Chelmsford Amateur Radio Society Intermediate Course (3) Using Multimeters

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.

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.

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!

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

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 - + Battery + - Bulb V + -

Analogue Diode Testing + Meter + RED LEAD - BLACK LEAD Internal Battery Display 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

Measure Potential Difference 120K 56K 47K 180K + 6 / 9 Volts - Volts Measure the Potential Difference across each Resistor and record the result for Digital and Analogue Meters