Multimeters

Experiment 4

Objective:

To learn how to use a multimeter to read current, voltage and resistance. To understand how to correctly use the meter in a circuit to obtain the correct reading.

Electrical meters are devices that measure important physical properties of electrical circuits. They can measure electrical component properties such as resistance of resistors, inductance of inductors, and capacitance of capacitors. They can also measure active properties of a circuit during its operation such as voltage and current. Our multimeter can function as a voltmeter (AC and DC), an ammeter (AC and DC), an ohm meter and a diode checker. For this lab, we will focus on the DC voltmeterand ammeter settings.

Current is the flow of charge through a device. In our case, this means the flow of electrons in a circuit. The unit of current is the Ampere, which represents the flow of one coulomb per second past a given point in the circuit. An ammeter measures the flow of charge (current) through a wire or electrical device. Two important facts about ammeters are:

1.Ammeters have very low resistance to the current so that, when placed in a circuit, the current is not impeded.

2.Ammeters have to be in series with the circuit whose current is to be measured since they measure the flow of charge.

Voltage differences are generated by devices such as batteries (chemical energy), and electrical generators (magnetic), and photovoltaic cells (solar). Voltage is a measure of the work per unit charge required to move a unit charge between two points. The unit of voltage is the volt, which is equivalent to a loss or gain in energy of 1 joule per coulomb. A voltmeter measures the difference in electrical potential, called the “voltage drop” (or usually the “voltage across” a device), between two points in a circuit. Two important points about voltmeters are:

1.A voltmeter has very high resistance to the flow of electricity so that, when it straddles two points in a circuit, a new path for the current is not created.

2.Voltmeters must go in parallel with the component being measured because of this.

An electrical circuit must be a closed loop. The wires and electrical components such as resistors, capacitors, and inductors, must all be connected together so that electrons can move around in a complete path. There are two kinds of circuits, series and parallel. Series circuits can be traced like a train, from one component to another to another. The current must move through a single component in order to get to the next component. Parallel circuits will have two or more components connected to the same two spots, creating a sort of “ladder” look to the diagram. The current moves through two or more components simultaneously. Examples of each are shown below in Figure 1.

(a) (b)

Figure 1: Two different resistor circuits: a series circuit (a) anda parallelcircuit (b).

To use the multimeter as a voltmeter in this lab, we want to turn the dial to Voltage, DC. This is the setting with the symbol: . The jacks we want to use are the two on the right – see Fig 2(a). To use the multimeter as an ammeter in this lab, we want to turn the dial to the Ammeter, DC setting:. Always first use common ground with the 10 A jack – see Fig. 2(b). If the reading is low (less than 0.3 A), then it is safe to remove that wire and place it in the 300 mA jack. Figure 2(a)shows the appropriate connectors for use asa voltmeter; Fig. 2(b) shows the appropriate connectors for use as an ammeter.

(a)(b)

Figure 2: The voltmeter configuration is on the left, the ammeter on the right.

(Note: the dial setting in the actual image is incorrect for both setups. Turn the dial to the indicated setting.)

EXERCISES:

  1. Build the circuits shown in the diagrams below, beginning with Fig. 3, using the various resistors, meters, and the source of electrical energy provided. Record the meter readings and explain what is happening in each circuit.
  2. The first circuit is to check the voltage across the power supply. Figure 3 shows the wiring diagram as well as the physical setup.

Figure 3: The left is the circuit diagram for the circuit.

The right shows how this is interpreted for actual wiring of the components in the lab.

  1. The second circuit is to set up a variable supply of voltage using a variable resistor (rheostat) and a fixed source of voltage. What happens to the reading on the voltmeter as you change the rheostat?

Figure 4: Measuring the voltage across the rheostat.

(Note that positive and negative signs appear in pairs in a circuit diagram indicating two points which are at higher and lower potentials relative to one another, with the plus sign indicating the point of higher potential.)

  1. Similarly, the voltmeter can read the voltage drop across the fixed resistor. What happens to the voltage reading when you change the rheostat now?

Figure 5: The voltmeter is reading the voltage across the fixed resistor

  1. Turn your meter to the DC ammeter setting and change the jacks – make sure you begin on the 10 A connectors. Configure your circuits according to the following two diagrams.How are the measurements related?

Figure 6: The ammeter measures the current first on one side of the resistor and then on the other.

  1. Now, using two meters in conjunction, we can measure both voltage and current in a circuit.

Figure 7: The voltmeter measures the voltage drop across the rheostat and the ammeter measures the current in the resistor and the rheostat.

  1. Reproduce the circuit shown below. What is the ammeter reading?Why does it differ so much from the circuit in Fig. 7 ?
  1. Reproduce the circuit shown below. What is the voltmeter reading?Explain the voltmeter reading.

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