# Simple Voltage and Current Measurement

Topics: Ammeter, Resistor, Electrical resistance Pages: 5 (958 words) Published: April 2, 2002
Experiment I
Simple Voltage and Current Measurement

Objective

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.

Theory

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.

Circuit Diagrams

For this experiment we used a Power supply source, voltmeter/ammeter/ohmmeter.
Fig 1.1 Power Supply and voltmeter in parallel Power SupplyVoltmeter

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Fig 1.2 Simple voltage measurement circuit.

aa

b a ba a
d
c
c d

c d

Fig 1.3 Simple current measurement circuit

Power Supply
(Be sure current I
control is at
maximum setting)

Fig 1.4 Circuit to measure resistance

Procedure

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 kÙ and 10 kÙ. We pretend that Rx in unknown. For 4 different values of Ix, we recorded Vx in an...