FET and vacuum-tube voltmeters

Chapter FET and vacuum-tube voltmeters

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • that a common VOM can measure is about 1 A. The maximum resistance is on the or-der of several megohms or tens of megohms. The lower limit of resistance indication isabout an ohm.FET and vacuum-tube voltmetersIt was mentioned that a good voltmeter will disturb the circuit under test as little as pos-sible, and this requires that the meter have a high internal resistance. Besides the elec-trostatic type voltmeter, there is another way to get an extremely high internalresistance. This is to sample a tiny, tiny current, far too small for any meter to directlyindicate, and then amplify this current so that a meter will show it. When a minisculeamount of current is drawn from a circuit, the equivalent resistance is always extremelyhigh.The most effective way to accomplish the amplification, while making sure that thecurrent drawn really is tiny, is to use either a vacuum tube or a field-effect transistor(FET). You needn’t worry about how such amplifiers work right now; that subject willcome much later in this book. A voltmeter that uses a vacuum tube amplifier to mini-mize the current drain is known as a vacuum-tube voltmeter (VTVM). If an FET isused, the meter is called a FET voltmeter (FETVM). Either of these devices provide anextremely high input resistance along with good sensitivity and amplification. And theyallow measurement of lower voltages, in general, than electrostatic voltmeters.WattmetersThe measurement of electrical power requires that voltage and current both be mea-sured simultaneously. Remember that power is the product of the voltage and current.That is, watts (P) equals volts (E) times amperes (I), written as PEI. In fact, wattsare sometimes called volt-amperes in a dc circuit.You might think that you can just connect a voltmeter in parallel with a circuit,thereby getting a reading of the voltage across it, and also hook up an ammeter in seriesto get a reading of the current through the circuit, and then multiply volts times am-peres to get watts consumed by the circuit. And in fact, for practically all dc circuits,this is an excellent way to measure power (Fig. 3-10).Quite often, however, it’s simpler than that. In many cases, the voltage from thepower supply is constant and predictable. Utility power is a good example. The effec-tive voltage is always very close to 117 V. Although it’s ac, and not dc, power can be mea-sured in the same way as with dc: by means of an ammeter connected in series with thecircuit, and calibrated so that the multiplication (times 117) has already been done.Then, rather than 1 A, the meter would show a reading of 117 W, because PEI1171117 W. If the meter reading were 300 W, the current would be 300/1172.56 A.An electric iron might consume 1000 W, or a current of 1000/1178.55 A. And alarge heating unit might gobble up 2000 W, requiring a current of 2000/11717. 1 A. Thismight blow a fuse or breaker, since these devices are often rated for only 15 A. You’veprobably had an experience where you hooked up too many appliances to a single circuit,blowing the fuse or breaker. The reason was that the appliances, combined, drew too54 Measuring devices