Analogue Multimeter, (VOM or VOA)
- A summary of the analog or analogue multimeter often called a VOA meter for making Volts, Amps and Ohms measurements or a VOM - Volts Ohm Meter.
The analog or analogue multimeter has been one of the mainstays used for electronics test. In view of the fact that analog or analogue multimeters measure Volts, Amps and Ohms they are often referred to as VOA meters. Another name for them is a VOM which stands for Volts Ohm Meter.
Although digital multimeters have become the option of choice these days as their costs have fallen, analog / analogue multimeters are still in widespread use. Analogue multimeters are extremely flexible and enable very many faults to be found in electronic circuits.
Analogue multimeters are still able to provide some functions that are not quite so easy when using digital multimeters. This is particularly true when monitoring voltages that may be fluctuating. Also analogue VOA / VOM meters can give a good indication of a measurement. Often when using a meter it is only necessary to see whether a voltage is present or not. Often it is easier to see this very quickly when using an analogue meter. The figures shown while the digital multimeter is settling can be a little less easy to compute to see if the reading is approximately correct. Once settled, though a digital multimeter gives a more accurate indication more easily.
Analogue multimeter history
The concept of the first analogue multimeter was born when an engineer working for the British Post Office who dealt with the telecommunications infrastructure in the UK became dissatisfied with having to use several instruments to measure voltage current and resistance.
The engineer named MacAdie developed an idea for a multifunctional instrument to a small company named the Automatic Coil Winder and Electrical Equipment Company. The instrument was converted from a concept into a real instrument and named an AVO standing for Amps, Volts and Ohms.
The first instrument, which as a DC only instrument was launched in 1923, but contained many of the features which were maintained in the AVO right up until the last analogue AVOs were sold in June 2008. At this time it was reputed to be the last professional analogue multimeter being manufactured in the world, making it the first and last.
An AVO 8 Mk III
The success of the AVO meant that many other companies designed ad developed their own versions of the AVO. They became so widely sued that the generic name multimeter was adopted although names such as VOA standing for Volts, Amps and Ohms or VOM standing for Volt Ohm Meter.
The analogue multimeters came in a variety of form factors from small and often very cheap instruments right up to robust professional instruments such as the AVO.
Analogue multimeter meter basics
The analogue VOA meter or VOM is based around the moving coil galvanometer - the basic analogue meters that can be found in many applications today.
Typical generic analogue multimeter
The meters will have a full scale deflection, FSD, i.e. the maximum reading calibrated on the scale of a certain current. Typically this might be a value of 50µA. The functionality of the basic meter is then extended by adding series and shunt resistors to enable voltage and current to be measured.
- Extending range of a meter for current measurements: To extend the current ranges of a basic analogue meter, a resistor is placed in parallel with the meter. In this way the shunt resistor takes current and for the same overall current flowing through the meter, more can flow through the overall circuit.
Analogue ampmeter using
a shunt resistor for higher current capability
The value of the shunt resistor is easy to calculate using Ohm's law. Using this it can be determined that the proportion of the current flowing in each leg is inversely proportional to the resistance. This if the moving coil meter has a full scale deflection of 50 µA and a resistance of 2 kΩ for a 1mA FSD, 0.95 mA needs to flow in the shunt resistor for the same voltage across the shunt resistor and the meter itself. Therefore the resistance of the shunt resistor needs to be: 5 / 95 x 2 kΩ = 105.3 Ω.
- Extending the range for voltage measurements: For voltage measurements, resistors are placed in series with the meter.
Analogue voltmeter using a moving coil meter
It is easy to calculate the value for the resistor. Knowing the resistance of the moving coil meter and its full scale deflection, it is possible to use Ohm's law to calculate the required values.
For example take a moving coil meter with a 50 µA FSD and a coil resistance of 2kΩ. For a voltage of 10 volts to enable 50µA to flow the total resistance must be V/I = R or 10 / 50 x 1-6 = 200 kΩ. Thus the series resistor required is 200 kΩ - 2 kΩ i.e. 198 kΩ.
- Resistance capability for a VOA meter: In order to provide the resistance measurement capability, an additional battery is required. This provides a current source to drive current through the external resistor. The amount of current flowing provides an indication of the resistance.
When making resistance measurements using an analogue multimeter, it is found that the high resistance indications are at the left hand section of the meter, i.e. when less current is flowing, and the low values of resistance are indicated at the right hand end of the meter scale, because a higher current flows. This may be a little confusing at first, but one quickly becomes accustomed to this. When using a resistance measurement on an analogue multimeter or analogue VOA meter, it is first necessary to "zero" the meter. This is needed to calibrate out any variations in the battery voltage. It is achieved simply by sorting out the two analogue multimeter probes and adjusting the control normally labelled "Zero" for zero ohms. Once this has been achieved the meter can be used accurately. A further point to note is that the negative terminal of the analogue multimeter is positive to the positive terminal, i.e. the polarity on the terminals is the opposite of what might normally be expected. For most measurements this is not of any consequence, although for some measurements of semiconductors it will have a bearing.
Analogue ohm-meter or resistance meter circuit
It can be seen that by adding the shunt and series resistors as well as a resistor network and battery, for resistance, it is possible to provide a considerable amount of additional capability for the basic analogue moving coil meter.
By Ian Poole
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