Page 69 - Teach Yourself Electricity and Electronics

Voltmeters 49

overcome by the magnetic force, causes the needle to fly past the actual current level
before finally coming to rest at the correct reading.
It is possible to use an electromagnet in place of the permanent magnet in the me-
ter assembly. This electromagnet can be operated by the same current that flows in the
coil attached to the meter needle. This gets rid of the need for a massive, permanent
magnet inside the meter. It also eliminates the possibility that the meter sensitivity will
change in case the strength of the permanent magnet deteriorates (such as might be
caused by heat, or by severe mechanical vibration). The electromagnet can be either in
series with, or in parallel with, the meter movement coil.
The sensitivity of the D'Arsonval meter, and of its cousins, depends on several fac-
tors. First is the strength of the permanent magnet, if the meter uses a permanent mag-
net. Second is the number of turns in the coil. The stronger the magnet, and the larger
the number of turns in the coil, the less current is needed in order to produce a given
magnetic force. If the meter is of the electromagnet type, the combined number of coil
turns affects the sensitivity. Remember that the strength of a magnetomotive force is
given in terms of ampere turns. For a given current (number of amperes), the force in-
creases in direct proportion to the number of coil turns. The more force in a meter, the
greater the needle deflection, and the smaller the amount of current that is needed to
cause a certain amount of needle movement.
The most sensitive ammeters can detect currents of just a microampere or two.
The amount of current for full scale deflection (the needle goes all the way up without
banging against the stop pin) can be as little as about 50 uA in commonly available me-
ters. Thus you might see a microammeter, or a milliammeter, quite often in electronic
work. Meters that measure large currents are not a problem to make; it’s easy to make
an insensitive device.
Sometimes, it is desirable to have an ammeter that will allow for a wide range of
current measurements. The full-scale deflection of a meter assembly cannot easily be
changed, since this would mean changing the number of coil turns and/or the strength
of the magnet. But all ammeters have a certain amount of internal resistance. If a re-
sistor, having the same internal resistance as the meter, is connected in parallel with the
meter, the resistor will take half the current. Then it will take twice the current through
the assembly to deflect the meter to full scale, as compared with the meter alone. By
choosing a resistor of just the right value, the full-scale deflection of an ammeter can be
increased by a factor of 10, or 100, or even 1000. This resistor must be capable of car-
rying the current without burning up. It might have to take practically all of the current
flowing through the assembly, leaving the meter to carry only 1/10, or 1/100, or 1/1000
of the current. This is called a shunt resistance or meter shunt (Fig. 3-5).
Meter shunts are frequently used when it is necessary to measure very large cur-
rents, such as hundreds of amperes. They allow microammeters or milliammeters to be
used in a versatile multimeter, with many current ranges.

Voltmeters

Current is a flow of charge carriers. Voltage, or electromotive force (EMF), or potential
difference, is the “pressure” that makes a current possible. Given a circuit whose resis-
tance is constant, the current that will flow in the circuit is directly proportional to the