Measuring the Value of a Capacitor

Background Theory

When an uncharged capacitor is connected to a d.c. supply through a resistor, the potential difference across its plates (Vc) increases rapidly at first but then slows down until the value is equal to that of the d.c. supply (Vs). When Vc = Vs then there is no current flowing through the resistor and the capacitor is fully charged. The Time Constant of the circuit, t, is defined as the time taken for the value of Vc to reach 63.2% of the final value which can be taken as Vs. We also have that t = RC where R is the value of the resistor in ohms and C is the capacitor value in Farads. For a full treatment of the mathematics behind this definition see the Appendix at the end of this note.

Practical Considerations

The ADC200 oscilloscope can give a time axis of almost any value and so the values of R and C can be any convenient value. It should be noted however that the input impedance of the ADC200 is approximately 1MW and will be connected in parallel with the capacitor. If this input impedance is not to significantly change the maximum value of Vc then the charging resistor should have a value < 1MW by a factor of about 100.

Experiment Setup

Arrange the components to give a series RC circuit as shown. Some means of discharging the capacitor between measurements is required and a flying lead will suffice. In our experiment a 1uF capacitor was used with a 3900W resistor and a 9V battery. The PC oscilloscope settings are typically: Timebase 2ms/div, Channel A input +10V d.c., Trigger Single shot, rising to capture a pulse when GO is selected. Use a sensitivity of 150mVor so at a pre-trigger value of about 5%.

Carrying out the Experiment

Discharge the capacitor by shorting across its terminals with the flying lead. Press the GO button on the oscilloscope screen and close the switch. A trace similar to that shown should be obtained. Adjust the timebase settings as required and repeat the run. To repeat a run open the switch, discharge the capacitor, press GO and close the switch again. Save and print out the screen image.

Calculations

Measurements can either be made from the computer printout or the screen cursors available in PicoScope can be used to determine the maximum value of Vc. Once the time constant value of Vc has been determined the cursors can be repositioned so that the time can be read from the display. Measure the maximum value of Vc reached on the printout. Call this value Vs. Calculate 63.2% of Vs and draw a dotted line across the graph parallel to the x-axis. From the point where the 63.2% line crosses the trace, drop a line down to the x-axis (see diagram above). Calculate the value of t from the graph.

If t = RC then C = t/R

From the example trace Vs = 9V and 63.2% is 5.7V giving a t value of 4.1ms

Since R = 3900 we get C = 4.1 x 10-3 / 3900

Which gives C = 1.05uF

Questions & discussion of results

Further study

The time constant can be derived from the discharge characteristic as well as the charging one except that the time is calculated from the time taken for Vc to fall to 36.8% of Vs. This is less easy to set up since the triggering is more difficult. It could be used with C values of the order of 1000m F where the times are several seconds so that manual triggering could be used. The circuit to be used is changed to that shown. The capacitor is first charged by switching to the battery and then discharged using the switch in the other position.

The circuit can be adapted to small C values if the switch and battery are replaced by a square wave generator. This alternately charges and discharges the capacitor giving both characteristics. Adjust the generator frequency and the timebase controls to get the best trace.

Credits

This experiment was written by the design team at Lascells Ltd. All the equipment to carry out these experiments and the further study projects is available from:

If you have any comments or suggestions for improvements please e-mail experiments@picotech.com

Appendix

Suppose the cell has EMF Vs and the voltages across the resistor and capacitor at any instant are Vc and Vr. As the capacitor charges, the value of Vc increases and is given by = q/C where q is the instantaneous charge on the plates. At this instant (time t) there will be a current I flowing in the circuit.

We know that current is given by the rate of change of charge.So

We also know that Vs = Vc + Vr and Vc = q/C

From Ohms Law we have that Vr = iR

Since Vs = Vc + Vr we get

re-arranged...

This equation can be shown to have the solution

When t = RC the equation becomes

But

i.e. the value of q is 63.2% of the maximum value of Q when t = RC