Electrical Instrumentation Unit 6

We have already learned about current (amps – A), voltage (volts - V) and resistance (R). In order to do his or her job properly, an electrician, electronics technician, maintenance technician, HVAC technician, or engineer must be able to measure all three of these quantities. The various crafts use an ammeter to measure current, a voltmeter to measure voltage, and a ohmmeter to measure resistance.

Multimeter An easier way to do this job is to use a meter that combines all three meter types into one device. This device is called a multimeter. There is a multimeter in the MB100 Kit. 

Most analog meters use a D’ Arsonval meter movement, also called a moving coil meter. To the right is a pictorial vies of the inside. A horseshoe magnet provides a magnetic field. A moving coil, suspended on a pivot is mounted between the poles of the magnets. The coil is wound around a light aluminum frame, called a bobbin, which is not shown in the picture.

Functions of a Multimeter An analog multimeter has a dial, an adjustment knob, and a combined display. The dial selects the type of measurement to be made: current (DCmA), voltage (DCV or ACV), or resistance (OHM). The adjustment know is used when measuring resistance (OHM). It is used to zero the scale before making a measurement. Part of the display is used with each setting. You will learn about these setting in the Experiments. Multimeters are the most important electronic testing instruments. They have many uses around the home and on the job. An electrician may use a multimeter when working with appliances and motor vehicles.

Two Types of Multimeters Digital Multimeters have a digital readout. They are very accurate and easy to read. They are best for finding the exact value of a current, voltage or resistance. Analog multimeters use a moving coil meter. They are less expensive and slightly less precise than digital types. Analog multimeters are best for observing the trend of a slowly changing current, voltage, or resistance.

Current Measurement In order to use the multimeter to measure current (amperage), there are two things you must understand. First, to measure current all the electricity must flow through he meter. So the meter is connected in series with the rest of the devices in a circuit as shown in the Figure. We will learn more about series circuits in Unit 12, but for now notice that only one path exists for the current to flow.

The second thing you need to remember is that the current does not have its own scale on the multimeter. It shares the scale for DC voltage and is marked V+mA. Since you will be measuring small electrical currents, the measurements are taken in milliamps (mA). The dial is marked DCmA for direct current milliamps and has two settings. When you use the multimeter dial, you will need to select the 10 or 250 DCmA scale. (You are not able to measure AC current with this meter).

When setting the multimeter to measure current, you should always start on the higher setting (250 DCmA) so you don’t ruin the meter. (The meter in our MB100 Kit is protected so that it cannot be ruined). When the meter is set this way, you read the scale shown in Figures 6-4a. Each small mark on this scale is 5 mA. When you switch the meter to the 10 DCmA scale, you read the scale shown on Figure 6—4b. Every small line is now 0.2 mA.

Voltage Measurement In order to use the multimeter to measure voltage, the multimeter must be connected across the device being measured as shown in the Figure This arrangement is called parallel. We will discuss this more in unit 13.

Resistance Measurement In order to use the multimeter to measure resistance, the meter must be connected across the device, in parallel, as for voltage. When measuring resistance, no source voltage can be connected to the component. A measure of resistance is also a measure of the circuit’s ability to conduct. In addition measuring the amount of resistance , you can also make a check of continuity. A very low reading, or near zero resistance, is an indicator of continuity between the points being examined. This fact is used when zeroing the meter. Before measuring resistance, the meter must be zeroed. There should be zero resistance when two probes of the multimeter are connected. In the Experiment, you will learn how to zero the meter.

Additional Electrical Instruments Fluke Clamp-On Amp Meter. Allows for the measurement of current by just clamping around the wire.

Fluke VoltAlert Tester Easy to carry and has continuous self-test so you always know it’s working, the Fluke 1AC- II non-contact voltage detector allows you to quickly test for energized circuits in the workplace or at home.

Electrical Multimeter Use Video Digital Multimeter Tutorial