DIGITAL MULTIMETERS

INTRODUCTION:

              Digital instruments convert analog signals into a digital or numeric readout. This digital readout can be displayed on the front panel and also used as an electrical digital output signal.

Digital instruments, particularly digital voltmeters or multi meters are used to measure analog quantity. It is necessary to convert the analog signal to an equivalent digital signal. Analog to digital converters are also used as a main element of a digital instrument.

DIGITAL  MULTIMETERS:

          Analog meters requires no power supply, they give a better visual indication of changes and suffer less form electric noise and isolation problems. These meters are simple and inexpensive.

   Digital meters offers high accuracy, have a high input impedance and are smaller in size. The output available is electrical in addition to a visual readout.

A basic digital multi meter is made up of several converters, circuitry for counting and an attenuator circuit.

The basic block diagram of digital multi meter as shown in below

    Figure shown in Digital Multi meter

SPECIFICATIONS OF DIGITAL MULTI METER:

  1. Display: 3 .5 digital LCD with a maximum reading of 1999.

  2. Polarity: automatic: (-) negative polarity indications.

  3 .Over range indication: (OL) or (-OL) is displayed.

  4. Operating environment:  0℃ to 50℃ at <70% R,H.

  5. Input impedance: 10MΩ in all DCV and ACV ranges.

  6. Power requirements: 9V Battery

  7. Dimensions: 91 mm(W) * 170 mm(L) * 40 mm(H)

  8. Weight: 330 gms

  9. DC volt:

      Ranges: 200mv, 2v, 20v, 200v, 1000v

      Resolution: 100 µV

      Accuracy: ± 0.5% 0f  reading

10. AC volts:

       Ranges: 200mv, 2v, 20v, 200v, 750v

       Resolution: 100 µV 

       Accuracy: ± 1% of reading (40-60Hz)

 11. DC current:

      Ranges: 200µA, 2mA, 20mA, 200mA, 2A, 10A

      Accuracy: ± 0.5% 0f  reading

  12. Ac current:

       Ranges: 2mA, 200mA, 10A

      Accuracy: ± 1% 0f  reading

  13. Resistance:

       Ranges: 200Ω, 2kΩ, 20kΩ, 200kΩ, 2MΩ, 20MΩ

       Accuracy: ± 0.5%  0f  reading

  14. Diode test:

       Test current: 0.8mA ± 0.3mA

       Open circuit volts: 3.0V

  15. Transistor:

        Ranges: 0-1000

       Base current: 10ΩA DC approx.

CURRENT TO VOLTAGE CONVERTER:

          Current is converted to voltage by passing it through a precision low shunt resistance while alternating current is converted into dc by employing rectifiers and filters.

For resistance measurement, the meter includes a precision low current source that is applied across the unknown resistance, again this is a dc voltage which is digitized and readout as ohms.

The current to be measured is applied to the summing junction at the input of the OP-AMP. Since the current at the input of the amplifier is close to zero because of the very high input impedance of the amplifier, the current IR is very nearly equal to Ii, the current IR causes a voltage drop which is proportional to the current, to be developed across the resistors.

This voltage drop is the input to the A/D converter, thereby providing a reading that is proportional to the unknown current.

Resistance is measured by passing a known current, from a constant current source, through an unknown resistance.

The voltage drop across the resistor is applied to the A/D converter producing an indication of the value of the unknown resistance. 

 The current to voltage converter figure shown in below