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A analog or analogue multimeter is one of the trusty workhorses of the electronics test industry. Analogue multimeters have been in use for very many years and sometimes go by the name VOA as a result of the fact that they measure volts, ohms and amps. These multimeters are extremely flexible and enable very many faults to be found in an electronic circuit.
... analogue multimeters have been available for many years and they are very flexible in their operation....
Analog multimeter ranges
Analog 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 quarter and all of the 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.
A typical meter may have the following ranges (note that the figures indicate the FSD):
- DC Voltage: 2.5V, 10V, 25V, 100V, 250V, 1000V
- AC voltage: 10V, 25V, 100V, 250V, 1000V
- DC Current: 50µA, 1mA 10mW, 100mA
- Resistance: R, 100R, 10 000R
There are several points to note from this typical analogue multimeter specification:
- 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. It is also for this reason that no 2.5V AC range was included.
- 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.
- 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 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 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 analog 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.
The operation of an analog multimeter is quite easy. With a knowledge of how to make voltage, current and resistance measurements (see the "Related Articles" on the left hand side of this page for further details) it is only necessary to know how to use the multimeter. If the meter is new then it will obviously be necessary to install any battery or batteries needed for the resistance measurements.
When using the meter it is possible to follow a number of simple steps:
- Insert the probes into the correct connections - this is required because there may be a number of different connections that can be used.
- Set switch to the correct measurement type and range for the measurement to be made. When selecting the range, ensure that the maximum range is above that anticipated. The range on the multimeter can be reduced later if necessary. However by selecting a range that is too high, it prevents the meter being overloaded and any possible damage to the movement of the meter itself.
- Optimise the range for the best reading. If possible adjust it so that the maximum deflection of the meter can be gained. In this way the most accurate reading will be gained.
- Once the reading is complete, it is a wise precaution to place the probes into the voltage measurement sockets and turn the range to maximum voltage. In this way if the meter is accidentally connected without thought for the range used, there is little chance of damage to the meter. This may not be true if it left set for a current reading, and the meter is accidentally connected across a high voltage point!
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