Page 74 - Teach Yourself Electricity and Electronics

54 Measuring devices

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 is
about an ohm.

FET and vacuum-tube voltmeters

It 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 internal
resistance. This is to sample a tiny, tiny current, far too small for any meter to directly
indicate, and then amplify this current so that a meter will show it. When a miniscule
amount of current is drawn from a circuit, the equivalent resistance is always extremely
high.
The most effective way to accomplish the amplification, while making sure that the
current 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 will
come 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 is
used, the meter is called a FET voltmeter (FETVM). Either of these devices provide an
extremely high input resistance along with good sensitivity and amplification. And they
allow measurement of lower voltages, in general, than electrostatic voltmeters.

Wattmeters

The 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 P EI. In fact, watts
are 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 series
to 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 the
power 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 the
circuit, 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 P EI 117
1 117 W. If the meter reading were 300 W, the current would be 300/117 2.56 A.
An electric iron might consume 1000 W, or a current of 1000/117 8.55 A. And a
large heating unit might gobble up 2000 W, requiring a current of 2000/117 17. 1 A. This
might blow a fuse or breaker, since these devices are often rated for only 15 A. You’ve
probably 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 too