Analogue Multimeter Specifications
- understanding the specifications of specs for an analogue or analog multimeter or VOA.
While there are very many similarities between a digital multimeter and an analogue multimeter, there are some significant differences in the specifications or specs.
The analogue multimeter specifications are different because of the way in which they work. Accordingly some of the analogue multimeter specifications may not be familiar to those who are more used to using digital multimters.
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):
|Meter range name||Typical FSD ranges|
|DC Voltage||3.0V, 10V, 30V, 100V, 300V, 1000V|
|AC Voltage||10V, 30V, 100V, 300V, 1000V|
|DC Current||50uA, 1mA 10mA, 100mA , 1A|
|AC Current||100mA, 1A|
|Resistance||R, 100R, 10 000R|
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.
By Ian Poole
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