Digital Multimeter Basics: Troubleshooting Essentials

In this article, we will discuss the digital multimeter or DMM.

This versatile tool appears everywhere, including unlikely places such as the toolbox of Heavy-Duty Mechanics. Yes, even mechanics are required to service mobile equipment loaded with electronic devices.

Like every other tool, a DMM is useless if you don’t know how to use it properly.

Until the early 1980s, the measuring device of choice was the overly complicated, somewhat dangerous, and currently seldom-seen analog multimeter.

Interpreting the reading on an analog multimeter was often daunting, and placing it incorrectly in a circuit could create serious safety issues.

Overview of the Fluke 87V

Ok…let's start with some basics.

DMMs measure voltage, current, and resistance. Some, such as the Fluke 87V, also perform diode testing and measure parameters such as continuity, frequency, and capacitance.

In this article, we will use the Fluke 87V as our DMM to zero in on voltage, current, and resistance measurements.

We talk more about digital multimeters and other common test instruments in the Industrial Electrical Maintenance Essentials 1 course. We cover the working principles of each tool and their applications.

How to measure voltage with a digital multimeter

Let’s start with voltage measurements. The Fluke 87V measures AC and DC volts, plus DC millivolts, each selectable.

A better measurement resolution is obtained by pushing the RANGE button.

The measurement probes are inserted into the V and COM jacks.

Voltage measurements are always taken with the meter connected to the voltage you are attempting to measure. This presents a potential problem. Basic circuit theory tells us that the total resistance of parallel components is always less than the lowest resistance component.

A voltmeter should have very high resistance so as not to change the resistance of the circuit under test when placed in parallel across a component. A perfect voltmeter would have infinite resistance, but that isn’t possible.

The Fluke 87V's input impedance, or resistance, is 10 megohms, which is standard for most DMMs today.

Importance of high input impedance

Let's look at a quick illustration of why a voltmeter should have a high input impedance.

We’ll start with a simple series circuit, which includes a twelve-volt supply and two one-thousand-ohm loads. The voltage across each resistor will be six volts.

What happens when we place the voltmeter parallel to one of the loads? Hopefully, the voltmeter reads six volts.

If we use the Fluke 87V, the voltmeter in parallel with the resistor won’t seriously change the circuit characteristics as 1000 ohms in parallel with 10 megohms is darn close to 1000 ohms. So, the voltmeter reads six volts.

But what if you have an inexpensive DMM with a low impedance value of 10,000 ohms? We won’t mention any brand names here. The circuit characteristics will change because you have created a new parallel resistance value.

Oops…now the reading is an incorrect 5.71 volts because of something called voltmeter loading.

Real-world voltage measurement tips

Ok… Is that how voltages are measured in the real world? In most situations, the black lead is attached to the zero reference point or ground, and the red lead is moved to points in the circuit.

Using only one hand is a very safe method for measuring voltages as it reduces the possibility of a serious shock to your heart.

Let’s demonstrate. If you want to know the voltage across R1, take a reading at Point B, and subtract it from the reading at Point A.

Point B will be 6 volts, and Point A will be 12 volts, so the voltage across R1 is 12 volts minus 6 volts = 6 volts.

How to troubleshoot with a digital multimeter

Here’s a troubleshooting question for you…

What is the problem if the voltage at Point A is 12 volts and the voltage at Point B is zero? That means that there are zero volts across R2, therefore R1 must be open.

So, how do you find an open circuit? Keep moving the red lead until you measure zero volts.

Older technicians will say you can locate this problem with a test light. That’s very true. Replace the DMM with a test light and repeat the process, observing the visual status of the test light.

This process works well for troubleshooting automotive lights but can cause more damage than the original fault in sensitive electronic devices such as 3-wire proximity sensors. Many workplaces have banned test lights because of their low resistance characteristics.

Here's another troubleshooting problem: The voltage at Point A is 12 volts, and 8 volts at Point B. How is that possible? The voltage at Point B is much higher than it should be.

This is a classic example of the effect of increased resistance due to corrosion.

If the loads were lamps, they would be dimmer than normal due to decreased circuit current and the voltage drop across the corrosion.

How to measure current with the Fluke 87V

Let’s move on to current or amperage measurement. Probably 25% of DMMs in the field today cannot measure current because an internal fuse is blown.

What’s the explanation for the blown fuse? At some time, the meter was incorrectly used to measure amperage. All ammeters are fused for protection.

The Fluke 87V can measure Amps, milliamps, and microamps by placing the selector switch appropriately.

On the Fluke 87V, the red lead is plugged into the appropriate A, mA/µA jack, and the black lead connects to the COM jack. Note the fuse ratings printed under each red jack.

It’s important to remember that an ammeter has a resistance of zero ohms and is supposed to be inserted in series. Why zero ohms? Anytime you add a resistance in series, the total resistance increases. We do not want an increase in resistance when attempting to measure amperage.

Let’s measure the amperage through a lamp.

We must break the circuit and insert the ammeter in series to measure the current.

Unfortunately, it’s much too easy to incorrectly place the ammeter in parallel with the lamp, creating a short circuit and blowing the fuse. More often than not, the test technician has no idea that the fuse has blown, making the ammeter useless.

How to measure resistance with a digital multimeter

Finally, we’ll wrap up this article with the resistance measuring function of the DMM.

The Ohmmeter is useful for measuring the resistance of passive devices such as fuses, relay coils, solenoids, heater elements, and lamps.

The Fluke 87V measures resistance by changing the selector switch to the appropriate position. Pushing the RANGE button obtains a better measurement resolution.

The measurement probes are inserted into the Ω and COM jacks.

The ohmmeter acts like a mini-power supply by applying a voltage across the device under test and calculating the resistance based on the resulting current flow. Every DMM has a battery that provides this voltage and powers the internal electronics.

The device under test must be the only device connected to the ohmmeter, and no circuit power must be applied.

Let’s look at an example to illustrate. We want to measure the resistance of R1.

First, we need to remove power from R1 by opening S1.

OK…now we place our ohmmeter across R1. The meter reads 50 ohms because we have not isolated R1 from R2. The meter will find all paths for the current flow. We want only one path, and that’s through R1.

So, we need to remove the connection between R1 and R2.

Now, the ohmmeter reads 100 ohms because R1 provides the only current path.

Summary

Ok…let’s summarize what we’ve discussed…

• Voltage is measured by connecting the DMM in parallel to the circuit.
• A voltmeter has input impedance to prevent errors like voltmeter loading.
• Amperage is measured by placing the DMM in series with the circuit.
• Blown fuses due to incorrect parallel connections often result.
• The resistance function uses the DMM's internal power to measure passive components like fuses and lamps.
• Proper isolation of the device under test is crucial for accurate resistance readings.

We’ll investigate more DMM functions in later articles. In the meantime, check out our courses on Industrial Electrical Maintenance. By completing each course, you’ll receive a certificate from RealPars.

If you're a plant manager looking to train your maintenance team, visit realpars.com/business. Just add your contact information, and our team will quickly reach out to discuss how we can support your team's development.

And if you want to learn more, here’s a list of the courses we mentioned in this blog post:

• Course #1: Industrial Electrical Maintenance Essentials 1: Safety, Inspection & Repair
• Course #2: Industrial Electrical Maintenance 2: Electromechanical and Semiconductors Essentials

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