Wonderful, that sounds like a better ansert for him - Thanks! May connect with you later (I'll let him get me a copy of the data Mark, mwillis@nwlink.com Thomas McGahee wrote: > > Notes interspersed below... > > ---------- > > From: Mark Willis > > To: PICLIST@MITVMA.MIT.EDU > > Subject: Capacitance/Inductance measurement boxes > > Date: Sunday, September 13, 1998 2:48 AM > > > > Hi all - A friend was looking at a recent magazine article (Nuts & > > Volts?) and wants to build a Capacitance measurement unit; The > > schematic I was looking at has some limitations, in the range of > > capacitance measurable IIRC. He wants to be able to measure up to > > larger values, and is dyslexic so can't reliably read cap. codes (and he > > does antique stuff so I think he's after the old electrolytics in power > > supplies, so needs ability to measure 2500 uF or more.) > > > > Anyone have a PIC box designed that does this sort of thing? > > (Intelligent pre-scaling of the capacitor range, then C-to-V conversion, > > display to a $8 DVM module, would be great.) He's buying (I could > > use an inductance unit early next year, pretty much the same problem > > really, but from the other side.) > > I have designed a number of capacitance meters over the last 35 years. > Many of these have been digital capacitance meters. The one that I am > currently working on uses a simple analog section that is controlled > by a PIC. The PIC determines what scale the analog section is working > at by operating a set of relays. The PIC receives a time period > back from the analog section that is directly proportional to the > capacitance. The PIC measures this time period and determines the > actual capacitance. The result is scaled and displayed with proper > units (pf nf ufd) on an LCD display. I am using a PIC 16C84 with > a 4.00 Mhz xtal, so the PIC has an instruction cycle time of > 1 microsecond. Very small time periods are handled by pre-scaling > the time period. This can be done by software or hardware... the > software method is cheaper, and so that is what I use. > > The current design is giving me 4 digit accuracy. Scales include: > .1 - 999.9 pfd > .1 - 999.9 nfd > .1 - 999.9 ufd > Plus an extended scale that measures capacitances up to 9,999 ufd. > > Note that in the current design implementation the extended scale can > require many seconds to achieve full scale counts. If I was > designing this meter to measure primarily the larger value > capacitances, then I would make a change to my analog section so > that I could measure capacitances up to 10,000 ufd in 1 second or > less. > > I have built a one-off pc board for the analog section and kluge > the analog section to a PIC on a ProtoBoard for right now. Accuracy, > resolution, and repeatability are all excellent. Unfortunately a > crash of one of my hard disks caused me to lose my original > pcb layout design, but I have a printed copy, so I can re-do it > without too much work. > > The software is still in a state of change, and I really do not want to > release it at this time, but I would be happy to discuss design issues > with anyone who is really interested. This project is on a back burner > right now, because I have some other designs that are taking up my > time at the moment. > > You can get a copy of one of my older capacitance meter designs in > either the October, 1976 issue of Popular Electroncs, or the > Popular Electronics 1980 Electronic Experimenter's Handbook. > The design in these issues goes up to 10,000 ufd. Replace all > 741 op amps with LF356 op amps for greater accuracy. > > If you cannot get a copy locally, I could probably dig up a copy > from my archives and scan and e-mail it, but check your local > library for back issues. > > > > > I don't really have time to help him - and the schematic he has won't > > do what he needs - so I said I'd ask & see what's out there. (He > > doesn't like my ideas for work-arounds very well, but doesn't want to > > learn enough to do it himself, either.) I'll build it for him if we can > > find something out there... > > > > Last schematic other than that on this project type I can remember was > > in Nuts & Volts in (?early?) 1995? or so. But I may have erased holes > > in my remembery here > > > > Mark Willis, mwillis@nwlink.com > > I hope this helps. > Fr. Tom McGahee