always best. If the item you are repairing accumulates a static charge, then it can discharge to gound thorugh you, thus zapping the MOSFET in question. You might actaually be better off attaching your wrist strap to the frame of the device that you are servicing. That way, both you and the device are at the same potential. Of course, there is a slight chance that if there is an initial discharge as you attach yourself to the device, the induced field in the frame could zap the MOSFET, but the chances of this are remote. 3. This reminds me of a story back when I used to work at a certian gov't. agnecy with a 3-letter name. They used ao have everybody wear thier photo IDs etched on a piece of aluminum hanging from a chain around the neck. You can imagine what happened when some guy bent over to tune the output tank circuit of a transmitter and the ID badge flipped in: yeee-ouch! They got smart and switched to polaroid pictures sandwiched in plastic-- but still on a neck chain. --Bill Bill Mayhew Division of Basic Medical Sciences Northeastern Ohio Universities' College of Medicine Rootstown, OH 44272 USA phone: 216-325-2511 (wtm@neoucom.UUCP ...!cbatt!neoucom!wtm)