In the last few circuit labs, we measured voltages and currents that did not change with time (DC, or Direct Currents). In fact, more common measurements of voltage, current, resistance, frequency, and the shape of time-varying electrical signals (if they repeat, they are AC, or Alternating Currents) are common to almost any laboratory, regardless of discipline. These laboratory exercises will (re-)introduce you to the use of a very common and useful electrical measuring device, the oscilloscope, and will compare it to uses of the multimeter.
The multimeter, which you have already used, is a device that will measure voltage, current, or resistance. Observe the controls for function selection and scale factor selection on the front of your multimeter. Note what ranges it will cover. Also, where the probe wires connect to your instrument, note the panel markings indicating the maximum allowable current and voltage that can be measured.
In making electrical measurements, always remember the following rules. To measure the voltage across a circuit element, the multimeter must be connected across (in parallel with) that circuit element. To measure current through a circuit element, the multimeter must be connected in series with that circuit element. To measure resistance, the resistor should be disconnected from the circuit and placed between the probes of the multimeter.
If ever the voltage, current, or resistance being measured by the multimeter exceeds the maximum value of the range selected, an over-flow indication "1. " will be displayed. You should then select a higher range. In general, you should start on the highest range possible if you have no idea of the value of the variable you are measuring.
When using the multimeter to read signals changing in time (AC signals), it reads the root mean square or rms voltage - this indicates a way of averaging over time. We will not go into the details here, but it is important to realize that the voltage reading on the multimeter gives the amplitude of an alternating signal divided by the square root of 2. Thus if the amplitude of an AC voltage is 5 V, the multimeter should only read 3.5 V. If you want a detailed reading of a time varying signal from a circuit, you need to use an oscilloscope.
The easiest and most efficient way to become familiar with an oscilloscope ("scope" for short) is to experiment with its various controls. In CircuitMaker, there is really not much to do with the scope. It will automatically appear when you run a circuit that has a time varying signal. You can then place your probe anywhere in the circuit, and the scope will measure. There are cursors on the time (horizontal) scale and voltage (vertical) scale that allow you to quantitatively measure any portion of the graph where you move them.
Background
The essential function of the oscilloscope is to display voltages which vary with time. One source of such voltage functions is a signal or function generator (also known as frequency generator or signal generator).
1. Connect the signal generator (set to output a sine wave) to a resistor in CircuitMaker. Place a ground somewhere in the circuit. Look at the signal by running the circuit. While it is running click with your probe at various places in the circuit. In the image below, the "A" indicates where the probe is when the signal is as shown below (sin wave). If you click a wire at the other side of the function generator, you will get a zero voltage since that is where it is grounded (by definition, ground is zero volts).
2. Attach a capacitor in series with the resistor in your circuit with the frequency generator. We want to measure the signal across the capacitor, so the ground needs to be on one side of the capacitor, and we will measure the signal on the other side of the capacitor. So the capacitor should be on the right of the resistor in the image above, and the ground should be between the capacitor and the signal generator (which is set on square waves). This is what is called an RC circuit, and can be used for timing or filtering, for example.
3. Run the circuit, and place the probe on the other side of the capacitor from the ground. The oscilloscope should show something like a distorted triangle wave. What you are seeing is the capacitor charging and discharging as the function generator goes high and low (on and off).
4. In order to see the capacitor fully charge (it will only charge so much before no more current flows) and fully discharge, you will need to slow down the charging and discharging cycles from the signal generator - this means making the period longer (and corresponding pulse width longer). Increase the period of the function generator by a few milliseconds at a time and run the circuit until you see the full charging and discharging cycle. (It fully charges when it asymptotes to a maximum value, and fully discharges when it asymptotes to zero).
The voltage on the capacitor follows an exponential growth in time according to this, derived in your text:
V = Vo [1 - exp(-t/RC)]
Notice that according to this equation:
when t = R*C,
V = Vo [1 - exp(-1)], and so
V = 0.63*Vo.
You can use this to determine the value of R*C, called the "time constant" for this circuit. Try to find a way to use this expected relation along with the actual measured oscilloscope output to determine the time constant for the circuit.
5. Use the a/b cursors to determine the amount of time it takes for the capacitor to charge to 0.63 of its fully charged value, and compare this to the theoretical value of RC.
