The fact that "some work and some don't" would support an oscillation or RFI problem.
Your comments reminded me a problem we had years ago with a video crosspoint switch. Small DMOS FETs were used as switches. They were packaged in small metal (TO-18) cans. The boards built in engineering worked fine but the units done by the assembly department were completely dead. The FETs were shipped with small conductive rubber "O" rings around the leads to protect the FETs from static. The assembler thought they were mounting pads and left them attached. When the FET was pushed down against the board, the "O" rings were not apparent. I'd hate to tell you how many engineering hours it took to find this very simple problem. The lesson here is when you hear hoof beats, check for horses before you go looking for zebras. Fred Townsend Declan Moriarty wrote: > On Wednesday 25 December 2002 06:08, somebody wrote > > Two thoughts come to mind. > > > > 1) Might the circuit somehow be oscillating? I have seen circuits of > > this kind that would sing. The problem went unnoticed because > > applying a meter or scope probe added enough capacitance to kill the > > oscillation so everything looked normal. The fix was to apply a nf > > or so of capacitance from the gates to ground. > > These fets are directly driven by latches. There is smoothing in > abundance > > > > > 2) Could this be a RFI issue? Are there strong fields around, such > > as a broadcast station in the neighborhood, that the circuit is > > picking up and causing the FETs to turn on? > > I hardly think so. 2 of these work, this one works but loses 10mA. > > > > > Fred Townsend > > > > Declan Moriarty wrote: > > > Here's one to tease yourselves over. I have a board to fix > > > and for once, I do not know how to go about it. > > > > > > I designed the thing. It's battery powered, and is consuming > > > batteries. There is a 1.0V line with nearly 40 class B fet stages, > > > using logic level fets (Irlml6401 p-channel on top and irlml 2502 > > > n-channel underneath) The gates are tied; > > > > > > That stage is pulling 10 mA on no load. It should draw basically > > > nothing. It draws 10 mA even with the load disconnected. The 'Off' > > > condition has -1.5V on the gates; the p channel fet sees the > > > voltage from +1.0 to -1.5V and is fully on, and the n channel > > > reverse biased. 'On' it is the reverse - the p channel is reverse > > > biased, and the n channel sees +2.5V. > > > > > > Now the tracks are too thick to use a millivolt meter and trace > > > where the 10mA is going. 10 mA doesn't show - I had difficulty > > > tracing 100mA. How do I find the $*�"! current leak?? > > > > > > I had one thing - a fet test program which applies logic 0 to all > > > gates and will drive one at a time high, as I press buttons. I > > > couldn't find any gate leakage. The 5V will usually leak onto the > > > 1.0V if a gate is gone, and I can find the fet pair that way. I > > > will go around the fet pairs with a voltmeter and check the values, > > > but expect to find nothing. > > > > > > -- > > > Regards, > > > > > > Declan Moriarty. > > > -- > > > Author: Declan Moriarty > > > INET: [EMAIL PROTECTED] > > > > > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > > > San Diego, California -- Mailing list and web hosting > > > services > > > ------------------------------------------------------------------- > > >-- To REMOVE yourself from this mailing list, send an E-Mail message > > > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > > > the message BODY, include a line containing: UNSUB CHIPDIR-L (or > > > the name of mailing list you want to be removed from). You may > > > also send the HELP command for other information (like > > > subscribing). > > -- > Regards, > > Declan Moriarty. > > -- > Author: Declan Moriarty > INET: [EMAIL PROTECTED] > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > San Diego, California -- Mailing list and web hosting services > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB CHIPDIR-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). -- Author: Fred Townsend INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
