Identifying, theory, testing . . . and how they work in a circuit.
                         -by Colin Mitchell

The diagram below shows the symbol for an ordinary diode (called signal diodes and powers diodes) and a zener diode. The "flag" on the cathode of the zener symbol indicates the diode is a zener. Sometimes zener diodes have a red body but there is no other special package for a zener diode. Sometimes the zener voltage is written on the diode, such as: BZX84-C5V6. This is a 5.6v zener.

A zener diode looks like an ordinary diode and in fact it is an ordinary diode except for one feature. It allows a fairly low voltage to appear across it, when it is reverse biased. In the animation below, both the zener diode and ordinary diode produce a characteristic 0.7v drop across them when they are forward biased. The diagram shows exactly how this 0.7v appears. As the voltage increases from 0v, nothing flows through either diode until 0.7v is reached. Anything over 0.7v passes through the diode and thus the voltage on the left-hand-end of the diode is always 0.7v lower than the supply voltage. We have used a supply voltage of 2v to show that the effect occurs at 0.7v and any voltage above 0.7v passes through the diodes.

The 0.7v drop is called the "characteristic voltage drop" of the diode or the "forward voltage drop" - and this applies to both diodes.  If we reverse the two diodes in the circuits above, we can show the difference between an ordinary diode and a zener diode. We will take a 5v6 zener diode and a supply voltage from 0v to 10v. 

The ordinary diode "resists" the voltage across it because it can withstand a voltage of 100v. (A signal diode can withstand approx 90v and a power diode can withstand 100v to 400v or more). The 5v6 zener can only withstand a voltage of 5v6 before it "breaks-down" and allows current to flow. When it "breaks-down," any voltage above 5.6v is passed to the other side of the diode and this "new" voltage appears across the load and causes current to flow through the load. 


Another component that acts exactly like a zener diode is a LED. A red LED is a 1.7v zener diode and it produces light according to the current flowing through it. It's a zener diode with a window, so you can see the current flowing.

The LED is a Zener diode!

When the voltage is below 1.7v, no current flows through the LED. As soon as the supply voltage reaches 1.7v, the LED starts to illuminate. As the supply voltage increases, the voltage across the LED remains the same  (1.7v) and the "left-over" voltage appears across the resistor. As the voltage rises across the resistor, more current will flow through the resistor and this current also flows through the LED and makes it glow brighter. 
In the animation above, the voltage on the left-hand voltmeter is always 1.7v lower than the supply voltage. 

We can re-arrange the zener circuit above very slightly to produce the standard zener regulator circuit. This is shown in the diagram below:

The two components we are discussing are the zener and resistor R. These two components are in series and the supply voltage must be above 5.6v for the zener regulator circuit to work. When the supply voltage is above 5.6v, the voltage across the zener remains at exactly 5.6v and this voltage is available for the "circuitry" we are supplying with this fixed voltage. The "circuitry" is called the LOAD. 
If the supply rises above 5.6v, the excess voltage is dropped across the resistor "R.

Now we come to the most difficult part of the discussion. The LOAD normally takes a varying amount of current. As different parts of the circuit turn on and off, the current requirement increases and decreases. 
Normally an electrolytic across the power rails and will deliver the extra current  - this will be added to the circuit above in the final design. 
For the moment, we need to show how the zener provides the extra current and how it maintains a constant voltage for the load. 
In the example below, the LOAD requires a current of 15mA and the supply voltage allows 20mA to flow into the circuit. This means 5mA will be flowing through the zener.

When the LOAD requires extra current, it comes from the ZENER! 
This is the way it works:
Suppose the LOAD requires an extra 1mA. Whenever a LOAD requires extra current, its resistance decreases and this means the voltage across the LOAD decreases. This slightly-lower voltage will be passed to the zener and it will turn-off slightly. The result is 1mA less will flow through the zener and the LOAD will take the extra 1mA.  
The reverse can also occur. If the LOAD requires 1mA less, the voltage across the zener will rise slightly and the zener will take 1mA more. If the LOAD is removed from the circuit, the zener will take the full 20mA. 

A LED can be placed in series with the zener diode to increase the zener reference voltage. A red LED has a characteristic voltage of 1.7v and other colours have 2.1v to 2.3v depending on the manufacturing process and the colour. The size of the LED does not influence the characteristic voltage it develops but sometimes the quality of the LED (such as HIGH BRIGHT) develops a slightly different voltage. 

The circuit above will put 5.6v + 1.7v = 7.3v across the LOAD. The supply voltage will have to be increase by 1.7v for the same current to be delivered.

An ordinary diode can also be added to the zener line to increase the reference voltage. An ordinary diode develops 0.7v across it when in the "forward-voltage" mode.  The voltage across the LOAD will be 5.6v + 1.7v + 0.7v = 8v. This is a very convenient way to create almost any zener reference voltage. 

In the circuit above, the zener is taking 5mA. This means the LOAD can take 15mA + 5mA = 20mA and the regulated voltage will remain constant. If the LOAD wants 21mA, the zener cannot supply the extra current and the result is the extra current flows through resistor "R" and creates an additional voltage drop across it. This means the voltage across the LOAD falls and thus the regulated voltage is lost. When this occurs, we say the zener has "DROPPED OUT OF REGULATION." 

As you can see from the circuits above, the current flowing through the zener is not being used for any practical purpose and is being WASTED
Zener regulators are very wasteful and should only be used if the project is being powered from a plug-pack. 

To test a zener diode, you will need a power supply that will deliver a voltage above the expected voltage of the diode. Most zener diodes will fall into the range 3v9 to 39v and the practical range is 5v6 to about 20v. 
This means you will a 24v supply.  The simplest answer is to connect 9v batteries in series or any old batteries that are nearing the end of their life. 
A current-limit resistor must be added in series to limit the current to between 1mA and 10mA. A small zener such as a glass-enclosed type will require about 1mA or 2mA to flow during the test procedure. A larger zener will need 5mA - 10mA to flow and "break-down" and create the zener voltage.

To test a zener, connect two or three 9v batteries and a 10k resistor as show in the  circuit above. Add the zener and watch the glow from the LED. It should be fairly bright. Change the resistor to 4k7 or 1k to get good brightness from the LED. This will prove a satisfactory current is flowing. Read the voltage across the zener with a voltmeter. If the supply voltage is 27v and the reading is above 25v, the zener is not breaking down and it is either a higher voltage device or it is open - and FAULTY!  If the reading is zero, the zener is also faulty. It has SHORTED! 
You can also add any combination of LEDs, zeners and ordinary diodes to obtain any desired voltage. 

Zeners can be placed in parallel but they really don't provide any real technical advantage.
However here's the voltage they provide and how to work it out.
In the diagram below, a 12v and 18v zener is placed in parallel. The resulting rail voltage is 12v.
Here's how to work it out.
As the voltage rises, the lower-voltage zener will break down and prevent the voltage rising any further. Thus the 18v zener will not come into operation and can be removed from the circuit.
If the 12v zener goes "open," the 18v zener will take over and the output voltage will 18v.

Zeners can be placed in series to obtain almost any voltage. Simply add the values to obtain the final rail voltage.
There is only one thing to remember. The wattage of each zener must be the same. The reason is the current required by a zener to maintain breakdown depends on the wattage of the device. If the 6v2 is a 5 watt zener and the 12v is a 1/4 watt device, the current required to keep the 6v2 in breakdown will be 10 times more than that required for the 12v and it will get very hot.


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