Zener Diodes can be used to produce a stabilised voltage output with low ripple under varying load current conditions. By passing a small current through the diode from a voltage source, via a suitable current limiting resistor (RS), the zener diode will conduct sufficient current to maintain a voltage drop of Vout.
Zener Diodes are widely used as Shunt Voltage Regulators to regulate voltage across small loads. Zener Diodes have a sharp reverse breakdown voltage and breakdown voltage will be constant for a wide rang of currents. Thus we will connect the zener diode parallel to the load such that the applied voltage will reverse bias it. Thus if the reverse bias voltage across the zener diode exceeds the knee voltage, the voltage across the load will be constant.
Here we are discussing troubleshooting of Zener diode based voltage regulators.
Zener Regulated DC Power Supply
The Below Figure shows a filtered dc power supply that produces a constant 24 V before it is regulated down to 15 V by the zener regulator. The 1N4744A zener diode used. A no-load check of the regulated output voltage shows 15.5 V as indicated in part (a). The typical voltage expected at the zener test current for this particular diode is 15 V.
Fig : Zener-regulated power supply test
In part (b), a potentiometer is connected to provide a variable load resistance. It is adjusted to a minimum value for a full-load test as determined by the following calculations. The full-load test is at minimum zener current (IZK). The meter reading of 14.8 V indicates approximately the expected output voltage of 15.0 V.
Zener Diode Open If the zener diode fails open, the power supply test gives the approximate results indicated in Figure. In the no-load check shown in part (a), the output voltage is 24 V because there is no voltage dropped between the filtered output of the power supply and the output terminal. This definitely indicates an open between the output terminal and ground. In the full-load check, the voltage of 14.8 V results from the voltage-divider action of the 180 ohms series resistor and the 291 ohms load.In this case, the result is too close to the normal reading to be a reliable fault indication but the no-load check will verify the problem. Also, if RL is varied, VOUT will vary if the zener diode is open.
Fig : Indications of an open zener
Incorrect Zener Voltage As indicated in Below Figure, a no-load check that results in an output voltage greater than the maximum zener voltage but less than the power supply output voltage indicates that the zener has failed such that its internal impedance is more than it should be. The 20 V output in this case is 4.5 V higher than the expected value of 15.5 V. That additional voltage indicates the zener is faulty or the wrong type has been installed. A 0 V output, of course, indicates that there is a short.
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