When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode’s reverse breakdown voltage. From that point on, the low impedance of the diode keeps the voltage across the diode at that value.

If the polarity is reversed, they conduct current and there may not be any current-limiting resistance in the circuit to prevent them from overload. Now the diode conducts current, and because the current flow in a forward-biased diode increases exponentially with the voltage, it overloads and melts.

Diodes usually come in glass or plastic cylindrical packages, marked with a stripe on one side to indicate polarity. In a perfectly ideal diode, current flows in one direction only, from the anode (positive side) to the cathode (negative side) which is marked with the stripe.

Zener diodes are used for voltage regulation, as reference elements, surge suppressors, and in switching applications and clipper circuits. The load voltage equals breakdown voltage VZ of the diode. The series resistor limits the current through the diode and drops the excess voltage when the diode is conducting.

Valence electrons which break free under the influence of the applied electric field can be accelerated enough that they can knock loose other electrons and the subsequent collisions quickly become an avalanche. This process of sudden release of electrons is known as zener breakdown.

The main cause of zener breakdown is high doping concentrations. The breakdown is a condition when upon increasing the voltage in small amounts; we experience a massive current flow in the device. Zener breakdown occurs when we apply reverse bias to a heavily doped diode.

The reverse breakdown voltage of a Zener diode, is called Zener voltage (Vz). The reverse current that results after the breakdown, is called Zener current (Iz). At breakdown, increase of VI increases II by large amount, so that V0 = VI– RI II becomes constant.

The maximum current is calculated by dividing the power rating by the Zener voltage: I=P/V = 3W/12V = 0.25A.

The Zener test current rating, abbreviated Izt, is another current level that will guarantee the Zener voltage, but it is not a minimum value. And the variation in the Zener voltage, Vz, with Zener current will be minimized when the variations in current flowing through the Zener is near Izt.

The Zener Voltage, VZ, of a zener diode is the breakdown voltage of a zener diode when it is connected in reverse-biased in a circuit.

Matching the Zener Diode and Resistor to the Situation

1. Choose a Zener Diode. Since we need 8 Volts we can choose between a 7.5V or an 8.2V zener diode.
2. Calculate the Maximum Current in the Circuit.
3. Select the Power Rating of the Zener Diode.
4. Select the Resisitor.
5. Select the Power Rating of the Resisitor.

The current, IZM, of a zener diode is the maximum current that can flow through a zener diode at its rated zener voltage, VZ. If the zener diode is given more power than the power rating, it may become damaged or destroyed. The maximum current, IZM, is there to prevent the diode from exceeding its power rating.

Current : The current, IZM, of a Zener diode is the maximum current that can flow through a Zener diode at its rated voltage, VZ. Typically there is also a minimum current required for the operation of the diode….Zener diode specifications.

Izk is the knee current of the zener diode. When the current flowing into the zener is larger than the specified knee current, it will permit significant current flow in the reverse direction.

Measure the reverse-biased voltage on the Zener diode by switching the multimeter probes. Place the positive lead on the marked or cathode side, and the negative lead on the unmarked or anode side. You should get a reading indicating infinite resistance or no current flow.