ENT 165 Instrumentation Lecturers Mohd Sofian Bin Mohammad

ENT 165 Instrumentation Lecturers: Mohd Sofian Bin Mohammad Rosbi Lecture 2

Current, Voltage & Resistance Measurement

Topics Outline 1. 0 Device for Current Measurement 1. 1 Analog ammeter 1. 2 Galvanometer 2. 0 Device for Voltage Measurement 2. 1 Analog voltmeter 2. 2 Oscilloscope 2. 3 Potentiometer 3. 0 Device for Resistance Measurement 3. 1 Ohmmeter 3. 2 Wheatstone bridge circuit 4. 0 Digital Multimeter

Objective As introduction to the student into some basic measurement device for current, voltage & resistance.

1. 0 Current Measurement

• Basic analog measurement of current –uses inductive force on the current carrying conductor in magnetic field. • This force can be used to measure the needle deflection on a display. • Direct Current (DC) – Charges flow in one direction – commonly found in many low-voltage applications, especially where these are powered by batteries • Alternating Current (AC) – Flow of electric charge changes direction regularly – Example: audio & radio signal – Home & school use AC

1. 1 Ammeter • An ammeter is an instrument for measuring the electric current in amperes in a branch of an electric circuit. • It must be placed in series with the measured branch, and must have very low resistance to avoid significant alteration of the current it is to measure. • connecting an ammeter in parallel can damage the meter

Ammeter – Principle of Operation • The earliest design is the D'Arsonval galvanometer or moving coil ammeter (respond to ac only) • It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field • The voltage drop across the coil is kept to a minimum to minimize resistance across the ammeter in any circuit into which the it is inserted. • Moving iron ammeters use a piece or pieces of iron which move when acted upon by the electromagnetic force of a fixed coil of (usually heavy gauge) wire (which respond to both dc & ac.

Ammeter Design

• An ammeter is placed in series with a circuit element to measure the electric current flow through it. • The meter must be designed offer very little resistance to the current so that it does not appreciably change the circuit it is measuring. • To accomplish this, a small resistor is placed in parallel with the galvanometer to shunt most of the current around the galvanometer. • Its value is chosen so that when the design current flows through the meter it will deflect to its full-scale reading. • A galvanometer full-scale current is very small: on the order of milliamperes.

Ammeter - Exercise

Ammeter - Exercise

1. 2 Galvanometer • It is an electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil. • Galvanometer has been applied to devices used in measuring, recording, and positioning equipment.

Galvanometer – Principle of Operation • Such devices are constructed with a small pivoting coil of wire in the field of a permanent magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny spring pulls the coil and pointer to the zero position. • In some meters, the magnetic field acts on a small piece of iron to perform the same effect as a spring. When a direct current (DC) flows through the coil, the coil generates a magnetic field. • This field acts with or against the permanent magnet. The coil pivots, pushing against the spring, and moving the pointer. The hand points at a scale indicating the electric current. • A useful meter generally contains some provision for damping the mechanical resonance of the moving coil and pointer so that the pointer position smoothly tracks the current without excess vibration.

Galvanometer – Application • Are used to position the pens of analog chart (example: electrocardiogram)

2. 0 Voltage Measurement

2. 1 Voltmeter • A voltmeter is an instrument used for measuring the potential difference between two points in an electric circuit.

• A voltmeter is placed in parallel with a circuit element to measure the voltage drop across it and must be designed to draw very little current from the circuit so that it does not appreciably change the circuit it is measuring. • To accomplish this, a large resistor is placed in series with the galvanometer. • Its value is chosen so that the design voltage placed across the meter will cause the meter to deflect to its fullscale reading. • A galvanometer full-scale current is very small: on the order of milliamperes.

Voltmeter – Principle of Operation • The moving coil galvanometer is one example of this type of voltmeter. It employs a small coil of fine wire suspended in a strong magnetic field. • When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. • The angular rotation is proportional to the current that is flowing through the coil. • For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.

Voltmeter - Exercise

2. 2 Oscilloscope • An oscilloscope is a piece of electronic test equipment that allows signal voltages to be viewed, usually as a two-dimensional graph of one or more electrical potential differences (vertical axis) plotted as a function of time or of some other voltage (horizontal axis • Perform some computations using data taken from the voltage waveform that is displayed such as: * Rms value * Average Amplitude * Peak-to-peak Amplitude * Frequency

Oscilloscope • An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. • Using for signal/wave display – Winamp Music Player, Electrocardiogram,

2. 3 Potentiometer • A potentiometer is a variable resistor that functions as a voltage divider • It is a simple electro-mechanical transducer • It converts rotary or linear motion from the operator into a change of resistance, and this change is (or can be) used to control any volume.

Potentiometer • Schematic symbol for a potentiometer. The arrow represents the moving terminal, called the wiper. • Usually, this is a three-terminal resistor with a sliding contact in the center (the wiper) - user-adjustable resistance • If all three terminals are used, it can act as a variable voltage divider • If only two terminals are used (one side and the wiper), it acts as a variable resistor

Potentiometer Circuit • Any current flow through the Galvanometer, G, wpuld be a result of an imbalance in the measured voltage, Vm and the voltage imposed across points A to B, VAB. • If Vm is not equal to VAB, a current will flow through the galvanometer, G. • Galvanometer detects current flow due to imbalance in voltage Vm and VAB. When Vm = VAB, there is a balance and no current, means no displacement in Galvanometer.

Potentiometer – Application • In modern usage, a potentiometer is a potential divider, a three terminal resistor where the position of the sliding connection is user adjustable via a knob or slider. For instance, when attached to a volume control, the knob can also function as an on/off switch at the lowest volume • Potentiometers are frequently used to adjust the level of analog signals (e. g. volume controls on audio equipment) and as control inputs for electronic circuits (e. g. a typical domestic light dimmer).

3. 0 Resistance Measurement

3. 1 Ohmmeter • The purpose of an ohmmeter, is to measure the resistance placed between its leads. • This resistance reading is indicated through a mechanical meter movement which operates on electric current. The ohmmeter must then have an internal source of voltage to create the necessary current to operate the movement, and also have appropriate ranging resistors to allow just the right amount of current through the movement at any given resistance.

Ohmmeter • The original design of an ohmmeter provided a small battery to apply a voltage to a resistance. It used a galvanometer to measure the electric current through the resistance. • The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance decreased, the current through the meter would increase. • A more accurate type of ohmmeter has an electronic circuit that passes a constant current I through the resistance, and another circuit that measures the voltage V across the resistance.

Ohmmeter • The standard way to measure resistance in ohms is to supply a constant voltage to the resistance and measure the current through it. • That current is of course inversely proportional to the resistance according to Ohm's law, so that you have a nonlinear scale. • The current registered by the current sensing element is proportional to 1/R, so that a large current implies a small resistance.

3. 2 Wheatstone Bridge Circuit • Use to measure capacitance, inductance and most often resistance. • Accurately measures resistance and detect small changes in resistance.

Wheatstone Bridge Circuit • Any one of the four resistors in the above bridge can be the resistor of unknown value, and its value can be determined by a ratio of the other three, which are “calibrated, ” or whose resistances are known to a precise degree. When the bridge is in a balanced condition (zero voltage as indicated by the null detector), the ratio works out to be this: • In condition of balance: R 2 R 1 = R 4 R 3

4. 0 Digital Multimeter • A multimeter or a multitester is an electronic measuring instrument that combines several functions in one unit. • The most basic instruments include an ammeter, voltmeter, and ohmmeter

4. 1 Digital Multimeter – Capabilities • • DC Voltage Measurements AC Voltage RMS Measurements DC and AC Current Measurements Resistance Measurements Capacitance/Inductance Measurements Frequency/Period Measurements Diode Measurements

Thank You