PHYSICS 220/230
Lab 4: DC Circuits

Introduction: 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.

Objective: You are asked to do the following with the equipment provided.  Devise your own procedure for parts 1) and 2) and record them in your notebook. 

1) Measurement of Resistance: Use the resistance mode of the FLUKE multimeter to measure the unknown resistances R1, R2, R3, RB, and RA.  Note that the multimeter has its own internal voltage source (a battery), so no external power supply is needed to make these measurements. 

2) Measurement of Current and Voltage:  Build a series/parallel resistor circuit using the Science Workshop Interface as your power supply and with resistors R1, R2 and R3 in series with each other, and with RB attached in parallel with R2.  Set the voltage of your source to 5V on the interface and measure its value.  Using the measured values of resistance from part 1), predict the results of your future measurements by calculating in your notebook the voltage across and current through each component (source, R1, R2, R3, and RB).  Then use the multimeter to measure the currents and voltages.  Use the FLUKE multimeter in its voltage mode to measure voltage.  Use the FLUKE meter in its current mode to measure current.  Note:  Many students find it a challenge to measure the current in R2.  You will need to modify your circuit in order to make this measurement.

3) Voltage divider:  Frequently in DC circuits a fixed supply voltage is available, but a lower voltage is needed.  One way to achieve a lower voltage is to build a circuit which divides the given supply voltage between two resistors in series. By appropriate choice of these resistors the given supply voltage can be divided into two voltages of any desired ratio. Such a circuit, known as a "voltage divider", is one of the most ubiquitous electronic components. 

To investigate the properties of a voltage divider, build a series circuit with the Pasco interface and RA and RB. This circuit is the prototype for the voltage divider. Assume that the voltage across RB is the output voltage of your divider and that the voltage from the Pasco interface is your (input) supply voltage. This output voltage would usually be connected to some device (called the load).           

                                                                   

Before you make any measurements, derive a theoretical expression and find a value for the ratio of the output voltage (Vout) across the load to the input voltage (Vinput) for this circuit, in terms of only RA and RB.  For this derivation, neglect the effect of a load connected in parallel with RB. (Ideally the resistance of the load is large compared to RB.) The result of this calculation will show you why the device has its name. Do your measurements confirm your prediction?

4) Potentiometer:  Now consider what you would do if you wanted to make a device that would allow you to continuously vary the output voltage across RB over a wide range, given the same input voltage. This would be what you would want to do for the sound level of a stereo, the output of the Pasco interface, or the light dimmer switch on the wall. You will quickly come to the conclusion that physically changing RB each time is not what you want to do. A more useful voltage divider can be built, however, from a single variable resistor called a potentiometer. Several potentiometers will be shown to you in the lab.  (See the picture at the top of the next page.)

These potentiometers have three terminals. The two outside terminals give you the fixed resistance of the whole device. The middle terminal (the "slider") allows you to use the resistance across only part of the device and the screw allows you to adjust this resistance to vary the voltage output.  Investigate the properties of potentiometers by measuring the fixed resistance of the two potentiometers that are on your lab desk.     

                                         

For each potentiometer, place the two outside terminals in a series circuit with the Pasco interface.  Adjust the output voltage between the middle  terminal and one of the outer terminals so that it is about 20% of the input voltage. Then simulate the addition of a "load" on the circuit by connecting RB across the output voltage. What does the addition of the load do to the value of the output voltage? Which potentiometer do you think would make a better voltage divider in a real circuit?