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Chelmsford Amateur Radio Society Intermediate Course (3) Using Multimeters Chelmsford Amateur Radio Society Intermediate Licence Course Carl G3PEM Slide Set-13 (3) Using Multimeters 1 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. Chelmsford Amateur Radio Society Intermediate Licence Course Carl G3PEM Slide Set-13 (3) Using Multimeters 2 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. Chelmsford Amateur Radio Society Intermediate Licence Course Carl G3PEM Slide Set-13 (3) Using Multimeters 3 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! Chelmsford Amateur Radio Society Intermediate Licence Course Carl G3PEM Slide Set-13 (3) Using Multimeters 4 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 Chelmsford Amateur Radio Society Intermediate Licence Course Carl G3PEM Slide Set-13 (3) Using Multimeters 5 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. - A + + Bulb Battery + V Chelmsford Amateur Radio Society Intermediate Licence Course Carl G3PEM Slide Set-13 (3) Using Multimeters 6 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 + Internal Battery + RED LEAD Meter Display Meter - BLACK LEAD LOW READING HIGH READING Chelmsford Amateur Radio Society Intermediate Licence Course Carl G3PEM Slide Set-13 (3) Using Multimeters 7 Measure Potential Difference + 6 / 9 Volts 120K 56K 47K 180K - Volts Measure the Potential Difference across each Resistor and record the result for Digital and Analogue Meters Chelmsford Amateur Radio Society Intermediate Licence Course Carl G3PEM Slide Set-13 (3) Using Multimeters 8
