The current through the zener diode of Fig. 5-16a is given by S Z S S R V V I (5-11)
This says the zener current equals the voltage across the series resistor divided by the resistance. Equation (5- 11) can be used to construct load line as previously discussed. For instance, suppose VS = 20 V and RS = 1 k . Then
the foregoing equation reduces to
1000
20 Z
S
V I
As before, we get the saturation point (vertical intercept) by setting VZ equal
to zero and solving for IZ to get 20 mA.
Similarly, to get the cutoff point (horizontal intercept), we set IZ
equal to zero and solve for VZ to get 20 V.
The following study aids will help to reinforce the ideas discussed in this chapter. For best results, use these study aids within 6 hours of reading the earlier material. Then review these study aids a week later and month later to ensure that the concepts remain in your long-term memory.
Figure 5–16 Zener Diode
Summary
Sec. 5-1 The Zener Diode
This is special diode optimized for operation in the breakdown region. Its main use is in voltage regulators, circuits that hold the load voltage constant. Ideally, a zener diode is like a perfect battery. To a second approximation, it has bulk resistance that produces a small additional voltage.
Sec. 5-2 The Loaded Zener Regulator
When a zener diode is in parallel with a load resistor, the current through the current-limiting resistor equals the sum of the zener current and the load current. The process for analyzing zener regulator consists of finding the series current, load current, and zener current (in that order.)
Sec. 5-3 Optoelectronic Devices
The LED is widely used as an indicator on instruments, calculators, and other electronic equipment. By combining seven LEDs in a package, we get a seen-segment indicator. Another important optoelectronic device is the optocoupler, which allows us to couple a signal between two isolated circuits.
Sec. 5-4 The Schottky Diode
The reverse recovery time is the time it takes a diode to shut off after it is suddenly switched from forward to reverse bias. This time may only be a few nanoseconds, but it places a limit on how high the frequency can be in rectifier circuit. The Schottky diode is a special diode with almost zero reverse recovery time. Because of this, the Schottky diode is useful at high frequencies where short switching times are needed.
Sec. 5- 5 The Veractor
The width of the depletion layer increases with the reverse voltage. This is why the capacitance of a varactor can be controlled by the reverse voltage. This leads to remote tuning of radio and television sets.
Sec. 5-6 Varistors
These protective devices are used across the primary winding of a transformer to prevent voltage spikes from damaging or otherwise polluting the in and out voltage to the equipment.
Sec. 5-7 Reading a Data Sheet
The most important quantities on the data sheet of zener diodes are the zener voltage, the maximum power rating, the maximum
current rating, and the tolerance. Designers also need the zener resistance, the derating factor, and a few other items.
Sec. 5-8 Troubleshooting
Troubleshooting is an art and a science. Because of his, you can only learn so much from a book. The rest has to be learned from direct experience with circuits in trouble. Because trouble-shooting is an art, you have to ask What if? Often and feel your way to a solution.
Vocabulary
In your own words, explain what each of the following terms mean. Keep your answers short and to the point. If necessary, verify your answer by rereading the appropriate discussion or by looking at end-of-book Glossary.
Light emitting diode (LED) temperature coefficient
open varactor
optocoupler varistor
photodiode voltage regulator
photodiode zener resistance
process zener voltage
Important Equations
The following formulas are useless if you don’t know what they mean in words. Suggestion: Look at each formula, then read the words to find out what it means.. Your chances of learning and remembering are much better if you concentrate on words rather than formulas.
Eq. 5-1 Current through Series Resistor
S Z S S R V V I _
This is an equation that you do not have to memorize. It says the current through the series resistor equals the voltage across the series resistor divided by the resistance. It is another example if Ohm’s law, where the voltage is the difference of the node voltages of the ends of a resistor.
Eq. 5-2 Thevenin Voltage
S L S L TH V R R R V
This is the voltage across the load resistor when the zener diode is disconnected. One way to remember it this: VS divided by RS + RL
is the load current. Multiply this load current by RL and you get
VTH. The value of VTH has to be larger than the zener voltage to get
voltage regulation.
Eq. 5-6 Zenner Current
IZ = IS - IL
This is disguised form of Kirchhoff’s current law. It says the zener current equals the difference between the series current and load current. To use it, you must already have carried out the two preceding steps in the process: 1. Find IS2. Find IL.
Eq. 5-7 Zenner Power
PZ =VZIZ
The zener power equals the zener voltage times the zener current. This power has to be less than the maximum power rating listed on the data sheet. Otherwise, you may burn out or seriously degrade the characteristics of the zener diode.
Eq. 5-8 LED Current
S D S R V V IS
This gives you the current through a resistor in series with a LED. It says the current equals the voltage across the series resistor divided by the resistance. Use 2 V for the value of VD, unless you have a