Zener Diode Circuits & Applications

– a compact note or tutorial that covers the requisites of Zener diode or voltage reference diode applied in various PSU (power supply units) and other circuits.

Zener diodes have diversity of uses in various circuits.

The most trivial Zener diode is that when it is applied like a voltage reference component. Among the diversity of shapes of powersupply circuits the Zener diode circuit uses persistent voltage as a reference.

A simple voltage regulator by Zener diode circuit:

When you are using Zener diode in regulator circuit, it necessarily has the limited current entering into it. If a faultless source of voltage was applied to it, it will pass too much current as soon as reaches breakdown-voltage. To overwhelm this, a current source must drive the ZenerDiode. And in this way current is limited to the selected value.

Practically in a circuit, resistor is the most simple form of source of current. This will bind the current-in-use by zener diode and thus guarantee that the working position of diode remains nearly fix.

1-simple-circuit-of-a-zener-diode-shunt-regulator Zener diode
Simple circuit of a Zener diode shunt regulator

It is very simple to calculate value of series resistors. It is just the voltage across resistor, distributed by the required current. You can choose the level for Zener current as it suits this circuit and Zener diode being used.


Value of Resistor (ohms)     =     (V1 – V2)   /   (Load current + Zener current)
V1 = Input voltage
V2 = zener diode voltage

Such a Regulator circuit is called a Shunt Regulator, where the regulating component is applied in parallel manner with the load, in the circuit. The Zener diode controls the voltages appearing around load letting a part of current to run through the diode and dodge the load to sustain the voltage around it. For large levels of power the shunt regulators are usually found as it is very ineffective, but they are very efficient for low power level. You can also use the zener diode as a Shunt regulator to generate a steady reference voltage, and then a regulator in series can use this voltage to yield the needed stable voltage. Analogue Regulated power supplies effectively use this technique.


Zener diode circuit for PSU with series transistor

The above shown simple Shunt regulator is not mostly effective and not feasible for higher current applicationz. The way out of it is to use a zener diode circuit with a seris pass-transistor. The simple circuit diagram is displayed below where the transistor is being used as emitter follower.

2-zener_diode_circuit_for_a_simple_regulated_power_supply Zener diode
Zener diode circuit for a simple regulated power supply

When using such a circuit, the current needed from the zener resistor potential divider must be calculated. It is the transistor’s emitter current distributed by the gain.

When selecting zener diode voltage, one should remember that emitter voltage will be lesser than zener voltage by an amount equal to base emitter voltige minus 0.6 volts, for Silicon transistor.


Zener-diode circuit to avoid OverVoltage

One more shape of zener-diode circuit is overvoltage shield circuit. Though power-supplis are usually dependable, however the effect of FET or series pass transistor, in case of shortcircuit, can be disastrous. In such case the maximum voltage (unregulated) would be applied to the circuits which use regulated power. And remember this could demolish all chips which are being powered.

This problem can be overcome by using “crowbar circuit”. Such circuit fire an SCR, as soon as it senses overvoltage circumstances. This rapidly grasps the Output-voltage and as shown below, it burns the fuse that cut off the power of input source.

3-scr_overvoltage_crowbar_circuit Zener diode
SCR overvoltage crowbar circuit

SCR overvoltage circuit works by firing SCR whenever overvoltage is noticed. Zenar diode is selected to ensure a voltage value beyond the regular operating voltage – adequate margin which is not fired below regular operating situations, but slight enough to permit current flow rapidly whenever the fault situation is sensed.

Under usual operating circumstances the Output-voltage is less than the revers voltage of zener diode and not any current flows through it and this way SCR gate is not afire.

On the other hand, if voltage increases above the acceptable voltage, zener-diode will start conducting, SCR will be fired and fuse would be blown.


Circuit tips

The zener diode is much elastic and advantageous circuit component. Still, similar to other electronic components, there are few clues and hints which empower the best of the zener diode. Some of them are listed here:

  • Choosing right voltage for greatest stability:

Whenever stability is required, with temperature change, zener voltage Reference diode must be selected to ensure a voltage value of about 5.5 V. And5.6 V is closest ideal value even though 5.1 V is a different widely held value according to its closeness to 5 V essential for certain logic families. When we need different level of voltage, we can use 5.6 V zener and the nearby microelectronics may be used for transmitting this to requisite output value.

  • Buffering the zener diode-circuit with a follower or emitter source:

To retain the voltage as of zener diode as steady as probable, the current from the zener diode should be retained fixed. Any deviation in value of current drained by load should be reduced as this will cause a change in the current from zener diode and results in minor voltage deviations. The variations make happen by load may be minimized through using follower- emitter- stage to decrease the current availed from zener-diode circuit and as a result the changes it perceives. And this also takes the advantage of using minor Zener diodes.

  • For best stability, drive using steady current source:

Using a good constant current source is also a method of improving the stability of Zener. Just a resistor is sufficient for various applications, but some improvements can be made with a more efficient current source as current could be retained almost irrespective of any changes in supply.

  • Guarantee adequate current necessary for Reverse-Breakdown:

This is essential to confirm that enough current is delivered from diode to certify that it rests in Reverse-breakdown. A current value of about 5mA should be retained for a usual 400 mw device. For perfect values of lowest current, one must consult the data sheet for the specific voltage and device.

  • Make sure the extreme current limits are not surpassed for Zener diode:

Although it is essential to confirm enough current is delivered via the Zener diodes, the extreme limits should not be surpassed. Typically it may be little like balancing in certain circuits as deviations in Load current would affect the Zener diode’s current. Attention should be given that neither the extreme current should be exceed nor the extreme power usage (Power = Zener diode current x Zener voltage). Even if this seems to be problematic, you can use an emmitter-follower circuit to buffer zener-diode and enhance the current ability.

If we make best use of Zener diodes, they can deliver highest performance. They frequently surpass the required performance, but keeping in view the low cost and ease of use; they make available a very efficient choice.



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