MULTIMETER

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MULTIMETER Powered By Docstoc
					ECE, ME – I SEM


   BASIC
ELECTRONICS
Multimeter
a)   Types (Analog & Digital)
b)   Voltage Measurement
c)   Current Measurement
d)   Resistance Measurement
Multimeter
   Electrical Test Equipment - The Multimeter
   A multimeter is used to measure voltage, current or
   resistance, depending upon the function selected
   – A voltmeter is used to measure voltage across a
   component or circuit
   – An ammeter is used to measure current through a
   circuit
   – An ohmmeter is used to measure resistance
Measuring Voltages and Currents
   when measuring voltage we connect across the component
   when measuring current we connect in series with the
   component
Loading effects – voltage
measurement
     our measuring instrument will have an effective resistance (RM)
     when measuring voltage we connect a resistance in parallel with
      the component concerned which changes the resistance in the
      circuit and therefore changes the voltage we are trying to
      measure
     this effect is known as loading
Loading effects – current
measurement
 Our measuring instrument will have an effective
  resistance (RM)
 when measuring current we connect a resistance
  in series with the component concerned which
  again changes the resistance in the circuit and
  therefore changes the current we are trying to
  measure.
 This is again a loading effect
Measuring a Resistor
   Remove power if necessary and isolate the
    component from the circuit under test
Principle of Operation
  The analog multimeter make use of permanent
   magnet moving coil (PMMC) mechanism. (Also
   called D’Arsonal Movement)
 It is basically a dc current meter.

 Three essential components

a) Deflecting device

b) Controlling device

c) Damping device
Typical d’Arsonval Meter Movement
Meter Deflection is Proportional to
Current
Digital Multimeters
   Digital multimeters (DMMs) are often
    (inaccurately) referred to as digital voltmeters
    or DVMs
       At their heart is an analogue-to-digital converter
        (ADC)




                      A simplified block diagram
Typical Digital Multimeters (DMM)
Digital Multimeter
   Measurement of voltage, current and resistance
    is achieved using appropriate circuits to produce
    a voltage proportional to the quantity to be
    measured
       In simple DMMs alternating signals are rectified as
        in analogue multimeters to give its average value
        which is multiplied by 1.11 to directly display the
        r.m.s. value of sine waves
       more sophisticated devices use a true r.m.s.
        converter which accurately produced a voltage
        proportional to the r.m.s. value of an input waveform
Digital Multimeter
Schematic Symbols for Meters

 The analog voltmeter and ammeter must be
  connected to the circuit with proper
  polarity to avoid damaging the meter
  movement.
 The ohmmeter may only be used on
  unenergized components
Key Points
   The magnitude of an alternating waveform can be
    described by its peak, peak-to-peak, average or r.m.s.
    value
   The root-mean-square value of a waveform is the
    value that will produce the same power as an
    equivalent direct quantity
   Simple analogue ammeter and voltmeters are based
    on moving coil meters
   Digital multimeters are easy to use and offer high
    accuracy
   Oscilloscopes display the waveform of a signal and
    allow quantities such as phase to be measured.

				
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posted:10/27/2012
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