Analogue,VOA, Multimeter

- A summary of the analog or analogue multimeter often called a VOA meter for making Volts, Amps and Ohms measurements.

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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.

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 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, VOA meter basics

The analogue VOA meter is based around the moving coil galvanometer - the basic analogue meters that can be found in many applications today..

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.

Analogue multimeter or VOA meter ranges

VOA meters or analogue multimeters, like digital ones have a variety of ranges. They are described in terms of Full Scale Deflection or FSD. This is the maximum that the range can read. In order to get the best reading, it is necessary to have the scale reading somewhere between about a 25 and 100% of FSD. In this way the optimum accuracy and significant number of figures can be read. As a result of this meters have a variety of ranges, that may appear to be reasonably close to each other - often there are two ranges per decade: 1, 3, 10, . . . etc.

A typical meter may have the following ranges (note that the figures indicate the FSD):

There are several points to note from these typical analogue multimeter or VOA meter ranges:

• The low voltage AC voltage, and in this example the 10V AC range may have a different scale to the others. The reason for this is that at low voltages a bridge rectifier is non-linear and this needs to be taken into consideration. Often as in the above example no 3 volt AC range may be included.
• High voltage ranges such as the 1000V or 1kV ranges will often use a different input connection to enable the reading to be taken through a different shunt and kept away from the rotary switch that may not be able to handle a voltage this high.
• AC current is often not included in the lower end meters because of the difficulties of undertaking the measurement without a transformer to step up any voltage across a series sensing resistor for rectification.
• Batteries inside the multimeter are used to provide a current for the resistance measurements. No other readings require the use of battery power - the meter is passive from that viewpoint.
• In this example, the three resistance ranges of varying sensitivity multiply the meter reading by 1, 100, or 10 000 dependent upon the range. This allows for low resistance measurements to be made as well as very high ones. Typically the higher resistance ranges may use a higher voltage battery than the one used for the low resistance ranges.

Analog multimeter sensitivity

One of the specifications for an analogue multimeter or VOA meter is its sensitivity. This comes about because the meter must draw a certain amount of current from the circuit it is measuring in order for the meter to deflect. Accordingly the meter appears as another resistor placed between the points being measured. The way this is specified is in terms of a certain number of Ohms (or more usually kOhms) per volt. The figure enables the effective resistance to be calculated for any given range.

Thus if a VOA multimeter had a sensitivity of 20 kOhms per volt, then on the range having a full scale deflection of 10 volts, it would appear as a resistance of 10 x 20 kohms, i.e. 200 kohms.

When making measurements the resistance of the meter should be at the very least ten times the resistance of the circuit being measured. As a rough guide, this can be taken to be the highest resistor value near where the meter is connected.

Normally the sensitivity of an analogue meter is much less on AC than DC. A meter with a DC sensitivity of 20 kohms per volt on DC might only have a sensitivity of 1 kohm per volt on AC.

Although analogue multimeters or VOA meters are not as widely used as they were, these analogue meters are still found in many laboratories and areas where a test meter is needed. As analogue multimeters are capable of providing the levels of accuracy needed for most test applications, they will undoubtedly be seen for many years to come.

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