The Zener Diode Regulator
The Zener Diodes can be used to produce a stabilized voltage output with the low ripple under varying load current conditions. By passing less current through the diode from the volt age source, via a appropriate current limiting resistor (RS), the zener diode will conduct enough current to maintain the voltage drop of Vout. We remember from the earlier topics that DC output voltage from half or full-wave rectifiers has ripple superimposed onto DC voltage and that as the load value changes so does the average output voltage. By connecting the simple zener stabilizer circuit as given below across the output of rectifier, a more stable output voltage is produced.
Zener Diode Regulator
The resistor, RS is linked in series with zener diode to limit current flow through the diode with the voltage source, VS being linked across the combination. The stabilized output voltage Vout is taken from across zener diode. Zener diode is connected with the cathode terminal of it connected to the positive rail of DC supply so it is reverse biased and will be operating in the breakdown condition of it. Resistor RS is selected so to limit maximum current flowing in circuit.
With no load connected to circuit, the load current will be zero, ( IL = 0 ), and all circuit current passes through zener diode which in turn dissipates its maximum power. The small value of the series resistor RS will result in the greater diode current when load resistance RL is connected and large as this will increase power dissipation need of the diode so care should be taken while selecting the suitable value of series resistance so that zeners maximum power rating is not exceeded under the no-load or high-impedance condition.
The load is connected in parallel with zener diode, so voltage across RL is always the same as zener voltage, ( VR = VZ ). There is a minimum zener current for which stabilization of the voltage is effective and zener current should stay above this value operating under load within its breakdown region at all the times. The upper limit of the current is certainly dependent upon power rating of the device. The supply voltage VS should be greater than VZ.
One small problem with the zener diode stabilizer circuits is that the diode can at times generate electrical noise on top of DC supply as it tries to stabilize the voltage. Usually this is not a problem for most of the applications but the addition of a large value decoupling capacitor across zeners output may be needed to give additional smoothing.
A zener diode is operated in its reverse biased condition always. A voltage regulator circuit can be designed by using a zener diode to uphold a constant DC output voltage across load instead of variations in input voltage or changes in the load current. The zener voltage regulator comprises of a current limiting resistor RS connected in the series with input voltage VS with the zener diode connected in parallel with load RL in this reverse biased condition. Stabilized output voltage is always selected to be same as the breakdown voltage VZ of diode.
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