Foundation Exp 1 Voltage, Current and Resistance

FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE

EXPERIMENT 1

VOLTAGE, CURRENT & RESISTANCE

1.0  PRELIMINARIES

1.1  INTRODUCTION

The voltage (V), current (I) and resistance (R) are fundamental elements of any electrical or electronic circuit. The voltage, measured in volts, will cause a current, measured in amperes, to flow around a circuit. A resistance, measured in ohms, is required to limit the current to safe levels. The three are related by Ohm’s Law, which can be expressed in three ways:

V= IR

I = V/R

R = V/I

Relationships for electric power (P), measured in watts, can be determined indirectly using Ohm’s Law:

P = V ´ I

P = V2/R

P = I2R

1.2  PURPOSE OF THE EXPERIMENT

This experiment is intended to:

-  Provide experience of conducting electrical experiments and presenting results

-  Provide experience in the use of basic laboratory equipment

-  Develop familiarity with the basic units of measurement of voltage, current and resistance.

1.3  PROCEDURE

There are several sections to this experiment. You should follow the instructions given so that you obtain the experimental data for each part. This must obviously be done in the lab. Outside the lab you may have to plot graphs and write the results up in your lab book as a report, adding conclusions and comments as suggested.

2.0  TEST 1: RESISTANCE MEASUREMENTS

2.1  PREREQUISITE

You will be using a Digital Multi Meter (DMM) on many occasions. The DMM in the lab is part of a multifunction workstation, which includes three other built in features: a function generator, a frequency counter, and a DC power supply. The DMM is located in the top left quadrant of the workstation. There are two types of workstation in the lab, both made by S.J Electronics, the Mark II and the newer Mark IV. Please identify which workstation you will be using, and make a note of this in your lab book.

2.2  PART 1 – SET UP AND RESISTANCE MEASUREMENTS

a)  The on/off switch for the whole workstation is independent of the on/off switch for the DMM. Locate the on/off switch for the DMM only, and press to activate it and hence its display panel. Set the DMM to read OHMs/W (using the dial selector on the Mark II, or the button selector on the Mark IV).

b)  Find two test leads (one red and one black) with 4mm plugs at both ends, and on one end of each connect a croc clip. The other end is to be plugged into the appropriate terminals on the DMM for resistance measurement, i.e. black to COM and red to V/W.

c)  Fetch

1)  A 680W (in the drawer marked ‘680R’) or 820W

2)  A 2.7kW (in the drawer marked ‘2k7’) or 3.0kW

3)  A 1.5MW (in the drawer marked 1M5) or 2.2MW resistor.

d)  Connect the 680W (or 820W) resistor between the two croc clips and record the DMM display. Repeat for the 2.7kW (or 3.0kW) and the 1.5MW (or 2.2MW) resistors, each time recording your results in your lab book.

Note how the DMM automatically sets the range for the different resistors measured.

The resistors in the lab have a tolerance of ±5%, i.e. a resistor whose stated (nominal) value is 1kW (1000W) may in fact have an actual value anywhere in between 1050W (+5%) and 950W (-5%). With this in mind, complete the table below in your lab books for the resistors you have chosen in part c):

Nominal value W / Max. possible value (+5%) W / Min possible value (-5%) W / Measured value W / Actual % error

Table 1

2.3  PART 2 – COMMENTS AND CONCLUSIONS

This part is to be written up outside lab time. Comment on the consequences of percentage errors and tolerances, especially in relation to Ohm’s Law.

3.0  TEST 2: BREADBOARD CONNECTIONS

3.1  PREREQUISITE

Breadboards can be purchased from the lab technician for a moderate fee. Most of the circuits you will be using will be constructed on breadboards (Figure 1), so it is essential to know how they function electrically.

Figure 1

3.2  PART 1 – CENTRAL BREADBOARD SOCKETS

a)  Set the DMM to read OHM/W.

b)  Find two test leads, one red and the other black, with 4mm plugs at both ends. On one end of each lead, attach a croc clip. The other ends are to be connected to the correct terminals on the DMM.

c)  Find two short lengths of wire (about 2 inches/5cm long) and strip the insulation from each end (about 5mm) and fit an end into each croc clip.

d)  Push the free ends of the two short wires into the pairs of breadboard sockets as indicated in Table 2, and thus measure the resistances between the various sockets

Connection 1 to socket / Connection 2 to socket / Resistance value W
D3 / A3
D3 / B3
D3 / C3
D3 / E3
D3 / F3
D3 / H3
D3 / D2
D3 / D4

Table 2

e)  Based on your observations, which of the following connections are correct practice when connecting the ends of a resistor into the breadboard, and explain why?

1. A3 and D3
2. D3 and B3
3. C3 and D3
4. D3 and E3
5. H3 and D3
6. D3 and D2
7. D3 and D4

3.3  PART 2 – OUTER EDGE BREADBOARD CONNECTIONS

Use a similar procedure to that above to find out where (a) there are, and (b) there are not connections between the lines of sockets along the long outside edges of the breadboard.

3.4  PART 3 – COMMENTS AND CONCLUSIONS

These comments and conclusions should be written up outside lab time.

a)  What is the resistance value, in Ohms, of a ‘short circuit’ and of an ‘open circuit’?

b)  How many sockets are there in total on your breadboard?

c)  State which groups of sockets have a short circuit between them, and where there are open circuits between sockets.

d)  Sketch two diagrams on how you could connect three resistors in series (adjacent to each other) using i) the coloured sets of connectors at the sides of the breadboard and ii) the lettered sets of connectors at the centre of the breadboard.

4.0  TEST 3: VOLTAGE, CURRENT AND POWER IN RESISTORS

4.1  INTRODUCTION

A basic electrical circuit will be connected up and the effects due to various changes in the circuit parameters investigated with the results presented in graphical form.

4.2  PART 1 – TESTS WITH CONSTANT RESISTANCE

a)  Fetch a 680W or 820W resistor (record which one you use) and connect it to sockets G2 and D2 (or equivalent) of a breadboard.

b)  Use the DC power supply located in the bottom right quadrant of the S.J Electronics (MK II and MK IV) workstation, as shown below in Figure 2.

Figure 2

c)  Using appropriate sockets and test leads (one red and one black), connect a resistor into the breadboard with croc clips placed on each leg. Do not connect anything to the DC power supply at this stage.

d)  The DC power supply, as shown in Figure 2, has three sets of terminals, each with a pair of red (+) and black (-) sockets. The top red and black terminal provides a constant 5V DC output that cannot be adjusted. The second pair of terminals provide a 15V DC output that also cannot be adjusted. The lower pair of terminals are capable of producing a maximum DC output voltage of 30V, which can be varied to anywhere between 0V and 30V by using the dial marked ‘VOLTAGE’, situated to the left of the fixed 5V DC output terminals. Ensure that the ‘CURRENT’ dial is in the 12 ‘o’ clock position, i.e. pointing ‘north’.

e)  Set the DMM to measure voltage. On the MK II, this requires that the dial is turned to the ‘V’ position, whereas on the MK IV, select the ‘DC V’ button so that ‘V’ is shown on the right of the display panel.

f)  Plug the 4mm red test lead plug into the V/W socket of the DMM, and the black plug into the ‘COM’ socket of the DMM. Set up the DMM and DC power supply to measure voltage across the resistor, as shown in Figure 3. Turn the ‘VOLTAGE’ dial to minimum (several turns anticlockwise), and have your circuit checked by a member of staff before proceeding.

Figure 3

g)  Switch on the DC power supply. Slowly increase the voltage (by turning the ‘VOLTAGE’ dial clockwise) to the first required value (2V). The power supply has its own voltmeter display, but use the DMM display panel to record your voltage in a table like Table 3 in your lab book.

h)  Without changing the ‘VOLTAGE’ dial, switch off the DC power supply and unplug the DMM from the circuit. You must now set the DMM to measure current. On the MK II, the DMM dial should be turned to the mA position (to read milliamps, ´ 10-3A), whereas on the MK IV, the ‘DC A’ button should be pressed until the green mA indicator appears on the right of the display panel. Connect your circuit as shown in Figure 4 below in order to be able to measure current

Figure 4

i)  Note that measuring current involves a different circuit configuration to that of measuring voltage. Here, the DMM in ammeter (device for measuring current) mode is situated in series with the resistor. This set up first requires ‘breaking’ the circuit and then ‘remaking’ it so the ammeter is in the correct position. To measure voltages, the DMM in voltmeter (used to measure voltages) mode is placed in parallel with the resistor. Once you have constructed the circuit as shown in Figure 4, have it checked by a member of staff. Then switch on the DC power supply and record the current as required in Table 3, in your lab book.

j)  Repeat the voltage and current measurements to complete Table 3. Take readings for power supply voltages of 2V, 4V, 6V, 8V, 10V and 12V. You should plot a ‘control graph’ of V versus I in the lab so that any dubious points can be immediately checked out (e.g. the voltage control might have been accidentally varied – note comment in Appendix 4). This needs to be done now as points cannot be checked after you have left the lab. Have your control graph verified by a member of staff. Do not forget to note down the value of the resistor you used for this part of the test.

Nominal voltage V / Accurate Voltage V / Measured current mA / V / I ratio / V ´ I product

Table3

4.3  PART 2 – TESTS WITH CONSTANT VOLTAGE

a)  Fetch the following six resistors: 2.2kW, 1.8kW, 1.5kW, 1kW, 820W and 680W.

b)  Draw Table 4 in your lab books.

Nominal Resistance W / Measured Resistance W / Measured Current mA / V / I ratio / V ´ I product

Table 4

c)  Use the DMM to measure the resistor values, and record your results in the table.

d)  Set the variable DC supply, VS, to be 5.0V £ VS £ 6.0V. Use the DMM to measure the actual value, and record it in your lab books. It is important that this stays constant for the entire test.

e)  Reset the DMM accordingly and use it to measure the current for each resistor value and for the same VS. Record all the values in tabular form (Table 4) in your lab books, and plot a control graph of I versus R in the lab so that any dubious points can be immediately checked out.

4.4  PART 3 – COMMENTS AND CONCLUSIONS

Note the instructions in Appendix 1 about presenting graphs. Plot the following five graphs:

a)  I versus V and power versus V for Test 3, Part 1

b)  I versus R and power versus R for Test 3, Part 2

c)  Power versus 1/R for Test 3, Part 2

Comment on the shape of the graphs, relating those to Ohm’s Law and the equations for power. Also explain the ‘first break circuit then remake’ technique needed to make a measurement of current (Test 3, Part 1).

5.0  TEST 4: RESISTOR COLOUR CODE SYSTEM

This part of the experiment can be done outside lab time. No experiments are required here. Draw these tables in your lab book, and complete them. Use the colour code system outlined in Appendix 3.

1st Digit
Band 1 / 2nd Digit
Band 2 / Multiplier
Band 3 / Tolerance
Band 4 / Resistor Value
Black / Brown / Black / Gold
Orange / White / Brown / Brown
Brown / Black / Red / Gold
Yellow / Violet / Green / Red
Orange / Orange / Orange / Silver
Brown / Black / Gold / Gold
Brown / Black / Yellow / Silver
1st Digit
Band 1 / 2nd Digit
Band 2 / Multiplier
Band 3 / Tolerance
Band 4 / Resistor Value
680kW, 5%
1.2kW, 10%
1.2W, 10%
820W, 1%
1.5MW, 10%
1W, 5%
3.9kW, 20%
Nominal Value / ± Tolerance / Minimum value / Maximum value
680kW / 5% / ? / ?
820W / 1% / ? / ?
3.9kW / ? / 3.822kW / ?

APPENDIX 1: GRAPH PRESENTATION PRINCIPLES

Graphical presentation of results is very important, and the standard presentation guidelines should be followed:

a)  Plot the quantity being directly controlled by the experiment along the horizontal (x) axis, and the quantity that responds along the vertical (y).

b)  Axes must be marked up with

1.  The numerical value of the main marker lines on both axes

2.  The quantity being measured

3.  Its unit in brackets

c)  Identify the measured points by drawing a small circle round each point.

d)  Provide a brief heading for each graph (e.g. Plot of x against y for z held constantly).

APPENDIX 2: COMMON SYMBOLS FOR MULTIPLIERS

Upper case / G / Giga / 1,000,000,000 / 1´109
Upper case / M / Mega / 1,000,000 / 1´106
Lower case / k / kilo / 1,000 / 1´103
Lower case / m / milli / 0.001 / 1´10-3
Greek letter mu / µ / micro / 0.000 001 / 1´10-6
Lower case / n / nano / 0.000 000 001 / 1´10-9
Lower Case / p / pico / 0.000 000 000 001 / 1´10-12

APPENDIX 3: RESISTOR COLOUR CODE