EMI104 - Basic Meter Movement & DC Voltmeter скачать в хорошем качестве

EMI104 - Basic Meter Movement & DC Voltmeter

Basic Meter Movement & DC Voltmeter Basic Meter Movement -D’Arsonval Principle The D’Arsonval movement employs a spring-loaded coil through which the measured current flows. The coil (rotor) is in a nearly homogeneous field of a permanent magnet and moves in a rotary fashion. The amount of rotation is proportional to the amount of current flowing through the coil. A pointer attached to the coil indicates the position of the coil on a scale calibrated in terms of current or voltage. It responds to dc current only, and has an almost linear calibration. The magnetic shunt that varies the field strength is used for calibration. Formation of DC instruments The basic meter movement becomes a DC instrument, measuring (i) DC current, by adding a shunt resistance, forming a microammeter, a milliammeter or an ammeter. (ii) DC voltage, by adding a multiplier resistance, forming a milli voltmeter, voltmeter or kilovoltmeter. (iii) Resistance, by adding a battery and resistive network, forming an ohmmeter. 2. The basic meter movement becomes an AC instrument, measuring (i) AC voltage or current, by adding a rectifier, forming a rectifier type meter for power and audio frequencies. (ii) RF voltage or current, by adding a thermocouple-type meter for RF. (iii) Expanded scale for power line voltage, by adding a thermistor in a resistive bridge network, forming an expanded scale (100 – 140 V) ac meter for power line monitoring. Basic DC Voltmeter Multi-Range Voltmeter Range Extension Voltmeters Solid state Voltmeters Differential voltmeters A basic D’Arsonval movement can be converted into a dc voltmeter by adding a series resistor known as multiplier. The function of the multiplier is to limit the current through the movement so that the current does not exceed the full-scale deflection value. A dc voltmeter measures the potential difference between two points in a dc circuit or a circuit component. To measure the potential difference between two points in a dc circuit or a circuit component, a dc voltmeter is always connected across them with the proper polarity From Ohm’s Law V=IR V=Im (Rs+Rm) Rs = (V/Im) Rm Im = full scale defl ection current of the movement (Ifsd) Rm = internal resistance of movement Rs = multiplier resistance V = full range voltage of the instrument