Zener Diode I-V Characteristics
Zener diodes are used in "reverse bias" or reverse breakdown, i.e. the anode of the diodes connects to the negative supply. From the above IV characteristic, we can see that the Zener diode has a region in its reverse bias characteristic where a negative voltage is almost constant regardless of the amount of current flowing through the diode. This voltage is virtually constant even with large current variations provided that the current of the Zener diodes remains between the breakdown current IZ (minimum) and the rated current IZ maximum (maximum). The self-control of this Zener diode can be used well in regulating or stabilizing a voltage source against changes in supply or load. The fact that the voltage across the diode in the breakdown region remains virtually constant proves this is an important property of the Zener diode as it can be used in the simplest types of voltage regulation applications. The function of a voltage regulator is to provide a constant output voltage to a load connected in parallel despite ripples in the supply voltage or variation in load current. The Zener diode will continue to regulate its voltage until the current holding diode drops below the minimum value IZ (min) in the reverse breakdown region.
Zener Diode Regulator
Zener diodes can be used to generate stabilized voltage outputs with less ripple under a variety of load and current conditions. By passing a small current from the voltage source through the appropriate current limiting resistor (RS) to the diode, the Zener diode draws enough current to maintain the Vout voltage drop.
We know that the DC output voltage of a half-wave or full-wave rectifier contains ripples superimposed on the DC voltage, and the average output voltage changes as the load value changes. A simple Zener stabilizer circuit, as shown below, can be connected to the output of the rectifier to produce a more stable output voltage.
Resistor, RS is connected in series with the zener diode to limit the current flow through the diode with the voltage source, VS being connected across the combination. The stabilized output voltage Vout is taken from across the zener diode. The zener diode is connected with its cathode terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor RS is selected so to limit the maximum current flowing in the circuit.
With no load connected to the circuit, the load current will be zero, (IL = 0), and all the circuit current passes through the zener diode which in turn dissipates its maximum power. Also a lesser value of the series resistor RS will result in a greater diode current when the load resistance RL is connected and large as this will enhance the power dissipation requirement of the diode so care must be taken when selecting the appropriate value of series resistance so that the zener’s maximum power rating is not exceeded under this no-load or high-impedance condition. The load is connected in parallel with the zener diode, so the voltage across RL is always the same as the zener voltage, ( VR = VZ ).
There is a minimum zener current for which the stabilization of the voltage is effective and the zener current must stay above this value operating under load within its breakdown region at all times. The higher limit of current relying on the power rating of the device. The supply voltage VS must be larger than VZ.
In zener diode stabilizer circuits, diode can sometimes produce electrical noise on top of the DC supply as it tries to stabilize the voltage. Usually this is not a problem for most applications but the addition of a large value decoupling capacitor across the zener’s output may be required to give extra smoothing.
Zener diodes are always reverse biased. Therefore, Zener diodes can be used to design simple voltage regulator circuits to maintain a constant DC output voltage across the load regardless of input voltage fluctuations or load current fluctuations.
The Zener voltage regulator consists of a current limiting resistor RS connected in series with the input voltage VS and a Zener diode connected in parallel with this reverse biased load RL. The regulated output voltage is always selected to be equal to the diode breakdown voltage VZ.