Voltmeters

Chapter Voltmeters

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

  • overcome by the magnetic force, causes the needle to fly past the actual current levelbefore 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 thecoil attached to the meter needle. This gets rid of the need for a massive, permanentmagnet inside the meter. It also eliminates the possibility that the meter sensitivity willchange in case the strength of the permanent magnet deteriorates (such as might becaused by heat, or by severe mechanical vibration). The electromagnet can be either inseries 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 largerthe number of turns in the coil, the less current is needed in order to produce a givenmagnetic force. If the meter is of the electromagnet type, the combined number of coilturns affects the sensitivity. Remember that the strength of a magnetomotive force isgiven 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, thegreater the needle deflection, and the smaller the amount of current that is needed tocause 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 withoutbanging 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 electronicwork. Meters that measure large currents are not a problem to make; it’s easy to makean insensitive device.Sometimes, it is desirable to have an ammeter that will allow for a wide range ofcurrent measurements. The full-scale deflection of a meter assembly cannot easily bechanged, since this would mean changing the number of coil turns and/or the strengthof 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 themeter, the resistor will take half the current. Then it will take twice the current throughthe assembly to deflect the meter to full scale, as compared with the meter alone. Bychoosing a resistor of just the right value, the full-scale deflection of an ammeter can beincreased 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 currentflowing through the assembly, leaving the meter to carry only 1/10, or 1/100, or 1/1000of 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 beused in a versatile multimeter, with many current ranges.VoltmetersCurrent is a flow of charge carriers. Voltage, or electromotive force (EMF), or potentialdifference, 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 theVoltmeters49