Measuring capacitance with a Picaxe

L

lornetw

New Member
Hi everyone.

I wish to make an adapter for my digital volt meter to measure capacitors (just ordinary ones, nothing fancy). I had a DVM that directly measured capacitors but now it doesn't work. I have done a search with "measuring capacitance" both on this forum and the Internet, but only found a couple of references (one of the threads in this forum said it could be done but didn't say exactly how). Anybody ever make one?

I found an old article in (I think) Popular Electronics that used a 555 as an oscillator. I will use that one if I have to but I wanted to use a Picaxe (maybe it would be overkill to use a Picaxe?). Ideally it would be as simple as possible, the value would be read on the DVM.

Any ideas?

Thanks,
Lorne Wilkins
 
W

womai

Senior Member
There are plenty of variations possible, and it also depends what capacitance range you are looking at.

For sufficiently large values, charging the capacitor through a resistor and measuring the time it take to get to e.g. half input level is a valid approach. You'd first discharge it through a much smaller resistor connected to an I/O pin on the Picaxe set to "low", the set this pin into input (making it high impedance) and charging it through a different pin suddenly set to high.

Another option is to generate a sine wave and send it through the capacitor which is in series with known resistor. From measurement of the voltage across capacitor and/or resistor you can figure out the capacitance.

One traditional (and very sensitive) method is a "Wheatstone bridge" (look on Google for this), in this case one side would consist of two known resistors (one variable and one fixed)and the other side of on known capacitor and the unknown capacitor. Again you feed it with a sine wave (or at least some AC signal). Tuning the resistor until the bridge voltage is zero (then the division ratio of the resistive leg is the same as the ratio of the capacitive leg), then you can calculate the unknown capacitance. That method has been around for ages (about as long as electronics itself). Switching between different reference components lets you move the measurement range over many orders of magnitude.

Wolfgang
 
J

jglenn

Senior Member
You might consider charging the cap with a constant current source, many ways to make them, the LM334 is one I have used. There is an opamp circuit too. This way, it eliminates the log curve which will complicate things.

Unless you like to do a lot of math! LOGARITHIMS.

They will charge with a linear function, like a ramp. So you just time from when you start charging, to a certain point, like 2.5V. Then figure out the relationship between charging time and capacitance. Need several ranges to cover from 10 pF to 1000uF.:D
 
L

lornetw

New Member
Thanks for your quick responses!

Thanks you everyone - that will give me enough to get started.

Regards, Lorne Wilkins
 
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