While learning about diodes I came across of a very interesting type of diodes, the Zener diode. Here is what I've learned about it.
What is a Zener diode
Normally a diode is made to conduct current in its forward bias condition (the Anode connected to Positive and the Cathode connected to Negative) and to oppose the current flow while on reversed bias condition (reversed connection) . Actually, it will oppose the current flow while on reversed bias condition up to a designed limit like 50VDC (1N4001) or 1000VDC (1N4007) but it would immediately start to conduct current when this limit is exceeded.
The same thing happens with the Zener diode with an exception: while on its reversed bias condition it would oppose the current flow up-to a given voltage called breakdown-voltage or Zener voltage (Vz), but after that voltage the diode starts conducting the current as normal while keeping the voltage across it to a steady voltage value (only if the current flow Iz is maintained under a specified threshold).
Source: www.electronics-tutorials.ws
For instance a 1N4733A Zener diode has a breakdown voltage of Vz=5.1V. What it means is that while connected on reversed bias and while the input voltage is less than 5.1V it opposes the current flow. However, as soon the voltage reaches the breakdown threshold of 5.1V it starts immediately conducting current while maintaining a steady voltage across it of about 5.1V. Keep in mind that the more current (Iz) it conducts the more it heats up. The more it heats up the less it opposes the current flow and thus we can see that it reaches a certain level when its resistivity is no longer stable and so the voltage across it (which obviously increases up-to the input voltage).
This is a little demo that shows how a Zener diode works:
Maximum reversed bias current
Usually the diode datasheet specifies its breakdown-voltage and the power dissipation. Knowing its breakdown-voltage (Vz) and the maximum power dissipation we can calculate the maximum current that can handle while keeping the voltage across it at a steady level:
Ptot=Vz*Iz => Iz=P/Vz
Example:
The 1N4733A Zener diode has a power dissipation of Ptot=1.3W and its breakdown-voltage is Vz=5.1V. By using the equation above we can deduce that the maximum current it can handle is Iz=1.3W/5.1V=255mA. So we can expect to see a quite steady voltage around 5.1V while the current flow is less than 255mA.
Zener diode as a voltage regulator
We saw that the diode connected on reversed bias would have a steady voltage across it if the current flow is less than a certain threshold. Thus the Zener diode can be used as a fixed voltage regulator for projects that requires a small current flow. In order to make sure we keep the current under a certain threshold we need to limit the current by using a resistor Rs connected in series with the diode. The minimum value of the resistor can be calculated by using the maximum current that the diode can handle in the reversed bias condition and the voltage drop across the resistor (which is the difference between the input voltage and the diode's breakdown voltage):
Rs=(Vinput-Vz)/Iz
Example:
Let's take the example of a 12VDC circuit when we use a 1N4733A Zener diode which breakdown-voltage Vz=5.1V and its maximum reversed bias current Iz=255mA. To limit the circuit to maximum 255mA we connect a resistor Rs in series with the diode.
The minimum value for the series resistor would be: Rs=(12V-5.1V)/255mA=27Ω
Zener diode applications
As we saw earlier one way we could use a Zener diode in our projects is as a voltage regulator for small currents.
Due to its reversed bias function we could use it to clip-off the negative or the positive part of an AC signal. For instance, by using a 10VAC power supply and a 1N4733A Zener diode in reversed bias mode while on series with a current limiting resistor we clip-off the negative part of the AC signal that is under the diode's forward voltage (VF=1.2V) and also the positive part of the AC signal that is greater than it's breakdown-voltage (Vz=5.1V).
The below screenshot confirms the above example:
Fig. 1 Clipping-off the AC signal with a reversed biased Zener diode
Conversely, if we would connect the diode in forward bias mode then it would clip-off the positive part of the AC signal that exceeds its forwarding voltage VF=1.2V and also its negative part of the AC signal that goes under its breakdown-voltage Vz=5.1V:
Fig. 2 Clipping-off the AC signal with a forwarded biased Zener diode
What would happen if we would replace the Zener diode with a series of 2 Zener diodes, one on forwarded bias mode and the other one on reversed bias mode and we would measure the signal across the series diodes?
Let's think about it for a while. When the AC signal traverses the first reversed biased Zener diode we would end up with a squared wave like in the Fig. 1 above. Conversely when the current passes the next series diode we would expect to get a squared wave like in the Fig. 2 above. By adding the both squared waves together we would expect to get something like the image below (see the yellow graph captured on CH1):
Fig. 3 Clipping-off the AC signal with a dual forwarded-reversed series Zener diodes
In the image above I captured both the AC input signal in its sinusoidal form and the AC signal across the series connected Zener diodes clipped-off to a squared wave form. Please note that we could also use different Zener diodes that have different breakdown-voltages.
Another arrangements that we could use is to connect the Zener diodes back to back but in parallel connection. In that case we would clip-off the part of the wave that exceeds the forwarding voltage VF=1.2V (see the yellow graph on CH1):
Fig. 3 Clipping-off the AC signal with a dual forwarded-reversed parallel Zener diodes
These Zener diodes AC signal clipping is sometimes called "the poor man's square wave generator".
Please note that a similar result we could get with a normal PN-junction diode except that the clipping point would be very close to the wave maximum and minimum amplitude (minus the diode forward voltage) where in case of a Zener diode it clipped at the diode's breakdown-voltage level.
Now, if you think that this article was interesting don't forget to rate it. It shows me that you care and thus I will continue write about these things.
Eugen Mihailescu
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