Measuring capacitance without a DMM that measures capacitance

[Albot]

I know there are Capacitance meters but is there a circuit i could build to measure capacitance with my DMM? like the one for Hfe/Leakage

[johngreene]

I know there are Capacitance meters but is there a circuit i could build to measure capacitance with my DMM? like the one for Hfe/Leakage

Yes, there is. But it would probably be easier/cheaper to just buy a meter that does it!

There are a couple of issues of using a DMM. One way of measuring capacitance is to time how long it takes to charge up to 90% of the supply voltage through a known resistance. The problem with this is the timing. A DMM updates the display pretty slowly so trying to use it for timing measurements would be very problematic. So the other option is to feed an RC 'filter' (that uses the 'C' under test) with a sine wave and rectify the output voltage with a diode. It would be best to use a precision rectifier circuit that uses an opamp to prevent loading of the circuit under test and removing the diode drop. Now you have a way of correlating the resultant DC voltage to capacitance.

--john

[frank]

You can also search on the net about the resistance decade box method.  What you will want is finding for given amplitude and frequency what is the "AC resistance".  You measure the current in the loop.  then you replace the cap with a decade box and try to find the resistance that will give the same current.  Then you will be able with those parameters to plug the data in math formulas for caps and find your value.

[DWBH]

There's a schematic somewhere on the gallery to measure capacitors with an ordinary DMM.

[jefe]

There's a schematic somewhere on the gallery to measure capacitors with an ordinary DMM.

Do you mean this?
http://www.aronnelson.com/gallery/main.php/v/NaBos-Layouts/cap_meter_layout_001.gif.html

I haven't tried it myself.

[frank_p]

Albot, check also the time it takes to charge and discharge the cap trough a RC filter.
Look figure 10.9 and 10.8 of this document.  Don't know if you could use a PC-scope (?)

http://teacher.nsrl.rochester.edu/phy_labs/Circuits/Circuits.html

[George Giblet]

Original circuit:
http://braincambre500.freeservers.com/indcap.htm

The cap meter looks pretty good for most people.  (I wouldn't rush out and build the inductance meter though)