Resistors

ANY ELECTRICAL   DEVICE HAS SOME RESISTANCE; none is a perfect conductor. Why, you might ask, would anyone want to put things into a circuit to reduce the current? Isn’t it true that resistors always dissipate some power as heat, and that this in- variably means that a circuit becomes less efficient than it would be without the resistor? Well, it’s true that resistors always dissipate some power as heat. But resistors can optimize the ability of a circuit to generate or amplify a signal, making the circuit maximally efficient at whatever it is designed to do.
Purpose of the resistor
Resistors can play any of numerous different roles in electrical and electronic equipment. Here are a few of the more common ways resistors are used.

Voltage  division

Voltage dividers can be designed using resistors. The resistors dissipate some power in doing this job, but the resulting voltages are needed for the proper biasing of electronic transistors or vacuum tubes. This ensures that an amplifier or oscillator will do its job in the most efficient, reliable possible way.

Biasing

In order to work efficiently, transistors or tubes need the right bias. This means that the control electrode—the base, gate, or grid—must have a certain voltage or current. Net- works of resistors accomplish this. Different bias levels are needed for different types of circuits. A radio transmitting amplifier would usually be biased differently than an oscillator or a low-level receiving amplifier. Sometimes voltage division is required for biasing. Other times it isn’t necessary. Figure shows a transistor whose base is biased using a pair of resistors in a voltage-dividing configuration.

Current limiting

Resistors interfere with the flow of electrons in a circuit. Sometimes this is essential to prevent damage to a component or circuit. A good example is in a receiving amplifier. A resistor can keep the transistor from using up a lot of power just getting hot. Without resistors to limit or control the current, the transistor might be overstressed carrying direct current that doesn’t contribute to the signal. An improperly designed amplifier might need to have its transistor replaced often, because a resistor wasn’t included in the design where it was needed, or because the resistor isn’t the right size. Figure shows a current-limiting resistor connected in series with a transistor. Usually it is in the emitter circuit as shown in this diagram, but it can also be in the collector circuit.

Power dissipation

Dissipating power as heat is not always bad. Sometimes a resistor can be used as a “dummy” component, so that a circuit “sees” the resistor as if it were something more complicated.  In radio, for example, a resistor can be used to take the place of an antenna. A transmitter can then be tested in such a way that it doesn’t interfere with signals on the airwaves. The transmitter output heats the resistor, without radiating any signal. But as far as the transmitter “knows,” it’s hooked up to a real antenna (Fig.).

Another case in which power dissipation is useful is at the input of a power amplifier. Sometimes the circuit driving  the amplifier (supplying its input signal) has too much power for the amplifier input. A resistor, or network of resistors, can dissipate this excess so that the power amplifier doesn’t get too much drive.

Bleeding  off charge

In a high-voltage, direct-current (dc) power supply, capacitors are used to smooth out the fluctuations in the output. These capacitors acquire an electric charge, and they store it for awhile. In some power supplies, these filter capacitors hold the full output voltage of the supply, say something like 750 V, even after the supply has been turned off, and even after it is unplugged from the wall outlet. If you attempt to repair such a power supply, you might get clobbered by this voltage. Bleeder resistors,  connected across the filter capacitors, drain their stored charge so that servicing the supply is not dangerous (Fig.).

It’s always a good idea to short out all filter capacitors,  using a screwdriver with an insulated handle, before working on a high voltage dc power supply. 

Impedance matching

A more subtle, more sophisticated use for resistors is in the coupling in a chain of amplifiers, or in the input and output circuits of amplifiers. In order to produce the greatest possible amplification, the impedances must agree between the output of a given amplifier and the input of the next. The same is true between a source of signal and the in- put of an amplifier. Also, this applies between the output of the last amplifier in a chain, and the load, whether that load is a speaker, a headset, a FAX machine, or whatever.

Impedance is the alternating-current (ac) cousin of resistance in direct-current

(dc) circuits. This is discussed in the next section of this book.

The color code

Some resistors have color bands that indicate their values and tolerances.  You’ll see three, four, or five bands around carbon-composition resistors and film resistors. Other units are large enough so that the values can be printed on them in ordinary numerals.

On resistors with axial leads, the bands (first, second, third, fourth, fifth) are arranged as shown in Fig. 6-12A. On resistors with radial leads, the bands are arranged as shown in Fig. 6-12B. The first two bands represent numbers 0 through 9; the third band represents a multiplier of 10 to some power. For the moment, don’t worry about the fourth and fifth bands. Refer to Table 6-1.

Table 6-1   Resistor  color code

Color of band	Numeral (Bands no.1 and 2.)	Multiplier Band no.3
Black	0	1
Brown	1	10
Red	2	100
Orange	3	1K
Yellow	4	10K
Green	5	100K
Blue	6	1M
Violet	7	10M
Gray	8	100M
White	9	1000M

See text for discussion of bands no. 4 and 5.

     Suppose you find a resistor whose first three bands are yellow, violet, and red, in that order. Then the resistance is 4,700 Ω or 4.7 KΩ. Read yellow   4, violet   7, red  

× 100.

As another example, suppose you stick your hand in a bag and pull out a unit with bands of blue, gray, orange. Refer to Table 6-1 and determine blue    6, gray    8, orange

  × 1000. Therefore, the value is 68,000 Ω = 68 KΩ.

After a few hundred real-life experiences  with this color code, you’ll have it memorized. If you aren’t going to be using resistors that often, you can always keep a copy of Table 6-1 handy and use it when you need it.

The fourth band, if there is one, indicates tolerance. If it’s silver, it means the resistor is rated at plus or minus 10 percent. If it’s gold, the resistor is rated at plus or minus

5 percent. If there is no fourth band, the resistor is rated at plus or minus 20 percent.

The fifth band, if there is one, indicates the percentage that the value might change in 1,000 hours of use. A brown band indicates a maximum change of 1 percent of the rated value. A red band indicates 0.1 percent; an orange band indicates 0.01 percent; a yellow band indicates 0.001 percent. If there is no fifth band, it means that the resistor might deviate by more than 1 percent of the rated value after 1,000 hours of use.

A good engineer always tests a resistor with an ohmmeter before installing it. If the unit happens to be labeled wrong, it’s easy to catch while assembling a complex electronic circuit. But once the circuit is all together, and it won’t work because  some resistor is mislabeled (and this happens),  it’s a gigantic pain to find the problem.