Definition of zener diode
Zener Diode : is a Diodes constructed to operate at voltages that are equal to or greater than the reverse breakdown voltage rating.
Zener diode principle
Figure 1(a) shows the two schematic symbols used to represent the zener diode. As you can see, the zener diode symbol resembles the basic P-N junction diode symbol in appearance; however, the zener diode symbol has a zig-zag bar instead of the straight bar. This zig-zag bar at the cathode terminal is included as a memory aid since it is “Z” shaped and will always remind us of zener.
Figure 1(b) shows two typical low-power zener diode packages, and one high- power zener diode package. The surface mount low-power zener package has two metal pads for direct mounting to the surface of a circuit board, while the axial lead low-power zener package has the zener mounted in a glass or epoxy case. The high-power zener package is generally stud mounted and contained in a metal case. These packages are.
identical to the basic P-N junction diode low-power and high-power packages. Once again, a band or stripe is used to identify the cathode end of the zener diode in the low- power packages, whereas the threaded terminal of a high-power package is generally always the cathode.
Characteristics voltage current
Figure 2 shows the V-1 (voltage-current) characteristic curve of a typical zener diode. This characteristic curve is almost identical to the basic P-N junction diode’s characteristic curve. For example, when forward biased at or beyond 0.7 V, the zener diode will turn ON and be equivalent to a closed switch; whereas, when reverse biased, the zener diode will turn OFF and be equivalent to an open switch. The main difference, however, is that the zener diode has been specifically designed to operate in the reverse breakdown region of the curve. This is achieved, as can be seen in the inset in Figure 2, by making sure that the external bias voltage applied to a zener diode will not only reverse bias the zener diode (+ —» cathode, - —» anode) but also be large enough to drive the zener diode into its reverse breakdown region.
As the reverse voltage across the zener diode is increased from the graph origin (which represents 0 volts), the value of reverse leakage current (Ir) begins to increase. Comparing the voltage developed across the zener (Vz) to the value of current through the zener (Iz), you may have noticed that the voltage drop across a zener diode (Vz) remains almost constant when it is operated in the reverse zener breakdown region, even though current through the zener (Iz) can vary considerably. This ability of the zener diode to maintain a relatively constant voltage regardless of variations in zener current is the key characteristic of the zener diode.
Generally, manufacturers rate zener diodes based on their zener voltage (Vz) rather than their breakdown voltage (VBr). A wide variety of zener diode voltage ratings are available ranging from 1.8 V to several hundred volts. For example, many of the frequently used low-voltage zener diodes have ratings of 3.3 V, 4.7 V, 5.1 V, 5.6 V, 6.2 V, and 9.1 V.
How to test zener diode with multimeter
Because a zener diode is designed to conduct in both directions, we cannot test it with the ohmmeter as we did the basic P-N junction diode. The best way to test a zener diode is to connect the voltmeter across the zener while it is in circuit and power is applied, as seen in Figure 3. If the voltage across the zener is at its specified voltage, then the zener is functioning properly. If the voltage across the zener is not at the nominal value, then the following checks should be made:
- Check the source input voltage. If this voltage (Vin) does not exceed the zener voltage (Vz), the zener diode will not be at fault because the source voltage is not large enough to send the zener into its reverse breakdown region.
- Check the series resistor (Rs) to determine that it has not opened or shorted. An open series resistor will have all of the input voltage developed across it and there will be no voltage across the zener. A shorted series resistor will not provide any current-limiting capability and the zener could possibly bum out.
- Check that there is not a short across the load because this would show up as 0 V across the zener and make the zener look faulty. To isolate this problem, disconnect the load and see if the zener functions normally.