Electronics Activity

The Capacitor

Objective

To determine the charging and discharging properties of an electrolytic capacitor in a simple resistive circuit

Background

Materials

  • Proto board
  • USB Oscilloscope
  • 100R & 330R resistors (R1)
  • 3 volt battery pack - Note: any battery voltage up to 12v can be used!
  • 1000f & 470f electrolytic capacitors (C1)
  • momentary contact switches (S1- S2)

Part 1 - Charging the Capacitor

Configure the proto board as in the following illustrations. Note: Some illustrations have a link to a larger image.

Because you have 2 capacitors and 2 resistors, there are 4 combinations that can be tested. It doesn't matter which you use to start.

Larger Image

Larger Image

Plug the USB probes into channel A. You can use the standard probes (alligator clips) or the high impedance probes (make sure they are set to x1 NOT x10!). Plug the USB cable into the computer. Run the Oscilloscope software.

Set the oscilloscope controls (see the following image to double check)

  • Channel A
  • Time Base: 50 ms/div
  • Voltage: 1 v/div
  • Trigger: Channel A, +ve
  • Trigger Level: 200mvge

  • Open the Samples menu and select 2K.

Take a test trace. Make sure the capacitor is fully discharged by pressing and holding switch S1.

Click the Run button. The screen should not show a trace. Press and hold S2 to charge the capacitor. You should see a trace similar to the following:

Click the Stop button. (Note: if you didn't get a trace check the circuit. Make sure the capacitor was fully discharged. Use a voltmeter to check the wiring and voltages.)

Analysis

It is assumed you have read the background information.

You need to find the maximum voltage across the capacitor when it is fully charged. From the trace shown above it is clear that the traced ends before it "flattens out". You can scroll the display by clicking and moving the scroll button below the display screen.

Moving the display to the left (by sliding the scroll button to the right) should reveal the point where the trace flattens or maxes out.

You could estimate the maximum voltage but the software has a set of cursors that can be used for that purpose. Click the Cursor A button to display the horizontal cursors.

Move one of the cursors (A1) so that it superimposes on the trace at the highest point where it flattens out. Read the voltage (at the bottom of the display).

Calculate 63% of the maximum voltage and move the cursor (A1) to that point.

Now you need to find the time it takes the capacitor to charge to 63% of the maximum voltage. Click the Cursor X button to display a set of vertical cursors. The vertical cursors intersect the X axis and therefore measure time

Place cursor 1 at the start of the curve and cursor 2 at the point where the cursor A1 crosses the trace.

The difference in these 2 cursors is displayed at the bottom of the screen as Delta (change in time). This is the time constant (τ) of this capacitor.

Record the details of this trial, R1, C1, and τ (measured) in the table (Part 1) which can be displayed and printed from the following link.

Recording Table

PART 2 - Discharging the Capacitor

When a capacitor discharges through a resistor, the same time constant (τ) can be used to describe how the voltage decreases with respect to time.

Set up the circuit as shown in the next illustration sequence.

Large Image

Note: the oscilloscope probes are on the resistor

This time the capacitor will be charged to the full voltage and then discharged through a resistor. Again, using the capacitors and resistors in the materials list you can do 4 trials.

Because the trace is starting from a high voltage you will need to know what maximum voltage to set the trigger level on the oscilloscope. You can get this voltage from part 1 or measure it with the oscilloscope or voltmeter before you begin.

Set the oscilloscope controls (see the following image to double check)

  • Channel A
  • Time Base: 50 ms/div
  • Voltage: 1 v/div
  • Trigger: Channel A, -ve
  • Trigger Level: 2.9 volts (if max voltage = 3v. Set it about .1v below max)

Take a test trace. Make sure the capacitor is fully charged by pressing and holding switch S1.

Click the Run button. The screen should not show a trace. Press and hold S2 to discharge the capacitor. You should see a trace similar to the following:

Analysis

Use the slider to check that the voltage drops off to 0.

Turn on Cursor X and Cursor A. Using one of the horizontal cursors (ie. A2) find the maximum voltage.

Calculate 37% of the maximum voltage (the capacitor should lose 63% of its voltage in the first time constant (τ)). Set the horizontal cursor at the calculated voltage

Position the vertical cursors at the discharge start point and where the horizontal cursor (A2 in the illustration) intersects the discharge curve.

Record R1, C1, and the difference in time (Delta) in the appropriate locations in the recording table (Part 2).

Repeat for the other combinations in your kit

Final Analysis

  1. Using the nominal values for R1 and C1, calculate the time constant τ and record in the τ (calculated) column.
     
  2. The components used have nominal values but their actual value can be different. Measure the resistor values with a digital multi-meter. If you have a capacitance meter measure the capacitor values and re-calculate the time constant and record in τ (precision).
     
  3. How do your measured values compare with the nominal and precision values?