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.