Zener diode - Wikipedia
The current-voltage relationship of an ideal diode. . When enough current runs in reverse through the zener, the voltage drop across it will hold steady at the. A PN junction will conduct current when the applied external voltage . Zener diodes have a well-defined low reverse breakdown voltage by design. . From the diode voltage equation, , we can see that it contains the. A Zener diode is a particular type of diode that, unlike a normal one, allows current to flow not only from its anode to its cathode, but also in the reverse direction, when the Zener voltage is reached. Zener diodes have a highly doped p-n junction. Normal diodes will also break down with a reverse voltage but the voltage.
A Zener diode with a power rating of 0. If excessive power dissipation is detrimental, then why not design the circuit for the least amount of dissipation possible?
Why not just size the resistor for a very high value of resistance, thus severely limiting current and keeping power dissipation figures very low? Less power dissipation means lower operating temperatures for both the diode and the resistor, and also less wasted energy in the system, right? A higher resistance value does reduce power dissipation levels in the circuit, but it unfortunately introduces another problem.
Remember that the purpose of a regulator circuit is to provide a stable voltage for another circuit. What it is supposed to do is maintain However, as we will see, it cannot accomplish this task. If we actually had This load current would have to go through the series dropping resistor as it did before, but with a new much larger! The situation is easier to comprehend if we temporarily remove the Zener diode from the circuit and analyze the behavior of the two resistors alone in Figure below.
Non-regulator with Zener removed. With a total voltage of 45 volts and a total resistance of Thus, the diode ceases to regulate voltage. The analytical technique of removing a Zener diode from a circuit and seeing whether or not enough voltage is present to make it conduct is a sound one.ZENER DIODE CHARACTERISTICS EXPERIMENT 2018
Remember that Zener diodes work by limiting voltage to some maximum level; they cannot make up for a lack of voltage. If the load resistance is too low, it will draw too much current, dropping too much voltage across the series dropping resistor, leaving insufficient voltage across the Zener diode to make it conduct.
When the Zener diode stops conducting current, it can no longer regulate voltage, and the load voltage will fall below the regulation point. To find this acceptable load resistance value, we can use a table to calculate resistance in the two-resistor series circuit no diodeinserting the known values of total voltage and dropping resistor resistance, and calculating for an expected load voltage of With 45 volts of total voltage and Being a series circuit, the current is equal through all components at any given time: Thus, if the load resistance is exactly Any load resistance smaller than With the diode in place, the load voltage will be regulated to a maximum of What we see is a tradeoff between power dissipation and acceptable load resistance.
The higher-value dropping resistor gave us less power dissipation, at the expense of raising the acceptable minimum load resistance value.
What Are Zener Diodes? | Diodes and Rectifiers | Electronics Textbook
If we wish to regulate voltage for low-value load resistances, the circuit must be prepared to handle higher power dissipation. Zener diodes regulate voltage by acting as complementary loads, drawing more or less current as necessary to ensure a constant voltage drop across the load. This is analogous to regulating the speed of an automobile by braking rather than by varying the throttle position: Despite this fundamental inefficiency of design, Zener diode regulator circuits are widely employed due to their sheer simplicity.
There is a second mechanism that disrupts the covalent bonds. The use of a sufficiently strong electric field at the junction can cause a direct rupture of the bond.
If the electric field exerts a strong force on a bound electron, the electron can be torn from the covalent bond thus causing the number of electron-hole pair combinations to multiply. This mechanism is called high field emission or Zener breakdown. The value of reverse voltage at which this occurs is controlled by the amount ot doping of the diode. A heavily doped diode has a low Zener breakdown voltage, while a lightly doped diode has a high Zener breakdown voltage.
What Are Zener Diodes?
At voltages above approximately 8V, the predominant mechanism is the avalanche breakdown. Since the Zener effect avalanche occurs at a predictable point, the diode can be used as a voltage reference. The reverse voltage at which the avalanche occurs is called the breakdown or Zener voltage.
A typical Zener diode characteristic is shown in fig. The circuit symbol for the Zener diode is different from that of a regular diode, and is illustrated in the figure.
The maximum reverse current, IZ maxwhich the Zener diode can withstand is dependent on the design and construction of the diode.
A design guideline that the minimum Zener current, where the characteristic curve remains at VZ near the knee of the curveis 0. The power dissipation of a zener diode equals the product of its voltage and current.
IZ max is a limiting factor in power supply design. When zener diode is forward biased it works as a diode and drop across it is 0. When it works in breakdown region the voltage across it is constant VZ and the current through diode is decided by the external resistance.
Thus, zener diode can be used as a voltage regulator in the configuration shown in fig.