DC Circuit Pre-lab Exercise
last updated Sept. 2, 2002
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This exercise is designed to help you understand how to operate the equipment used in the DC Circuit lab. Read the selection below and answer the questions at the end.  You may wish to print this page and keep it for future reference, as we will use some of this equipment in later labs.

1. Circuit elements - in the DC Circuit lab, we will be using three basic circuit elements: batteries, resistors, and wires.
Circuit Element
Photo of Element
Circuit Symbol
Description
Wire
wire
wire symbol
A circuit element used to connect other circuit elements. Assume that wires have no resistance.
Resistor
resistor
resistor symbol
A resistor is has many uses. In this class, it is used as a protective or dissipative device. The resistance, measured in Ohms, can be found by looking at the color bands. 
Battery
battery
battery symbol
A battery is a DC voltage source. The long line means + side.

2. Using a tester board - We could connect everything in the DC Circuit Lab by twisting wires together, but this can lead to a huge mess of tangled wires. To avoid this and to make the circuit easier to construct and test, we use a tester board (sometimes called a breadboard or a perf. board).
tester board
detail of the tester board
The tester board consists of 4 rows of electrically connected holes.
 The above picture shows the five holes in a row which are electrically connected. These five holes are not connected to the row below them, nor to the row next to them. (Electrically connected means that you can count this as a wire connecting all the holes.)

Examples of the tester board in use:
the correct circuit
good circuit on tester
This is a good circuit
bad circuit
bad circuit on tester
These are 2 bad circuits. The wire doesn't connect any elements and is not a circuit. The resistors have been shorted. (Shorted means that one side is directly connected to the other.)

3. Using multimeters:
A multimeter is a device that measures voltage, current, and resistance (i.e. it can act as a voltmeter, an ammeter, and an ohmmeter). The picture below shows one of the two multimeters we will be using in this class.
multimeter It can read:
  • voltages between a few millivolts to 1500 V, both AC and DC 
  • currents from a few microamps to a maximum of 10 A, both AC and DC. (Do not, under any circumstances, exceed 10 A!!) 
  • resistances from a few Ohms up to 20 megaohms.
How to use the multimeter:

multimeter face detail
1. Select the setting you want to use. (DCV, DCA, ACV, etc. as shown).
2. Select the range of voltages or currents you are going to measure. If you are uncertain of the range, then select the highest possible range available.  The range is indicated on the dial. If the number says 200 in the DCV setting, then that means that the multimeter can only read up to 200 V in that setting.
In both of the examples below you are reading the volatge of an AA battery.
Example #1: You know that the voltage you are reading is no more than 2 V, so you turn the dial to DCV to the 2. If you accidentally switch to the 200 mV setting, it wouldn't damage the meter, but the reading would say "OL" meaning overload.

Example #2: If you had no idea what the voltage was, you could start at DCV at the 1500 V range. The reading at this range would probably say "2", with no decimal places. You can then switch to the 20 V setting. On the 20 V setting, the voltage would probably read "1.50" V. Now you know that the voltage is under 2 V, and you can switch down to the 2 V setting. On the 2 V setting the voltage reading would be given to three decimal places: "1.500" V.

3. Plug in two wires with banana connection s into the terminals.
multimeter terminal detail
  • Plug in one of your wires into the black COM terminal. 
  • If you are reading a voltage or a resistance, then plug in the other wire into the red (V - omega )  terminal. 
  • If you are reading a current under 2 A, plug in the other wire into the A terminal
  • If you are reading a current under 10 A, plug in the other wire into the 10 A terminal 

4. Connect your wires to your circuit.
Connect your multimeter as shown below.
Reading a voltage or resistance
Reading a current
reading a voltage
reading a current
One lead goes to either side of the element:
  • You will always get a positive reading for resistance
  • A positive reading of voltage means that you have connected the + terminal (red) to the side of the element with the higher voltage.
  • A negative reading of the voltage means that you have connected the + terminal to the side with the lower voltage
The ammeter replaces the connection between the element and the rest of the circuit
  • A positive reading of current means that the current is flowing into the + terminal and out of the COM (black) terminal
  • A negative reading of current means that the current is flowing into the COM and out of the + terminal
Questions:
1. Draw lines (wires) to connect the elements into the circuit given: (the A with the circle around it means the galvanometer = green boxy thing)
dc ohmmeter
galvanometer
battery
Rk: resistor
Rx: resistor
tester board


2.  Connecting the multimeter
(a) Show how you would connect the multimeter to measure the voltage of the resistor by drawing lines to indicate connections.
 (b) Show how you would connect the multimeter to measure the current going through the resistor by drawing lines to indicate connections.
multimeter terminals
multimeter terminals

resistor
resistor
3.  Circle all that apply. When connecting a voltmeter to a circuit which may have high voltage you should:
a. Connect your terminals to either side of the element
b. Connect your voltmeter by replacing the connection the element has to the rest of the circuit.
c. Use your fingers to hold the connection in place.
d. Use a clip or other "hands off" connection to hold the voltmeter in place.
e. Turn the circuit off before connecting the voltmeter.
f. Turn the circuit on before connecting the voltmeter
g. Use the voltmeter on the highest 1500 DCV setting.
h. Use the voltmeter on the lowest setting, 200 mV.
i. Run screaming from the room because the voltage is just too high.