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Transcript
VOLTMETERS Done by: Naser M.Jamal Ibrahim Shahbaz WHAT IS A VOLTMETER? A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. WHAT IS A VOLTMETER? Some voltmeters are intended for use in direct current (DC) circuits; others are designed for alternating current (AC) circuits. HOW DOES IT WORK? The main principle of voltmeter is that it must be connected in parallel with the element we want to measure the voltage across. HOW DOES IT WORK? The parallel connection is used because a voltmeter is constructed in such a way that it has a very high value of internal resistance. HOW DOES IT WORK? So if that high resistance is connected in series than the current flow will be almost zero which means the circuit has become open. If it is connected in parallel, than the load impedance comes parallel with the high resistance of the voltmeter and hence the combination will give almost the same the impedance that the load had. T YPES OF VOLTMETERS Analog Voltmeter Digital Voltmeter The readings are shown on a scale. The readings are displayed on an LCD or LED display panel. T YPES OF VOLTMETERS 1. Analog voltmeter • Permanent Magnet Moving coil Voltmeter. • Moving Iron Voltmeter. • Electro Dynamometer Voltmeter. • Rectifier Voltmeter. • Induction Voltmeter. • Electrostatic Voltmeter. 2. Digital Voltmeter. PERMANENT MAGNET MOVING COIL VOLTMETER 1.With the probes unconnected, the meter is like an open circuit, no current can flow into the meter or the coil inside it. 2.With no current flowing, the coil generates no magnetic field and the pointer stays at zero. PERMANENT MAGNET MOVING COIL VOLTMETER 3. Connect the meter probes to something you're testing (such as a circuit board), and current will immediately start to flow through the meter and the coil inside it. PERMANENT MAGNET MOVING COIL VOLTMETER 4. The moving current creates a temporary magnetic field around the coil that repels the magnetic field created by the permanent magnet causing the movement of the pointer. The strength of the magnetic field is directly related to the size of the current that flows through the coil. PERMANENT MAGNET MOVING COIL VOLTMETER 5. The greater the voltage we are measuring, the greater the magnetic field produced by the coil, and the higher up the dial the pointer moves. MULTI-RANGE VOLTMETER The range of the basic voltmeter can be extended by using number of multipliers and a selector switch. Such a meter is called multi-range voltmeter. NOTES When using a voltmeter we have three points to take into consideration: 1.Polarity 2.Sensitivity 3.Loading effect POLARIT Y The analog voltmeter indicates polarity by the moving direction of the needle (CW or CCW). while the digital voltmeter indicates polarity by the sign of the numerical value displayed on the LED. SENSITIVIT Y The sensitivity is known as “ohms-per-volt” rating of the voltmeter and is given by: S=1/(full scale current) Voltmeters with low sensitivity may give correct readings when measuring voltages in low resistive circuits, but unreliable readings in high resistive circuits. LOADING EFFECT The voltmeter will have an internal 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. How to minimize the loading effect?