First there was the galvanometer, then came the avometer and today, scientists, electricians and anyone else working with electricity use a multimeter, also known as a DMM (for digital multimeter).
The multimeter is basically a digital version of an AVOmeter, which was designed in the early 1920s by British Post Office engineer Donald Macadie to measure amps, volts and ohms (hence "avo"). There are still plenty of analog volt-ohm-milliammeters (VOMs) around, but DMMs are more common and have more functionality.
The applications of multimeters are varied and not confined to the measurement of voltage, current and resistance. You can use a multimeter to test for continuity in a circuit and, depending on the model, to measure capacitance. With most models, you can also test batteries, diodes and transistors and distinguish between DC and AC current.
Getting to Know Your Multimeter
In terms of usability, accuracy and functionality, there's a big difference between an analog and digital multimeter. An analog VOM relies on electromagnetic induction to move a needle, but a DMM has internal circuitry that is more sensitive to minute impulses, and reading an LED display with decimal fractions is more reliable than gauging the position of a needle between meter gradations.
Every multimeter can measure volts, amps and ohms, and most have a dial that allows you to adjust the sensitivity. On a reasonably priced meter, you'll find DC voltage settings from 200 millivolts to 1,000 volts and AC voltage settings from 200 millivolts to 750 volts.
The meter also detects both AC and DC current from 2 milliamps to 20 amps and measures resistance from 200 ohms to 200 megohms. If the meter measures capacitance, it does so on scales that extend from 2 nanofarads (10-9 farads) to 200 microfarads (10-6 farads). Some meters adjust the sensitivity internally. All you have to do is set the dial on the quantity you're measuring and the meter does the rest.
Most DMMs have a setting for testing diodes, designated by the diode symbol. Some also have a setting for testing transistors, labeled hFE. Your meter may also have a setting for testing batteries, but you don't really need this. You can test any battery by using the DC voltage setting in the range of the battery's charge.
How to Use a Multimeter
Every multimeter comes with a a pair of probes, one black and one red, and three or four ports. One of the ports is labeled COM for common, and that's where the black probe goes. Two of the other ports are labeled A for amps and mA/µA for milliamps/microamps. The fourth port, if there is one, is labeled VΩ for volts and ohms. The fourth port is sometimes incorporated into the third one, which is then labeled mAVΩ.
If the meter has four ports, plug the red probe into the VΩ port to measure voltage and resistance, plug it into the mA port to measure current in milliamps and into the A port to measure current in amps. To test a diode, use the VΩ port. You can also use this port to test a transistor, or if the meter has a multi-pin input port, you can plug the transistor into that.
To make a measurement, set the dial to the quantity you're measuring and choose the appropriate scale. If the scale is too large, you'll get an approximate reading, and if the scale is too small, the reading will be off the scale. Either way, no harm will come to the meter. Touch the probes to the terminals of the device or circuit you're testing and read the measurement from the LED display or the analog scale.
The Main Applications of a Multimeter
Any scientist who works with electrical equipment needs a multimeter, but so do tradespeople, such as electricians and appliance repair professionals. A multimeter is also something that should be in every home tool chest, because it's an invaluable tool for diagnosing problems with household circuitry and home appliances.
Every multimeter can measure voltage, current and resistance. These functions are necessary for diagnosing circuit problems and detecting worn out components.
- Testing voltage: Use the voltage setting to measure voltage drop across circuit components and to measure total voltage across a circuit. You'll need the DC voltage setting for most small circuit components and for testing batteries and the AC voltage setting for testing residential circuit components, such as light switches, light fixtures and outlets. Note that you can measure voltage without disconnecting the circuit. Simply touch one probe to the negative terminal or, if testing AC voltage, to the hot terminal. Touch the other probe to the other terminal and record the reading.
- Testing current: You normally use the mA scale for testing current through a electronic circuits and the A scale for testing residential current. To test current, the meter must be part of the circuit. In most cases, you have to make a break in the circuit, and then connect one wire to one of the meter probes and the other wire to the other probe.
- Testing resistance: The meter has a built-in power source that is activated when you choose the resistance scale. It sends a small current from one probe, and the smaller the current recorded by the other probe, the higher the resistance. If the second probe records no current, the meter displays infinite resistance or the letters OL, which means open line. This function is useful for continuity testing. You can also use it to check a diode by checking the resistance in one direction across the device, then reversing the probes and checking resistance in the other direction. If the diode is good, you should get low resistance in one direction and near infinite resistance in the other.
Uses of Multimeters
The uses of multimeters are many, even if you aren't a professional tradesperson or a lab worker. It comes in handy when you want to do any of the following things:
- Test batteries: Just use the DC voltage setting and touch the probes to the battery terminals to determine how much of its original voltage the battery supplies.
- Determine if a power cable is broken: Measure the resistance between the hot and neutral wires of any residential electric cable. If the resistance is infinite, or the meter reads OL, the cable is damaged.
- Test a switch: If a light fixture isn't working, or is flickering, testing the switch is often the first and easiest step to diagnosing the problem. To check a switch, choose the 200-volt range, place a probe on the terminal connected to the load and place the other probe on the ground screw. You should get a voltage reading around 120 volts when the switch is closed and o volts when it's open.
- Test an outlet: To check a household outlet, choose the 200-volt range and insert the probes into the outlet slots. If you don't get a reading of around 120 volts, there's a problem with the outlet or the circuitry.
- Test old incandescent light bulbs: Adjust the meter dial to test for resistance or continuity. Touch one probe to the screw thread and the other to the foot on the bottom of the bulb. The bulb is bad if the display shows OL or the meter shows infinite resistance.