MEASUREMENTS & INSTRUMENTATION ANALOG AND DIGITAL METERS
ANALOG Metering devices Provides monotonous (continuous) movement.
ELECTRICAL MEASURING INSTRUMENTS ANALOG METERS A d Arsonval galvanometer (Moving coil galvanometer)
ANALOG METERS Pointer & Coil spring arrangement Permanent Magnet Moving Coil Meter
Theory of operation The deflecting torque can be shown equal to:
It can also be shown that: Key point: The deflection is directly proportional to the current passing through the coil.
Moving coil galvanometer The deflection of the pointer is proportional to the current I. This device can be used to measure currents upto I FSD. (I FSD = Full Scale Deflection Current) Normally, I FSD 1 ma
How to measure currents > I FSD? Equivalent circuit of a moving coil galvanometer I = 0 ~ I FSD Rm = Meter Resistance (coil resistance ~ 100Ω) G Ideal Galvanometer
To measure upto 10 ma? Modify the meter by adding a shunt (parallel) resistor. I FSD R m I MAX = 10 ma I MAX I FSD Rm = 100 Ω, I FSD = 1 ma Need to measure upto 10 ma G R 1 Calculate R 1 so that a current of (10 1) ma will flow through it when the meter shows FSD.
Similarly, calculate R 2, R 3, and R 4 to make it possible to measure upto 50 ma, 100 ma, and 1 A respectively. I FSD I MAX = 10 ma I MAX I FSD G R 1 R 2 R 3 R 4 Multi Range Ammeter
Problems Ammeter 1. A Galvanometer with a Full-Scale Deflection current of 2 ma and internal resistance of 100 Ω is to be converted to an ammeter to measure upto 150 ma. Calculate the shunt (parallel) resistor required. 2. Calculate the resistors R 1,R 2,andR 3 to obtain measurement ranges 0-5 ma, 0-50 ma, and 0-500 ma respectively as shown in the following circuit of a multi-range ammeter constructed using a moving coil Galvanometer with a meter resistance of 150 Ω and a Full-Scale Deflection current of 1 ma. G R 1 R 2 R3 2.5 Ω 0.5 Ω
The Galvanometer as a Voltmeter G I FSD V The same Galvanometer mechanism can be modified to measure upto different voltages V by adding series resistors as shown. Multi Range Voltmeter
Problems Voltmeter 1. A Galvanometer with a Full- Scale Deflection current of 2 ma and internal resistance of 100 Ω is to be converted to a voltmeter to measure upto 500 mv. Calculate the series resistor required. 2. Calculate the resistors R 1, R 2, and R 3 to obtain measurement ranges 0-1 V, 0-5 V, and 0-50 V respectively as shown in the following circuit of a multirange voltmeter constructed using a moving coil Galvanometer with a meter resistance of 150 Ω and a Full- Scale Deflection current of 1 ma. G R 3 R 2 R 1
AMMETERS have a very small shunt resistor in them to reduce the effect of introducing the meter resistance into the circuit being measured. VOLTMETERS (ΔV) have a very large series resistor in them to reduce the amount of current drawn from the circuit being measured.
Measuring Voltages and Currents Measuring voltage and current in a circuit when measuring voltage we connect across the component when measuring current we connect in series with the component
Measuring Voltages and Currents Loading effects voltage measurement our measuring instrument will have an effective resistance (R M ) 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
Measuring Voltages and Currents Loading effects current measurement our measuring instrument will have an effective resistance (R M ) 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
The Galvanometer as an Ohmmeter In order to use the same Galvanometer for resistance measurement, it is necessary to use an outside power source (to drive a current through the unknown resistor R x ), and a zero adjustment resistor to limit the current to I FSD, in the event that R x =0. V G Power source (External battery) Zero adjust resistor Unknown Resistance (R x )
Ohmmeter: Allows measurement of resistances from 0 Infinity. The scale starts with infinity (left), descends towards 0 (right). Non linear scale.
Ohmmeter scale:
Problems Ohmmeter 1. A Galvanometer with a Full-Scale Deflection current of 1 ma and internal resistance of 100 Ω is to be used as an Ohmmeter, along with an internal battery of 1.5 V as the power source. Calculate the additional resistance needed in the circuit to obtain FSD (0 Ω reading), when the meter terminals are touched together. V G
The Multimeter
The Multimeter Combines all three major measurements (Current, Voltage, Resistance) into one operational unit. Needs to select the correct mode, operation range, terminals, and proper polarity (direction) before connecting to a circuit or making a measurement. The pointer movement is measured the scale corresponds to the meter range selected must be used to take the reading. Also called an AVO meter.
The Multimeter Measures both DC and AV voltages and currents. A rectifier circuit is used to convert AC to DC, and average or the RMS value is measured.
DIGITAL Metering devices Provides a discrete (stepwise) numeric indication.
How it works.. The digital multimeter converts the analog voltage corresponding to a certain measurement to a numeric value through an Analog to Digital conversion process. Then it displays the number thus obtained on a LED or a LCD display.
Test leads are used to connect the multimeter to the circuit to be tested.
Function/Range Switch DC Voltage AC Voltage Battery Test Resistance DC Current Continuity Test
Display It s important to understand the many different symbols.
Display
Function Dial OFF AC Volts & Hertz V DC Volts V Continuity (tone) )))) Ohms Ω Diodes Capacitance AC Amps A DC Amps A Display backlight On/Off
The voltmeter is used to measure the voltage potential across a component in an active circuit. It can be used to measure either DC or AC voltages. AC voltage can ONLY measure sinusoidal (SINE) waveforms. Voltage Meter
Test Lead Connections Voltage Plug the RED test lead into the jack labeled V/Ω and the BLACK test lead into the jack labeled COM (common).
Ohmmeter The ohmmeter is used to measure the resistance of a component. POWER CANNOT be applied to the component being tested. The ohmmeter actually applies a small voltage and uses Ohm s law to calculate the resistance from the measured current.
Test Lead Connections Resistance Same As Voltage Measurement
Ammeter The ammeter is used to measure the current through a component in an active circuit. It can be used to measure only DC current. Some multimeters can also measure AC current.
Test Lead Connections Current Plug the RED test lead into the jack labeled ma for milliamps or into the jack labeled 10A for circuits with high current levels. Plug the BLACK test lead into the jack labeled COM. Be certain to select the range according to the test lead connections. Observe proper polarity.
Basic Instructions Be sure the test leads are connected appropriately for the intended measurement. Turn the meter s Function/Range switch to the function (ACV, DCV, DCA or Ω) to be measured. Select a range which is higher than the expected value. Connect the test leads to the circuit/component being tested. Observe proper polarity. Turn the meter s Function/Range switch to reduce the range and increase the accuracy. Record results. Remember to turn the power OFF when finished.
Special Functions Continuity Tester Used to test the continuity between two points. Will produce an audio tone. Diode Tester Used to test the resistance in a diode for forward bias and reverse bias. Battery Tester Used to test 9 volt and 1.5 volt (AAA,AA, C, D) batteries.
Safety Features Auto Ranging Reversed Polarity Protection 1000 volts protection on leads (higher than maximum range of AC or DC volts)
If the Meter Malfunctions : Check meter s battery Review instructions to make sure all the meter s settings are accurate for the type of testing being conducted Check to see if test leads are broken or not connected properly Inspect and test the fuse