Chelmsford Amateur Radio Society Intermediate Course (3) Using Multimeters

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Chelmsford Amateur Radio Society Intermediate
Course(3) Using Multimeters
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Multimeter Purpose

• Used to measure Voltage and Current around a
  circuit.
• Multimeter is the generic name given to the
  universal, Volt, Amp, and Ohmmeter.
• There are several on the market and can be either
  Analogue or Digital in operation.

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Operational Considerations

• Take care to set the meter to appropriate range
  before connecting to the circuit.
• Set to highest range and work down, to protect
  the meter.
• There may be a need to switch off equipment
  before connecting to High Voltages.
• ALWAYS HAVE ONE HAND IN THE POCKET OR BEHIND
  BACK, AND REMOVE RINGS IF WORKING ON HIGH
  VOLTAGES.

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Meter Polarity

• There are many cases for observing the correct
  polarity when making measurements.
• When taking measurements of Volts or Amps always
  check the probes are connected to the correct
  terminals.
• This is easy if the probes have RED and BLACK-
  leads, but meters purchased from rallies or
  privately may just have leads of same colour.
• NB On Resistance ranges Analogue meters may put
  out opposite polarity!

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Analogue vs Digital

• Digital meters normally have high input
  impedance.
• Accuracy of analogue meters are specified in Ohms
  per Volt.
• Digital meters are simple to read - the display
  is normally clear.
• Analogue meters are good when making adjustments,
  when you are looking for a change in values.
• The reading of an Analogue scale can be harder.
• Analogue meters will draw some current from the
  circuit being measured, therefore care is
  required when interpreting the results.
• Digital meters auto-sense polarity. Be careful
  with Analogue ones

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Measurements

• To measure a Voltage or Potential Difference a
  Voltmeter is connected in parallel with the
  circuit.
• To measure current the Ammeter is connected in
  series with the circuit to be measured.

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Analogue Diode Testing

• Analogue meter on Resistance Range
• Diodes are sensitive to internal battery polarity

• Some Digital meters have dedicated functions for
  this with normal polarity

LOW READING
HIGH READING
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Measure Potential Difference
Measure the Potential Difference across each
Resistor and record the result for Digital and
Analogue Meters