Declan, Hmmm, sounds like the original design is marginal. There is 5 volts applied to the mosfets to "pump em up", but the 4049 is also powered by the same 7805. That means the very highest effective trigger would be 10 volts, not 12. Right now it is only about 7 volts, and that will not dependably switch the mosfets. There are problems on the adjacent board where it is only delivering a 2 volt pulse (I suspect it should be 5 or more).
I have just about decided to trash part of the circuit and put in a much simpler and dependable circuit. The entire wire wrap section should go; it simply is not dependable enuff for oilfield service. Thanks for the insight. > Syd, You say it worked fine with the 820. 5V isn't enough. The things > should be switched hard on (12V) and hard off. There are logic level > fets which will work at 5V but the IRF820 isn't one of them. Beware also > savage capacitance inside the fets, notably Drain/gate but also > gate/anything. This is good for providing unwanted switch on and > component failure on switch off. > > The gate can be pumped up like a capacitor. Is there a 5V regulator, > e.g. 7805 on the switch on? > > As has been mentioned, if your fet blew drain/gate, your firing circuit > is toast. > > If you are trying to make money & fix it, eliminate uncertainties. > > > On Sun, Nov 16, 2003 at 11:24:20AM -0800, Syd Levine (AnaLog) > enlightened us thusly > > > > The trigger pulse is generated on a different board, then is > > conditioned with a 4049 cmos chip. It seems weak, but there are so > > many other problems, I am having trouble figuring out what it is > > supposed to look like. The 4049 is powered by a separate 5 volt > > regulator, but the input to it is only a 2 volt pulse, which seems too > > weak. > > > > > > How is the trigger pulse generated and what does it look like right > > > now? If the original MOSFET was shorted, it probably also shorted > > > either drain or source to the gate. This might have killed the > > > driver circuit either by overloading it (short to ground) or by > > > bypassing the high voltage pulse directly into the driver.... > > > > > > > > > SLA> the 840 with regard to the trigger pulse? Most of these type > > > tools > > use an > > > SLA> SCR; this is my first experience with a mosfet fired > > > transmitter. > > -- > > With best 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: Syd Levine \(AnaLog\) 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).
