Schematic description

Wattmeter schematic explained

The schematics for Wattmeter is based on the PIC 16F876 device. The PIC microprocessor has a crystal of 4MHz in order to have a true calculation power of 1 MIPS. There are four other parts, the power supply, the RS232 interface, the input signal conditioning and the display driver.
The wattmeter is equipped by a standard 9 Volt battery and the power supply for PIC and the other components is a linear regulator LM78L05, capable to source 100mA at 5V. The PIC interfaces the PC serial port using a MAX232 that translate the TTL levels to true RS232 levels. The version of the MAX232 used allow the usage of 100N capacitors for charge pumps. The RS232 connection is used for future expansions (PC remote control and monitor, boot loader).
The input voltage is measured by the PIC, via two simple resistors and a MC33201 operational amplifier, that gives a measurement range from 0 to 19 V. The current transducer is the LEM LTS 25 NP with a range of Â±25A (the usable range is about 3 times this limit). Another MC33201, with the same configuration used for voltage measurement, is used for current/voltage conversion.
The display driver is an ICM7211 with 4 digits and 3 position for the dot. The display driver requires a BCD input and a digit Strobe.
The display can shows the power (P=V*A) in the form PPP.P watt (resolution 0.1W), the voltage in the form TT.TT volt (resolution 10mV), or the current in the form Â±CC.C ampere (resolution 0.1A). The operator can choose one of the previous measure by a three position switch. The central point of two resistance connected in serial mode (the other terminals are connected to +5V and GND respectively) is joined to an analog Pic input. The switch can shunt one of the two resistance (or no one) obtaining a +5V, +2.5 V or 0V input for the AN2 Pic input.

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Wattmeter is an instrument used to measure electric power. The most commonly used type of wattmeter is called an electrodynamic wattmeter. The electrodynamic wattmeter has two coils of wire. A fixed coil is arranged to receive the current of the circuit to be measured. A movable coil, supported by jeweled pivots that permit it to turn, is arranged to receive a current proportional to the voltage of the circuit to be measured. When the circuit is energized, the magnetic fields produced by the currents make the movable coil try to turn so that its axis is parallel to the axis of the fixed coil. Springs stop it in a position that depends on the power in the circuit. A pointer shows the power in watts on a scale.

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From Wikipedia, the free encyclopedia

Wattmeter

The wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) inwatts of any given circuit.

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Low-cost watt meter/recorder
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USB-compatible unit makes key power measurements with ±1.5% accuracy and calculates consumption costs

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Electronic Educational Devices
Denver, CO
Brad Volin 877-928-8701
Fax 303-282-6411
bvolin@wattsupmeters.com
http://www.doubleed.com

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Wattmeter

An instrument that measures electric power. See Electric power measurement

A variety of wattmeters are available to measure the power in ac circuits. They are generally classified by names descriptive of their operating principles. Determination of power in dc circuits is almost always done by separate measurements of voltage and current. However, some of the instruments described will also function in dc circuits, if desired.

Probably the most useful instrument in the measurement of ac power at commercial frequencies is the indicating (deflecting) electrodynamic wattmeter. It is similar in principle to the double-coil dc ammeter or voltmeter in that it depends on the interaction of the fields of two sets of coils, one fixed and the other movable. The moving coil is suspended, or pivoted, so that it is free to rotate through a limited angle about an axis perpendicular to that of the fixed coils. As a single-phase wattmeter, the moving (potential) coil, usually constructed of fine wire, carries a current proportional to the voltage applied to the measured circuit, and the fixed (current) coils carry the load current. This arrangement of coils is due to the practical necessity of designing current coils of relatively heavy conductors to carry large values of current. The potential coil can be lighter because the operating current is limited to low values. SeeAmmeter, Voltmeter

A thermal converter consists of a resistive heater in close thermal contact with one or more thermocouples. When current flows through the heater, the temperature rises. Thermocouples give an output voltage proportional to the temperature difference between their junctions, in this case proportional to the square of the current, and so make suitable transducers for the construction of thermal wattmeters. See Thermal converters, Thermocouple,Thermoelectricity

The electrostatic force between two conductors is proportional to the product of the square of the potential difference between them and the rate of change of capacitance with displacement. A differential electrostatic instrument may therefore be used to construct a quarter-squares wattmeter. In spite of the problems of matching the capacitance changes of the two elements and the small forces available, electrostatic wattmeters were used as standards for many years.

Digital wattmeters combine the advantages of electronic signal processing and a high-resolution, easily read display. Electrical readout of the measurement is also possible. A variety of electronic techniques for carrying out the necessary multiplication of the signals representing the current and voltage have been used. Usually the electronic multiplier is an analog system which gives as its output a voltage proportional to the power indication required. This voltage is then converted into digital form in one of the standard ways. Many of the multipliers were originally developed for use in analog computers. See Analog computer

The instruments described are designed for single-phase power measurement. In polyphase circuits, the total power is the algebraic sum of the power in each phase. This summation is assisted by simple modifications of single-phase instruments. See Alternating current

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