Multimeters are the perfect tool for troubleshooting electrical problems in anything from industrial to household devices. There are hundreds of different types of multimeters available with different capabilities and price ranges, however, the basics of how to use them should be the same. Multimeters measure voltage, current, and resistance using either analog or digital circuits. The most common meter is the Digital Meter.
Basic meters will usually have three ports to measure the following:
Some DMMs have only one port for Amps, while some have milliamp ports and Amp (shown below). The milliamps port is used for measuring currents less than 300 mA (typical) for a more exact reading.
The rotary selector, shown here above is used to select the type of measurement you wish to perform.
- The “V” with the wavy line above it is for measuring AC voltage.
- The “V” with the dotted and solid lines above it is for measuring DC voltage.
- The “mV” with the dotted and solid lines above it is for measuring DC milli-volts.
- The “Ω” position is for measuring resistance.
- The next position has a “diode” symbol. This position allows us to test diodes.
- The “mA/A” position is for measuring milliamps and Amps.
- The last position, “µA” is for measuring micro-amps.
Making Simple Measurements
The best way to understanding how to use a DMM is by making some simple measurements.
DC/AC Voltage Test: Battery
1. Plug the blackjack into the COM port on the multimeter and the red jack into the Voltage port (often indicated as VΩmA).
2. Set meter dial to DC voltage (indicated by a V)
3. Place the red probe to the “+” and the black probe to the “-” pole on the battery.
Your meter should display the DC voltage. You can follow the same steps to measure AC voltage, resistance and continuity. Just make sure the dial is correctly placed to measure the appropriate unit (eg. VAC, VDC, Ω, Diode, etc.)
Note: When measuring resistance, it’s usually required to measure the component outside the circuit, as shown below, and not in the circuit. Taking measurements in-circuit introduces the risk of inaccuracy due to resistance from neighboring components.
Measuring current is a little different than simply placing the probe on contact as with voltage. First, plug the red jack into the Amp port (usually marked with the letter A).
To measure current it’s required to loop the meter into the circuit, as shown in the image below. The circuit is opened and the probes are inserted within the closed-circuit so the current runs through the meter completing the loop. Another, and possibly more accurate, the technique is to measure the voltage across a resistor and use Ohm’s law (V=IR) to calculate the current.
Diagram of current measurement
Multimeter Features & Types
There are two basic types of multimeters – analog and digital.
Analog multimeters are still commonly used and cost about the same as a digital multimeter (DMM) but do not provide the same accuracy as a DMM. Analog meters are useful in displaying real-time readings with voltage and current drops, where it is necessary to constantly observe the effects of adjusting parts of the circuit such as the load. Analog meters’ distinguishing feature is a display with a spring-loaded needle pointer resting on a wire coil that is surrounded by a magnet. With the flow of current, it produces a force that pushes the magnet to twist the coil causing its rotation.
Digital multimeters are more commonly used because of their accuracy and ease of use. Unlike analog meters, DMMs have no form of resistance within, offering a precise reading. It is also not limited in terms of the size of a readout dial. DMMs can be easily calibrated and have an automatic range adjustment.
Other common DMM features include:
- Auto-ranging to select a specific range for the quantity under test
- Sample and hold, to display the most recent reading after the meter is removed from the circuit under test
- Current-limited tests for the voltage drop across semiconductor junctions for testing diodes and transistors
- A bar graph display of the tested quantity, which makes go/no-go testing easy, and also allows spotting of fast-moving trends
- Low-bandwidth oscilloscope functionality
- Automotive circuit testers
- Simple data acquisition to record maximum and minimum readings over a given period, or to take samples at fixed intervals
- Integration with tweezers for use with SMD technology
- A combined LCR meter for small-size SMD and through-hole components
DMMs can also be directly connected to a computer by IrDA links, RS-232 connections, USB, or an instrument bus such as IEEE-488. The interface allows the computer to record measurements as they are made.
What to look for in a basic multimeter?
- Continuity testing with a piezo buzzer
• Resistance test from 10 ohms (or lower) and up to 1 Megaohm (or higher)
• DC voltage test from 100mV (or lower) and up to 50V
• AC voltage test from 1V and up to 400V (or 200V in the US/Canada/Japan)If you plan to get fairly advanced in electronics and want a meter you won’t
outgrow soon, here are a few other features you’ll want to look for in a meter:
- Auto-off: Prolongs the life of the batteries
- AC and DC current test, from 10mA and up to 10A for some models
- Kick Stand: keeps the meter upright and hands-free on work surfaces
- Hold Button: keeps the value shown on the display so you can probe without looking at the meter
- Common Battery: 9V or AA, pocket meters use hard-to-replace coin cells
Safety First: Common Mistakes When Using a Multimeter
Remember that you are dealing with electricity, so it’s important to use caution when using a multimeter. Some of the most common mistakes people make include:
- Not remembering to switch the test leads (jacks) when switching between current sensing and voltage/resistance testing
- Exceeding the max input of the meter
- The function switch is in the wrong function for attempted measurement
- Using a meter in an area above the stated rating
- To avoid false readings, which could lead to possible electric shock or personal injury, replace the battery as soon as the battery indicator appears. Also, keep your fingers behind the finger guards on the test probes when making measurements.
- Some simple mistakes can cause serious injury, even death. Be sure to always test the leads and make sure the dial is in the
the correct position, and never use a meter where the leads have been damaged.
- Never use a meter on circuits that exceed 4800 watts and be careful when working with voltages above 60 VDC or 30 VAC
RMS, as they may pose a shock hazard. Always pay close attention to the safety ratings of a tool and never use one that
isn’t rated properly.
Next Step Meters
In addition to the essential hand-held multimeter, for a full-service workshop, your projects may eventually take you to benchtop test stations like the ones shown below. Understanding that cost is a major issue when building a workspace, Jameco has multiple labs in one. With multiple instruments packed into one piece of equipment, these lab centers can go a long way in your testing procedures.