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.
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? 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). -- 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).
