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