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Chelmsford Amateur Radio Society Advanced Course (10) Measurements Part-1 - Meters

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... to the required range by a Shunt Resistor, which be calculated by OHMS LAW. ... Apply OHMS LAW: R = V / I, with V = 1 - 0.1 = 0.9 Volts and I = 1 mA ... – PowerPoint PPT presentation

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Title: Chelmsford Amateur Radio Society Advanced Course (10) Measurements Part-1 - Meters


1
Chelmsford Amateur Radio Society Advanced
Course(10) MeasurementsPart-1 - Meters
2
Types of Movement Constructional Details
  • Meter movements for DC and AC measurements can be
    moving-coil, moving-iron, hot-wire and
    thermocouple. The latter are good for measuring
    RF amps.
  • For DC and low frequency AC, we will only
    consider the moving coil type.
  • For AC a diode rectifier is needed in the meter
    circuit
  • The maximum movement possible is called Full
    Scale Deflection, FSD

3
Ammeter
  • The basic meter movement is typically 1mA for FSD
    and has internal resistance of 100 ohms.
  • Its range has to be extended to the required
    range by a Shunt Resistor, which be calculated by
    OHMS LAW.
  • Example for 1 Amp FSD scale-
  • Calculate the voltage drop across the meter from
    V I.R
  • We get V 110-3 x 100 0.1 Volts.
  • If 1mA flows through the meter then 0.999 amps
    must flow through shunt.
  • There is 0.1 volts across the shunt.
  • As R V / I, then the shunt is 0.1 / 0.999
    0.1001 Ohms.

4
Voltmeter
  • To measure voltages consider the basic meter
    movement with typical 1mA FSD and internal
    resistance of 100 Ohms. This is extended to the
    required voltage range by a series resistor.
    Called a Multiplier Resistor.
  • The volts drop across the meter is 0.1 Volts so
    to have a full scale of 1V it is required to
    drop (1 - 0.1) Volts across the series resistor
  • Apply OHMS LAW R V / I, with V 1 - 0.1 0.9
    Volts and I 1 mA
  • Therefore the multiplier resistor is V / I 0.9
    / .001 900 Ohms.
  • - and the meter is a total of 1000 Ohms input
    resistance

5
Ohmmeter
  • To set for ZERO OHMS the two probe leads red and
    black are joined and the calibration R adjusted
    for FSD.
  • Measuring a resistance introduces another series
    resistor into the circuit which reduces the
    current through the meter.
  • Thus the scale can be calibrated in Ohms.

6
Meter Loading
  • Potential Divider opposite should have a voltage
    at the midpoint of 0.5 Volts
  • Total resistance of the meter is 1000 Ohms 100
    Ohms internal resistance plus 900 Ohms
    multiplier resistance.
  • The lower 470k is shunted by 1000 Ohms so the
    bottom pair becomes 470k x 1000 Ohms / (470 k
    1000) Ohms 997.79
  • The volts measured will be 997.79 / 470997.79 x
    1 0.002 volts. Oops!
  • Care must be taking when measuring voltages
    across large resistors.
  • Use a high impedance digital voltmeter or a very
    high ohms per volt analogue voltmeter. NB DVMs
    may be gt10MOhm impedance
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