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A Low-Capacitance Adaptor for DMMs

This neat little adaptor allows a standard digital multimeter to measure low values of capacitance - from less than one picofarad to over 10nF. It will allow you to measure tiny capacitors or stray capacitances in switches, connectors and wiring.

By Jim Rowe

Specifications

Three measuring ranges –  

Range A:  0.1pF = 1mV, [gives a range from below 0.3pF to above 100pF.]
  
Range B:  1pF = 1mV, [gives a range from below 1pF to above 1000pF (1nF)]
  
Range C:  10pF = 1mV, [gives a range from below 10pF to above 10.0nF.]

Accuracy:  Within approximately 2% of nominal full scale reading,
(assuming you can calibrate ranges using capacitors of known value).

Power: 9V alkaline or lithium battery.

Current drain: less than 5mA.

Although some modern digital multimeters do provide capacitance measuring ranges, these are generally not particularly useful when it comes to measuring low value capacitors or the stray capacitance associated with connectors, switches and other components.

For most of these small capacitance measurements you normally need to use a dedicated low-value capacitance meter and these can be a bit pricey.

The Adaptor is easy to build, with all of the components mounted on a small PC board. The board fits into a box which is small enough to be used as a dedicated ‘low capacitance probe’ for the DMM, making it well suited for measuring stray capacitances. Just about any modern DMM is suitable for the Capacitance Adaptor, provided it has an input resistance of 10MΩ or 20MΩ.

Click for larger image
Click for larger image

How it works

Essentially the Adaptor works as a capacitance-to-DC-voltage converter, as shown in Fig.1.

First we generate a square wave ‘clock’ signal with a frequency of between 110kHz and 1.1kHz (depending on the measuring range) using a simple relaxation oscillator based on capacitor C1, resistor R1, trimpot VR1and a Schmitt trigger inverter. This square wave signal is then passed though a Schmitt buffer stage to ‘square it up’ and produce a waveform with very fast rise and fall times.

The output from the Schmitt buffer is then split two ways and passed through identical resistors R2 and R3. Then they are fed to the two inputs of an exclusive-OR (XOR) gate. The signal which passes through R2 has a small trimmer capacitor VC1 connected from the ‘output end’ of R2 to ground, while the signal which passes through R3 has the capacitance which is to be measured connected from the output end of R3 to ground (ie, between terminals T1 and T2).

So each signal is fed to the inputs of the XOR gate via an RC delay circuit. The combination of these two RC delay circuits and the XOR gate form a simple ‘time delay comparator’.

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