Capacitor Measurement

[Dave]

AMay, I personally would tend to use a constant current source with the RCTIME function. That way you can measure the timing between 2 points with the cap charging at a fixed rate (much more linear). Remember that the resultant range can not be greater than 0 to 65535 so you will probably have to use different current levels to maintain accuracy over large ranges 100Pf to 100 uF. I have used this methode for measuring the backup storage of airbag modules capacitors in production equipment with great success. LOL

Dave Purola,
N8NTA

[Melanie]

>> caps, but they are still off 50 - 100%.

Bear in mind that unlike Resistors, Capacitors are pretty much guaranteed NOT to be the value printed on them. With most Capacitors, if you're within 20% of what they're supposed to be, you've got a good one!

So, if your gizmo is telling you it's 50% off... it could actually be telling you the truth!

[AMay]

Thanks for the response.

I can imagine a constant current source.
I can imagine starting it, and stopping it with a comparator.
I can't imagine how to time the interval in PIC basic. The books I have don't help, or I am not looking in the right place. (I am just a retired bookkeeper having fun with this.)

I don't want to get involved with a separate counter. The author who got me started on this ran into all kinds of stability problems with his on the small caps.

[Acetronics2]

Hi,

Some times ago, Elektor published a nice project about a capacitance meter; It's not PbP, but fairly works ...

Project N° 020144 ... Programmed PIC and PCB available.

Good choice ... if for measuring !!!

Alain

[bbarney]

here's a good one but it's not basic either
http://www.ubasics.com/adam/pic/archive.shtml
Autoranging Digital Capacitance Meter .01pF to 16,777.215 uF
by Fr. Tom McGahee
it's at the bottom of the page

[T.Jackson]

>> caps, but they are still off 50 - 100%.

Bear in mind that unlike Resistors, Capacitors are pretty much guaranteed NOT to be the value printed on them. With most Capacitors, if you're within 20% of what they're supposed to be, you've got a good one!

So, if your gizmo is telling you it's 50% off... it could actually be telling you the truth!

Many books quote it, I even recall one of my old tech teachers saying it: "Electrolytic capacitors have a huge varying degree of tolerance" But in practice, I have never measured any capacitor with a +/- 20% tol. I have however measured many of them faulty with very high ESR, which is a common fault with electrolytic capacitors used in switch mode circuitry. Sometimes, an extremely high ESR – near open circuit – will result in a somewhat lower capacitance measurement. All and all, I think modern-day components have traveled miles ahead. In reality, today, +/- 20% tol with anything would be very seldom.

Regards,
Trent Jackson

[ronsimpson]

I have old Capacitor substitution boxes dating back 50 years. (20 years for the Heathkit boxes) Large capacitors 2 to 470 uf do not last many years. I replaces all the large capacitors in my HeathKit radio 10 years ago. I have some capacitor boxes where one decade failed because of a bad switch. “Anything below 100 pf returns 0, even with a 10meg discharge resistor.“

I think you should go to the store [DigiKey] and get some capacitors you know are good

[ronsimpson]

1)
I have used a NE555 IC + R + unknown Capacitor. The 555’s time is dependent on capacitor. The PIC measures time and drives a LCD display. A one-shot [multivibrator] converts capacitance to time. [74LS221]
2)
I built a C & L meter based off a kit. I measure well under 0.1pf.
http://my.integritynet.com.au/purdic...er-project.htm
It uses an oscillator and reference coil and two reference capacitors.
The oscillator oscillates at C1= 680pf and L1= 68uH or .01uf and 68uH.
An unknown capacitor is placed across the reference capacitor. If the unknown cap is 1pF then the frequency changes by square root (681/680). If the unknown cap is 1000pf then the new frequency is a product of 1680pf and 68uH.

The math is hell.
I measure the inductance of inches of wire or tiny ‘gimmick’ capacitors (1pf).

[Acetronics2]

http://www.electronics-diy.com/lc_meter.php

here ... soft available !!!

Alain