barcode in vb.net 2010 FET Voltmeters in Software

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A good voltmeter disturbs the circuit under test as little as possible, and this requires that the meter have high internal resistance. Besides the electrostatic-type voltmeter, there is another way to get high internal resistance. This is to sample a tiny current, far too small for any meter to directly indicate, and then amplify this current so a conventional milliammeter or microammeter can display it. When a minuscule current is drawn from a circuit, the equivalent resistance is always extremely high. The most effective way to accomplish voltage amplification, while making sure that the current drawn is exceedingly small, is to use a field-effect transistor, or FET. (Don t worry about how such
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Wattmeters 45
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amplifiers work right now; you ll learn all about that later in this book.) A voltmeter that uses a FET voltage amplifier to minimize the current drain is known as a FET voltmeter (FETVM). It has extremely high input resistance, along with good sensitivity and amplification.
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Wattmeters
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The measurement of electrical power requires that voltage and current both be measured simultaneously. Remember that in a dc circuit, the power (P ) in watts is the product of the voltage (E ) in volts and the current (I ) in amperes. That is, P = EI. In fact, watts are sometimes called volt-amperes in dc circuits. Do you think you can connect a voltmeter in parallel with a circuit, thereby getting a reading of the voltage across it, and also hook up an ammeter in series to get a reading of the current through the circuit, and then multiply volts times amperes to get watts consumed by the circuit Well, you can. For most dc circuits, this is an excellent way to measure power, as shown in Fig. 3-10. Sometimes, it s simpler yet. In many cases, the voltage from the power supply is constant and predictable. Utility power is a good example. The effective voltage is always very close to 117 V. Although it s ac, and not dc, power in most utility circuits can be measured in the same way as power is measured in dc circuits: by means of an ammeter connected in series with the circuit, and calibrated so that the multiplication (times 117) has already been done. Then, rather than 1 A, the meter will show a reading of 117 W, because P = EI = 117 1 = 117 W. If the meter reading is 300 W, the current is I = P/E = 300/117 = 2.56 A. An electric iron might consume 1000 W, or a current of 1000/117 = 8.55 A. A large heating unit might gobble up 2000 W, requiring a current of 2000/117 = 17.1 A. You should not be surprised if this blows a fuse or trips a circuit breaker, because these devices are often rated for 15 A. Specialized wattmeters are necessary for the measurement of radio-frequency (RF) power, or for peak audio power in a high-fidelity amplifier, or for certain other specialized applications. But almost all of these meters, whatever the associated circuitry, use simple ammeters, milliammeters, or microammeters as their indicating devices.
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3-10 In a dc circuit, power
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can be measured with a voltmeter and an ammeter, connected as shown here.
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3-11 A utility meter with four rotary analog dials. In this example, the
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reading is a little more than 3875 kWh.
Watt-Hour Meters
Electrical energy, as you now know, is measured in watt-hours or kilowatt-hours (kWh). Not surprisingly, a metering device that indicates energy in these units is called a watt-hour meter or a kilowatt-hour meter. The most often used means of measuring electrical energy is by using a small electric motor, the speed of which depends on the current, and thereby on the power at a constant voltage. The number of turns of the motor shaft, in a given length of time, is directly proportional to the number of kilowatt-hours consumed. The motor is placed at the point where the utility wires enter the building. This is usually at a point where the voltage is 234 V. At this point the circuit is split into some circuits with 234 V (for heavy-duty appliances such as the oven, washer, and dryer) and general household circuits at 117 V (for smaller appliances such as lamps, clock radios, and television sets). If you ve observed a kilowatt-hour meter, you have seen a disk spinning, sometimes fast, other times slowly. Its speed depends on the power being used at any given time. The total number of turns of this little disk, every month, determines the size of the bill you will get, as a function also, of course, of the cost per kilowatt-hour. Kilowatt-hour meters count the number of disk turns by means of geared rotary drums or pointers. The drum-type meter gives a direct digital readout. The pointer type has several scales calibrated from 0 to 9 in circles, some going clockwise and others going counterclockwise. Reading a pointer-type utility meter is a little tricky, because you must think in whatever direction (clockwise or counterclockwise) the scale goes. An example of a pointer-type utility meter is illustrated in Fig. 3-11. Read from left to right. For each meter scale, take down the number that the pointer has most recently passed. Write down the rest as you go. The meter shown in the figure reads a little more than 3875 kWh.
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