Roman Black has described an easy to build Pic based high resolution capacitance meter on his web site. In need of a way to measure my junk box cap collection, I quickly saw the advantage of his design and built a copy.
DMM's are out
When using a DMM cap tester it is soon apparent that they work well on the larger caps, which is not really that helpful as the large caps are usually well marked. Small caps, in the pf and nano range, are a different story, tiny cryptic markings, if at all, and inconsistent, non-repeatable readings on the DMM.
What a treat then to use Roman's 0 to 50 MFD cap tester and be able to read down to several pf, with rock steady, repeatable results.
Easy to build
Simplicity itself, six caps, six resistors, a Pic 16f628 and a 2 x 16 LCD...
Roman suggests that for battery use a low quiescent current, 5 volt regulator be used. I didn't bother with this as I wanted to use the back light so used a regular LM 7805 (see Mods)and a wall wart for power.
The circuit is simple enough to be built on a vero board, if you wish. I like Target 3001 (free version) and so made a small PCB, using the blue film technique, shown here tin plated. A couple of foobah's but usable. When the unit is first powered up it reads the internal capacitor. The 270 pf that I had selected with the DMM was only 210 pf, so used the partially built unit to find a better fit cap. I found one that read 274 pf after settling, so used that. However, once in the box it now reads over 280 pf. I am not sure how or if this affects the accuracy so haven't worried about it!
I am well pleased with the results. I used a 1% resistor for the 10K but the 1k5's were just 5%. For greater precision either the 10K or the 1k5 between RA3 and RA2 can be trimmed with trim pots. The "as built" accuracy of my unit is more than adequate for my needs.
At first I was a bit concerned with accuracy, as most of my 1 uF caps measured low. But as I tested more caps my confidence rose. Mylar caps were best for being nearly right on the label value, electrolytic's the worst. A lot to do with manufacturing to the low tolerance, so that was why many of my 1 uF caps measured 965 nano F.
Here is a short video...
I am using a wall wart that is rated 9 volts @ 200 mA. This was fine to run the project minus the back light, However I found that the 12 volt open circuit voltage dropped to only six volts when the back light was connected. My next in line wall wart was rated at 9 volts 400 mA but the open circuit voltage was 18 volts! Didn't want to risk that so used the 200 mA one and put a 12 ohm resistor in the back light circuit. This brought the voltage down to 8.9 V so felt comfortable with that. However the regulator was starting to just get warm so took the regulator off the board, cut a window in the end of the box, with an aluminum plate over and bolted the regulator to that. Now any heat is dissipated outside of the box, much better.
Kudos to Mr R B for this Pic based high resolution Capacitance meter and for sharing the details and the Hex code with us.
Simply an outstanding project
Continue reading. Click here to view the presentation.
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