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Volt-Ohm-Milliammeter (VOM): Definition, Functions & Types

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Definition: A common and simple multimeter used in electronic circuits is the volt-ohm-milliammeter or VOM. A VOM is a voltmeter, ammeter, and ohmmeter, all in one.

The VOM has the advantages of being inexpensive and portable. It does, however, usually have a low input resistance (in ohms per volt) on the lowest voltage range. This factor can cause accuracy problems.

When an electronic device called a field-effect transistor was developed, a VOM was designed to overcome the low input impedance problem. The field-effect transistor-VOM (FET-VOM) measures ac and dc voltage, ac and dc current, resistance, and decibel ratings.

Some multimeters are also equipped with accessories such as temperature probes. The leads of the temperature probe are inserted into the meter while the probe itself can be placed in front of an air-conditioning duct, near a furnace heater, or submerged in a hot liquid.

The scale on the meter reflects the temperature in Celsius and/or Fahrenheit. Other optional accessories for VOMs include adapters for reading higher than normal voltages and larger than normal meter current values.

Digital Multimeters

Digital multimeters (DMM) are the most commonly used meters in the electronics field today. They are rapidly replacing the analog meter, which operates on the principle of magnetism and rotating coil.

The DMM uses modern electronic circuitry to take electrical measurements and display values, usually on a liquid crystal display screen. See Figure 1.

DMM using a liquid crystal display

Figure 1. DMM using a liquid crystal display. (Knight Electronics)

Digital meters are more rugged and smaller in size than analog meters. They are also very accurate and very portable. However, some technicians still prefer the analog meters for taking certain types of readings involving solid-state circuits.

The liquid crystal display shows the meter reading in digits rather than on a scale. Some DMMs simultaneously display digits as well as a bar graph that simulates a linear scale reading, Figure 2.

Digital meters not only measure volts, ohms, and current but can also test electronic components such as transistors and diodes, Figure 3. The digital multimeter can come with a rotary dial (like the analog meter) to select functions or a keypad that is pressed with the fingertips.

Most digital meters use an international standard of labels to indicate various meter functions such as AC, DC, and combinations of symbols, Figure 4.

The graphic symbols for AC and DC are often combined with metric prefixes to identify the function or range of the meter setting.

Many digital multimeters are equipped with protective circuitry to prevent accidental damage when the wrong function is used to take readings. Not all digital meters have this capability, but it is available.

This DMM can display values in a numerical form or in a graphical form

Figure 2. This DMM can display values in a numerical form or in a graphical form. (Fluke Corp.)

At the lower right of selector switch of this DMM is a setting to test diodes and capacitors.

Figure 3. At the lower right of selector switch of this DMM is a setting to test diodes and capacitors. (Fluke Corp.)

The international graphic symbols for AC and DC combine with other electrical prefix symbols to indicate the meter setting.

Figure 4. The international graphic symbols for AC and DC combine with other electrical prefix symbols to indicate the meter setting.

Polarity

Polarity is usually not an issue when using a digital meter. The meter will automatically adjust for an incorrect polarity, or it will flash a message or symbol on the liquid crystal display, warning the user of the wrong polarity.

Auto Range Feature

Some digital meters have an auto-range feature. This means that it is not necessary to determine which range to select when using the meter. On these meters, this function is done automatically through the internal electronic circuitry, Figure 5.

Resistance reading using a DMM still requires you to disconnect the circuit from the power source to prevent damaging the meter.

These meters select range automatically

Figure 5. These meters select range automatically. (Fluke Corp.)

Computer Display Meters

Some manufacturers offer interface cards that can be installed in a personnel computer and have test leads similar to meter leads. After the interface board is installed in an expansion slot in the computer, software is loaded.

Now the computer will display simulated meters on the monitor. The computer can then be used to take voltage, current, and resistance readings. Using the computer is very similar to using a digital meter, but with the computer, you can use the memory and hard drive system to store measurements and retrieve them later.

 This type of metering equipment is commonly installed in new industrial applications to monitor high-speed assembly equipment or to test electrical products.

AC Meter Readings

When ac is applied to the meter movement, the needle does not deflect. Remember that ac rapidly changes direction. The meter coil current changes direction in pace with the applied ac voltage.

The result is that the magnetic field rises and collapses and then reverses so rapidly that the coil cannot deflect the needle. The coil simply vibrates under the influence of applied ac voltage. To remedy this, a meter changes the ac voltage into dc by means of a rectifier that converts the applied ac current to a dc current of equal value.

When the applied ac current is rectified to an equal dc value, the value is referred to as the rms value. The abbreviation, rms, stands for root mean square. This is a formula used to equate ac voltage to dc voltage.

The rms value is the equivalent dc value of an ac waveform. Many meter scales and some DMMs use this abbreviation.

Some meters have a special location to plug in the meter lead when reading ac voltage or current. Many meters have a scale marking printed as rms. This simply means that the readings taken on that scale are equal for dc or ac voltages.

Resolution

Resolution is a term that describes the degree of change that must take place before the meter will display the value. For example, if a meter has a resolution of 1/1000th, it can measure a voltage down to 1 millivolt.

In general, the better the meter resolution, the more expensive the meter.

In digital meters, the resolution is also determined by how many digits the meter can display. A digital meter with a five-digit readout has a better resolution than a digital meter with a four-digit readout.

Important Meter Information

Multimeters are very useful tools. There are a number of important details to remember when using these meters.

  • When measuring voltage, the meter must be connected in parallel to the device being read. Start on the highest range when measuring an unknown voltage and move slowly to a lower range for increased accuracy as needed. Remember to observe correct polarity! The red, or positive, lead goes to the positive side of the circuit. The black, or negative, lead goes to the negative side of the circuit.
  • When measuring current, an ammeter must be connected in series with the circuit. A wire must be disconnected to insert the meter. It is wise to make a rough current calculation using Ohm’s law to determine the proper current range on the meter. When troubleshooting a circuit, it is best to start on the highest possible setting. A faulty component can cause higher currents than would be normally expected. Again, observe correct polarity: red to the positive side of the circuit and black to the negative side of the circuit.
  • When measuring resistance, be certain that no power is applied to the circuit. It is best to disconnect the voltage source before taking resistance measurements. In general, it is not necessary to observe polarity when taking resistance measurements. However, as you advance through your studies, polarity must be observed when checking certain solid state devices. Always adjust and zero the meter on the proper range before measurements are made. The ohmmeter should be readjusted after changing ranges or after prolonged use. An open circuit will have an infinite reading
  • A meter has its greatest accuracy at about two-thirds deflection on the meter scale. Use the range that reads as close to this deflection as possible.

Caution

When using a multimeter or DMM, it is easy to connect the meter to a voltage source immediately after taking a resistance or current reading. This is the most common mistake made when using a multimeter or DMM. This action will result in damage to the meter or personal injury.

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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.