Low frequency capacitance meter adapter

May 21, 2015

Contents

0.1 How to use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

0.2.1 Low or no voltage output . . . . . . . . . . . . . . . . . . 10.2.2 High voltage output . . . . . . . . . . . . . . . . . . . . . 20.2.3 Fluctuating output . . . . . . . . . . . . . . . . . . . . . . 2

0.3 How does it work . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.4 appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

0.1 How to use

Turn on capacitance meter using switch.Attach banana plugs to the red and black connectors at the end of the box.Use red for positive and black for negative. Attach the capacitor to the greenconnectors at the top. This can be done using banana-to-crocodile cables or byusing the screw holes on the connector.Ensure the voltmeter has a full scale deflection of 200mV. Adjust the sensitivitydial to give a so that a readable value is shown on the voltmeter.The sensitivities are shown as the maximum possible readable values Use thetable below to convert the voltage to capacitance.

Sensitivity Capacitance per millivolt20uF 100nF2uF 10nF200nF 1nF20nF 100pF2nF 10pF200pF 1pF

0.2 Troubleshooting

0.2.1 Low or no voltage output

potential Causes:-device may be off-battery may be flat-sensitivity may be too low

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Solutions:-ensure device is switched on-ensure sensitivity is high enough-replace battery

0.2.2 High voltage output

Potential causes:-sensitivity too high-if at lowest sensitivity then capacitor may be above measurable range

Solutions:-reduce sensitivity

0.2.3 Fluctuating output

Potential causes:-bad connection

Solutions:-ensure all connections are secure

0.3 How does it work

This circuit contains a Schmitt trigger/integrator oscillator that is used to out-put a stable triangle wave. This output is then passed into a differentiatorthat gives an output that is proportional to both the capacitor being measuredand the resistance vout = RC dVindt , which can be changed by modifying thesensitivity. Differentiating a triangle wave gives a square wave output with anamplitude between 0 and 200mV. By passing this through a precision rectifier, aconstant DC output can be obtained. The device is powered using a 9V batterywhich, by use of a buffer amplifier, creates 3 voltage rails of 4.5V, -4.5V and 0V.

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0.4 appendices

Circuit diagram3

Component ValueR1, R2 1MR3 1.8MR4, R5, R7, R8, R9, R10 10KR6 20KSELA 100SELB 1KSELC 10KSELD 100KSELE 1MSELF 10MC1 20nFD1, D2 1N0041LED1, LED2 Yellow(voltage drop 1.8V)op-amps 2x tl074

Component values table

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