Simple Voltage and Current Measurement
The objective of this experiment was to measure the Voltage and Current. Upon completion of this experiment I was able to:
1)Set the DC power supply to a specific voltage.
2)Properly connect the voltmeter to measure voltage.
3)Measure current with the ammeter.
4)Measure resistance with the ohmmeter.
5)Determine the accuracy of a given meter reading.
The theory required for this experiment was an understanding of Ohm’s Law. Ohm’s Law is the algebraic relationship between voltage and current for a resistor. Resistance is the capacity of materials to impede the flow of current or electric charge. Ohm’s Law expresses the voltage as a function of the current. It was also necessary that the concept of measurement accuracy be understood. This is discussed below.
Accuracy is of primary importance in an experimental work. The tolerance
quoted by the meter manufacturer allows us to calculate the accuracy of any reading
taken with that particular meter. For example, assume that the dc voltage scale on a
particular multimeter is rated at 3% of full scale. This means that a reading on
the 10V scale is accurate to ( 0.03%)(10) = 0.3V. Thus, a reading of 9V on
the10V scale indicates a true voltage, which lies between 8.7 and 9.3 V. A reading of 1V
on the scale would indicate a true voltage between 0.7 and 1.3 V. At this point, the error
is 30%! Any reading less than 10% of full scale should be viewed with suspicion
since most meters are very inaccurate n this range.
For this experiment we used a Power supply source, voltmeter/ammeter/ohmmeter.
Fig 1.1 Power Supply and voltmeter in parallel
Fig 1.2 Simple voltage measurement circuit.
b a ba a
Fig 1.3 Simple current measurement circuit
(Be sure currentI
control is at
Fig 1.4 Circuit to measure resistance
To measure the voltage output of the power supply, we connected the voltmeter as shown in Fig. 1.1. We made sure that the voltmeter is always connected in parallel with the voltage being measured. Before turning on the power, we set the voltmeter voltage range to a DC value higher than the highest voltage we expected to measure. This precaution must be observed with all meters in order to avoid the possibility of burning out an expensive instrument.
We then set the current control to maximum current and adjust the output voltage of the supply to values of 2.5, 10, and 15. We made sure to set the range switch of the voltmeter to a suitable scale before changing the voltage. By using 3% tolerance for the meter, we determined the range of the true voltages at each setting. The values are showing in Table 1.1.
We then set up the circuits shown in Fig. 1.2. We measured and recorded the voltage and placed the values in Table 1.2 for:
(a) Vab = Vdc = 3V
(b)Vab = 5V, Vdc = 10V
To measure the voltage output of the power supply, we connected the ammeter as shown in Fig. 1.3. In order to measure the current, we made sure to insert the ammeter in series in the wire in which the current was flowing. By varying the supply, Vs, between 0 and 10 V we measured and recorded the current, I, and placed the values in Table 1.3.
To determine the resistance we set up the circuit in Fig.1.4 where RX is any resistor with a value between 1 kU and 10 kU. We pretend that Rx in unknown. For 4 different values of Ix, we recorded Vx in an appropriate table by measuring with voltmeter/ammeter. We then plot a Ix x Vx graph. We made sure that the points on the graph lie on a straight line of slope 1/Rx . We noticed at the end that the resistor was 2 k.
Table 1.1 Power Supply versus Voltmeter voltage (Fig 1.1)
Meter Reading, VTrue Voltage Range
Table1.2 Simple voltage measurement (Fig 1.2)
Vab Vdc Vac Vad
3 3 6.21 0
5 10 0 -4.96
Table 1.3 Current measurement (Fig 1.3)
Resistance kU Vs I (mA)
1 2 2.04
1 4 4.9
1 6 5.99
1 8 8.01
1 10 9.98
Table 1.4 Resistance measurement (Fig 1.4)
Ix (A) Vx (V)Rx (U ) Slope of the line
1.02 2.04 2.003
2.02 4.04 1.991
3.04 6.04 1.993
4.04 8.01 1.993
In this experiment I learned the basics theory behind Ohm’s Law, and I put it to use in the experiment. I also learned how to use measurement devices such as voltmeters, Ohmmeter, and ammeters. Some error in the lab was cause by the inaccuracy of the dial on the power supply that’s set the initial voltage.
Answer to Questions
1) The precautions that must be taken before connecting any meter into a circuit is to set the meter at its maximum current so we do not damage the meter.
2) A voltmeter is connected in parallel to measure unknown voltage.
3)It is bet to obtain a meter reading as far upscale as possible because the needle of the meter castes a shadow on the meter and it makes it difficult to read.
4) An ammeter is connected in series to measure unknown current