Construction & Working of Multimeter or Avo Meter

Construction & Working of Multimeter or Avo Meter Points : Construction & Working of Multimeter or Avo Meter The “heart” of the meter consists of a d.c. moving coil instrument taking very low current of the order of 1mA for full scale deflection. On d.c. a tapped universal shunt is used for current measurements and a tapped series resistance for voltage measurements. On a.c. a transformer feeds into a metal rectifier, the out put from which goes to the d.c. movement. Thus the same scales are used for d.c. and a.c. measurements. The resistances are measured by applying a set of voltage from an internal battery to the resistance under test measuring the resultant current. Since the resultant current is, by “Ohm,s Law”, inversely proportional to the resistance, so the scale can be calibrated directly in Ohms. Kilo and mega-ohms. Selection of Range The selection of the particular range required is affected by a rotary switch. All the accessories required for the various measurements to be carried out by the meter are housed in the meter casing, making the test-set completely self-c.

Scale Reading Techniques The scales of a typical multimeter are drawn in fig. (the photographic view of a typical multimeter). A selector switch is provided to select the function required. Reading Current Let us consider the d.c. current ranges provided by the VOM are as under:
To measure d.c. current values from 1mA to 500mA, the test leads are plugged into the —common and + terminals. If the 1OA range is employed, the test leads are plugged into the —10A and +10A terminals. We know that measurement of current requires that the circuit under test be opened and the meter connected in series with the circuit. When the polarity of the circuit is proper the pointer will move on right side. In this way the reading is taken on the scale for particular selector switch.

Reading Voltage: Let us consider the VOM with following ranges.
The VOM is provided with 2.5, 10, 50, 250, 1000, and 5000V ranges. A selector switch and a function switches (-dc, +dc) are also provided. For measurement of dc voltages up to 1000V, the test leads are plugged into the —common and 5000V d.c. terminals. Depending upon (he scale used we can measure the voltage directly or by multiplying factor with meter constant with pointer reading.

Reading Ohms Let us consider the ranges of Ω on the meter are as under
As seen the ranges are Rx1, Rx100 & R10,000. These are basically multiplying factors for the ohm scale. For example, we read the scale directly when VOM is set to its Rx1 position. If the instrument is operated on its Rx100 range, we will multiply the scale reading by 100. The test leads arc plugged into the —common and + terminal when the ohmmeter function is employed.
Before making a resistance measurement, the test leads must be connected together (short-circuited), and the zero 2 control adjusted to bring the pointer exactly to zero on the Ω scale. Otherwise the reading will be incorrect.