Capacity & Inductance meter....

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cactusface

Senior Member
Hi All,
Has anyone tried or even built a capacity and/or inductance meter?? I thought about using some logic gates to form an osc, then putting the test cap CX in parallel with the main cap and noting the change in freq, minus the main freq. If you know the freq you should be able to calc the CX value! But the maths might be a bit tricky as it usually involves things like PI...

Any help or ideas would be very welcome..

Regards
Cactusface
 
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mrburnette

Senior Member
cactusface said:
Any help or ideas would be very welcome..

Regards
Cactusface
Click to expand...
@Cactusface, this has been around for years: super and will give you some idea of the hardware interface required. The rest is just software. I have not looked at the assembly routines to see how much of the functionality could be replicated by the PICAXE; but because of the critical timing required in software, I am guessing that attempting without an external crystal would not be wise.

- Ray
 
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womai

Senior Member
Here's a pretty good kit - I have one myself; very inexpensive of you don't need the enclosure or the tweezer probes:

http://www.ascel.de/ae20204.html

Assembly instructions are VERY extensive and clear, measurement accuracy is pretty good. Shipping was fast.

It works the same as many other designs out there - just as you describe it creates a resonant tank circuit and measures the change it frequency when an additional capacitance or inductance is added.
 
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g6ejd

Senior Member
It can all get a bit complicated. You can try a simple RC circuit, so apply a logic 1 (5V) to an output pin connected to a R, that is then connected to your test C to ground. At the junction of the RC, connect another pin, this time an input ADC and then start a timer to measure the time it takes to get to some value, say 3.162V (see later), then from the expression:

Vo = Vin (1-EXP(-t/CR) you can work out the value of C because you know R and t, EXP = 2.7182818 Eulers #, so:

C = t / (R * LN(1/(1-Vo/Vin))) Where t is in Seconds, R is in ohms, C in Farads and LN is a natural log, on most calculators.

As the expression LN(1/(1-Vo/Vin)) is a constant if the reading always stops at 3.162V and the supply is 5V, then the equation becomes:
C = t / R and simple integer maths will give a close enough result. The LN bit equates exactly to 1 at that 3.162 volts reading, so just keep testing your ADC until that value is reached and then take the timer value. Some folks will see that the result is the time constant of an RC series network i.e. T = CR

When you have that working you can expand on to higher precision maths, etc.

You can do almost exactly the same thing with inductors, I can show you how to do that if you want.
 
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