Capacitance Meter

Did you make sure that the capacitors were fully discharged before you started?

How accurate is your resistor?

Did you accurately measure your voltages for charging and discharging.

There are a lot of variables that you need to control for accuracy.

DAB

Bulk e-lytics aren't manufactured precisely.  If this always happens, maybe one of your trims is off.

• Some large capacitors have relatively high inductance that can limit high frequency response. If your measurement circuit uses very high frequencies your readings may be low.
• If your circuit uses small currents for short durations to measure capacitance, the voltage on a large capacitor will hardly change at all - if you are measuring these small voltage changes, your measurement (A/D) may not have enough resolution to be accurate.
• If your circuit is measuring the charging time of the capacitor, a large capacitor will spend a long time in the vicinity of the threshold voltage where you decide that the capacitor is charged. Any electrical noise on the charging signal can cause the threshold to be detected significantly early.

It might help if you provide the measurement method - circuit and algorithm.

Hi Michael - It might be interesting to graph your readings versus stated values for a range from small to large capacitances. This might give you a feel for how the readings are diverging and the rate of divergence. Sometimes this type of information can help locate and fix the problem. If you are using the Arduino for timing you might look closely at how you are doing the timing. I learned recently on this forum that delay() can sometimes throw things off.

John

The circuit is measuring the charging time of the capacitor.
Here is the algorithm: http://code2flow.com/xp9FLZ.code.png

Hi Michael - I looked at your schematic and I am questioning why the 5 volt output from your powersupply is being fed to the Vin input on the Arduino board. This routes through a second 5 volt regulator on the board. It may present a problem for the arduino to regulate its 5 volt supply if it only has 5 volts on the input. It may be better to route your power supply output directly to the 5 volt pin on the Arduino. Whether this is having any effect on your readings is unknown.

John

After writing this I decided to check a little further and it seems that the UNO R3 has a parallel regulator that may handle situations where Vin is close to 5 volts. I will leave my post in place as Doug will be able to tell us for sure if this is the case. In the mean time don't make any changes.

Thanks

Forgive John, he achieves mightily given the burden he is under:  Lurking by the light of his twin Brett Farve - Bart Starr desklamps in the rec-room of his doublewide igloo, the literally beady eyes of the wall fauna gazing sternly, if uncomprehendingly, upon him.  Red State blanket on the plaid couch.  At least he isn't wasting his life constructing snippy little TLAs referencing his neighbors to the south as so many do.  Or constructing ghastly/cool dioramas of squirrels playing cards by the flicking light of old beer signs.

And now a word from RAP, PRAB!

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CCMQFjAC&url=http%3A%2F%2Fninova.itu.edu.tr%2Ftr%2Fd…

Don gets a little giddy everytime he survives a weekend in Chi-town. Bullets wizzing everywhere.

What type of capacitors are you measuring?

Some capacitors such as MLCC have values that change depending on applied voltage.