An adaptation of Trades Access Common Core Line E: Electrical Fundamentals Competency E-4: Use of Multi-Meters
Multimeters 101: Basic Operation, Care and Maintenance and Advanced Troubleshooting for the Skilled Trades by Brent Pfifer is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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I
A technician is only as accurate as the measurement equipment they are using. If the equipment is used incorrectly or is faulty, then the measurements will be inaccurate. If the measurements are inaccurate, then the technician will draw the wrong conclusions. To avoid getting inaccurate readings, you need to handle, use, and store meters properly. When you are done using a multimeter, it should always be turned off to extend battery life.
These precautions apply equally to digital and analog meters.
The two major types of meters are analog and digital (Figure 1). Although both meters perform the same functions, they look different.
As you can see, the difference is in the display unit. Digital meters are usually simpler to use and are more accurate than analog meters, and therefore have become more popular. We will focus on the digital multimeter (DMM), as it is the most common type in use, although analog multimeters may still be preferable in some cases, for example when monitoring a rapidly varying value.
When handling a multimeter it is wise to be sure the meter is held securely. Dropping a multimeter, especially an analog meter, even from a small height can affect future readings and the accuracy of the multimeter. While taking readings the user is most likely going to need the use of both hands to complete the task. As a result it is necessary to be sure the meter is set securely in a safe place where it can be read without having to change the user’s position. If no suitable spot is available, a second person to hold the meter and/or record the readings may be necessary. Some meters also can be equipped with magnetic straps or tethers to aid in their use by a single technician.
Multimeters should be stored in a dry place where they will not be subject to physical damage. Most multimeters will come with a case in which to store them. This case not only keeps all the pieces together in one place (meter, leads, batteries, magnetic strap etc.) it will also protect the vulnerable parts, such as the display, from any damage that may occur.
When storing a multimeter for a prolonged period, removing the batteries will ensure that corrosion will not accumulate at the battery connections.
The leads on multimeters can be varying in length. Additional care must be taken to ensure the internal connections of the leads do not become damaged by tightly winding them around themselves.
Even though you may normally deal with small voltages and currents, the values are never far away from lethal levels. You can receive a shock or burn from any common electrical circuit. The severity of the electrical shock depends on a number of factors:
Normal household currents (plugs and light circuits) are generally limited by a circuit breaker to a value of 15 amperes. This device has been designed to trip and open a circuit if the 15 ampere value is exceeded. It is designed to protect against property damage and not necessarily personal injury. The possibility of causing a fatal injury can occur with a current flow of only 50 milliamperes (mA) or five one-hundredths of an ampere (.05A). The body is sensitive to relatively small values of current. In comparison, a 100-watt lightbulb draws approximately 0.85 ampere (850 mA) of current when connected to a 120 volt source. Remember, there are 15 amperes available in each standard house circuit. Electrical shocks, electrical burns, and other related injuries occur far too often and in most instances, go unrecorded. If you come across someone who you think has received or is receiving an electrical shock, always keep in mind:
It is vital to eliminate the possibility of the machine being energized unexpectedly. In order to create a safe work environment, workers need to guard against contact with electrical voltages and control electrical currents by de-energizing the circuits providing power to the equipment you will be working with.
Make the environment safer by doing the following:
All digital multimeters combine the features of an ammeter, a voltmeter, and an ohmmeter. Figure 1 shows a typical DMM, although different makes and models may have a different number of digits in the display unit and the input/output jacks may be in slightly different positions. Since a DMM is an important tool, you will want to learn how to use one correctly.
The upper portion of the DMM houses the display unit. The middle portion of the DMM houses the function switch, and the bottom portion contains the jacks for test leads.
The function switch normally has positions that will allow a technician to measure:
In addition, some DMMs have function switch positions that will allow a technician to measure and to test diodes and capacitors. Some DMMs require manual setting of ranges; others have an auto ranging feature.
All DMMs may be used to measure voltage, current, and resistance. More advanced DMMs may measure frequency, relative power differences, or other important circuit parameters. Each measurement function has similarities and differences that you need to learn about.
Many meters will use symbols on the display, switch, and connections. Figure 2 lists some of the common symbols you may see.
AC | |
DC | |
Ω | Ohms |
AC or DC | |
Hz | Hertz |
+ | Positive |
– | Negative |
µF | MicroFarad |
m | Milli |
M | mega |
Low battery | |
Manual range or automatic touch hold | |
Continuity beeper | |
Diode | |
Ground | |
Fuse | |
Double insulation | |
Capacitor | |
OL | Overload |
Voltage measurements are very easy to make with a DMM. On meters with manual range selection, start with a value one setting higher than expected. An auto range DMM will automatically select the range based on the voltage present. Figure 1 shows the process.
Follow these steps to measure voltage:
Auto ranging units display the unit of measurement in the top right corner. In manual ranging units, the meter will use the range selected. Auto range will determine the highest setting and automatically display that unit.
As you have seen, the procedure for voltage measurements is relatively straightforward. The leads are simply connected across, or are parallel with, the points of voltage to be measured.
For current measurements, however, the process is slightly more complex. First, the circuit must be opened at the test points and the meter inserted in series at that opening (Figure 5). The total current must flow through the meter. To allow the measurement to be made without disturbing the circuit itself, the current meter has very little internal resistance.
This is where an inexperienced technician must be particularly alert. If the meter is inadvertently connected across a point of P.D. (potential difference) or in parallel with a component instead of in series, the small internal resistance will permit a very large current to flow through the meter resulting in a short circuit. This will most certainly damage the meter severely and perhaps the circuit as well. More alarming is the possibility of causing a dangerous arc flash. The severity of an arc flash depends on a number of factors including but not limited to the distance from the arc flash, the safety equipment worn or more specifically the lack of, the duration of the arc flash as well as the length of the arc flash. For more information on arc flash safety visit www.esasafe.com.
With current measurements, shutting off the power before connecting the meter is essential. You will be disconnecting one end of a wire or component to connect the meter in series. If you leave the power on, you could easily receive a dangerous shock or damage the circuit.
On meters with manual range selection, start with the highest current setting and work your way down.
Follow the steps below to measure current in circuits of 0-30V:
Figure 6: Ammeter connections to measure the same current at different points in a circuit
When measuring current on circuits with voltage values greater than 30 V or where “breaking” the circuit is impractical or dangerous, a clamp-on ammeter or amprobe can be used. These ammeters have two spring-loaded expandable jaws that allow you to clamp around a single conductor (Figure 7). This feature allows you to measure the magnetic field created by the current flowing through the wire to give an ampere reading without having to make physical contact or interfering with the circuit.
You have studied voltage and current measurements, but you will find resistance measurements different in several ways. Resistance is measured with the circuit’s power turned off. The ohmmeter sends its own current through the unknown resistance and then measures that current to provide a resistance value readout.
Even though it reads out resistance, the ohmmeter is still a current-measuring device at heart. The ohmmeter is created from a DC current meter by the addition of a group of resistors (called multiplier resistors) and an internal battery. The battery supplies the current flow that is eventually measured by the meter. For this reason, use an ohmmeter only on de-energized circuits.
In the process of measuring resistance, the test leads are inserted in the meter jacks. The leads are then attached to the ends of whatever resistance is to be measured. Since current can flow either way through a pure resistance, there is no polarity requirement for attaching the meter leads. The meter’s battery sends a current flow through the unknown resistance, the meter’s internal resistors, and the current meter.
The ohmmeter is designed so that it will display 0 Ω when the test leads are clipped together (zero external resistance). The meter reads infinite (I) resistance or over limit (OL) resistance when the leads are left open. When a resistance is placed between the leads, the readout increases according to how much current that resistance allows to flow.
To conserve its battery, an ohmmeter should never be left on the ohms function when not in use. Since the current available from the meter depends on the state of charge of the battery, the DMM should be zero adjusted to start. This may require no more than a test of touching the two probes together.
Figure 8 shows how resistance measurements are taken.
Follow the steps below to measure resistance:
You can use the same connection procedure used in measuring current to verify that a circuit, wire, fuse, or switch is complete without breaks in the circuit. This is called a continuity test, and most DMMs will have an audible continuity setting (). If there is no audible alarm, then the circuit is broken. You can test the audible setting by touching the leads together while the leads are inserted into the common and continuity () jacks. A good example is testing a heating element when you suspect the heating element might be “burned out”. If the heating element is complete without breaks, the audible sound will ring out when tested. However it is important to note that the portion of the circuit to be measured needs to be isolated to ensure a false reading is not obtained. Failure to do so can mean that other parts of the circuit may be read inadvertently thus providing an inaccurate reading.
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II
The trick to effectively troubleshooting electrical equipment and circuits is to zero in as quickly as possible on the problem. Using a multi meter will allow you to effectively test the components that are most likely causing of the problem before you unnecessarily dismantle the equipment and replace parts.
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There are really only two rules for troubleshooting using a voltmeter. They are simple and always true:
With digital meters, voltage readings that are considered as zero will often indicate very small voltage readings. For example, when reading across a closed switch, a very small reading could indicate a very slight resistance across the switch contacts or even a meter inaccuracy.
Notice that the first rule does not say that if you read zero volts across a switch, the switch is closed. There are many situations in which you might read zero volts across an open switch.
The second rule indicates that the load has failed. This only means that the problem is with the load and you don’t have to look anywhere else for the problem. The actual remedy still has to be determined. This may require a replacement of the load, but there may be other possibilities. For example, there may be an overload that needs resetting.
Always look for the easy fix first. Check components that are easily accessible first that might explain the symptom that you have observed. For example, one of the first checks is to verify the power supply.
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You can troubleshoot a problem using either a volt or ohms test. It is most practical to choose voltage testing. With a resistance test, you have to first disconnect the component being tested from the circuit, and while you are removing the wiring you could jostle things and possibly change the circuit, which may temporarily remedy the problem. In other words, you may not really find the problem.
When you use your voltmeter to troubleshoot, you will find either a switch that is open or a load that has failed. You can do this without moving any wires and without changing the circuit in any way. You may then remove the device and double check it with your ohmmeter.
In series circuits, the total voltage is the sum of the individual voltage drops in the circuit, and the equation E = IR is used to calculate the voltage drop across each resistor. Since the current is the same through each resistor, the voltage drop across each resistor is directly proportional to the value of resistance. In other words, the greater the value of a resistor in a series circuit, the higher the voltage drop. Consider the simple series circuit in Figure 1.
From the values given above, you can easily calculate the voltage drop across each resistor by:
E1 = I1 × R1 = 2 A × 40 Ω = 80 V
E2 = I2 × R2 = 2 A × 20 Ω = 40 V
The voltage drop of 80 V across the 40 Ω resistor is twice the voltage drop across the 20 Ω resistor.
Refer to Figure 2. If an open is introduced between resistors R1 and R2 (for example, by disconnecting a lead), current flow through the circuit is, of course, interrupted. If there is no current flow, the voltage drop across each of the resistive elements is zero (since E = I × R).
However, the potential difference of the source still exists across the open. If a voltmeter is connected across the open, the reading is the same as if it were connected directly across the terminals of the supply source.
In a series lighting circuit, you could easily determine which lamp was burnt open simply by measuring the voltage across the lamp-holder terminals, in succession, until you have measured the total source voltage.
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Sometimes you will be required to troubleshoot a piece of equipment that has stopped working. The first thing you would check for is power. Is the breaker off? Is the switch off? Is there a general power outage?
Once you have determined that power is still available you can begin using the multimeter to locate the problem. Starting with the first component or the one easiest to check, work your way through the circuit until you reach the component that shows no voltage reading. This is known as hopscotch voltage readings. Figure 3 illustrates this process. The dashed line indicates where the probe has already been placed and removed.
Follow these steps to complete the voltage test procedures with an autorange meter:
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This test, using a digital multimeter, determines whether:
Follow these steps to complete the resistance test procedure:
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This is a quick audible alarm test using a digital multimeter to determine whether an electrical circuit or wire is complete or broken.
This test can be applied to a circuit as a whole or in sections—on individual components or sections of wiring. A break in continuity can be caused by mechanical damage, corrosion of components, or simply a switch being left open.
Follow these steps to complete the continuity test procedure with an autorange digital meter:
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Just as in series circuits, electrical current flows “from negative to positive” through each of the load components in a parallel circuit. As illustrated in Figure 6, electrons leave the negative terminal of the source and flow from negative to positive through each of the load resistors. Note that the polarity of each of the resistors is the same as the polarity of the source.
Polarity is always expressed from one point of a circuit relative to another point with a different electrical potential. Note that in Figure 6 the top side of each resistor, which is marked negative, is in effect the same point. No difference in potential exists between any of these like terminals.
Also notice that the individual currents through each resistor (I1, I2, I3) together constitute the total current (IT) drawn from the source. When the total current required to operate each of these parallel loads exceeds the current output rating of the one source, you will need to increase the source output.
Voltage sources are connected in parallel whenever it is necessary to deliver a current output greater than the current output a single source of supply can provide, without increasing voltage across a load.
An advantage of parallel-connected power sources is that one source can be removed for maintenance or repairs while reduced power to the load is maintained. If a 6 V battery has a maximum current output of 1 A, and if it is necessary to supply a load requiring 2 A, then you can connect a second 6 V battery in parallel with the first.
If there is any doubt about the polarity of the two batteries, then you can do a simple voltmeter test for correct polarity.
If the polarity is correct (Figure 7a), then the voltmeter indicates 0 V because the EMFs oppose each other. You may connect a paralleling jumper between these two points.
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