Now we come to the point of stability.
You made the following comment:
"The Website I referred to earlier says that the input voltage needs to be a few volts more than the output for the zener to be stable. How does the zener become unstable?"
The whole purpose of a zener regulator is to turn an unregulated voltage into a stable voltage.
An unregulated voltage is one that rises and falls.
The zener regulator will also turn a "supply" the dips under load into a supply that does not "dip under load."
But firstly and answer to the question: When does a zener become unstable. It becomes unstable when the current through it is less than the 1mA we mentioned before. Some zeners need 5mA minimum some need 15mA and some need 30mA or more. It depends on the voltage of the zener and the current rating. This is a value you find in the data sheet.
Now we answer the question:
The input voltage must be a few volts above the zener voltage.
The voltage of the supply is worked out in a very technical way.
You need to know the minimum and maximum load. In our case the minimum load will be 1x 220R and the max load will be 3 x 220R in parallel.
The next thing you have to do is work out the current taken by the single 220R resistor and the current taken by the 3 x 220R. Suppose the minimum current is 10mA and the maximum current is 30mA. The difference between these is 20mA.
You can see the zener will have to absorb the 20mA when the load is light. Depending on the voltage of the zener, the wattage of the zener is: its breakdown voltage x 20 = _____mW. The smallest zener is 400mW.
Note: you are getting a 30mA supply for the cost of having to absorb only 20mA.
Now, to work out the resistance of the feed resistor, the current will always be 31mA. This current will be divided between the zener and the load as 1mA through the zener and 30mA through the load or 10mA through the load and 21mA through the zener.
Now we can work out the value of the “feed” resistor.
We know the current flow = 31mA We also know the voltage of the zener = Vz.
Now we come to an amazing situation. The resistance of the feed resistor can be worked out easily if the supply is constant. But of the supply fluctuates, we have to take this into account.
If the supply is constant, the resistance of the feed resistor is worked out by determining the voltage across it, thus: Voltage of supply – Vz = Vfeeder
If the voltage of the supply is 3v higher than the zener voltage, the resistance of the “feeder” is: R = 3 / 0.031 = ______ohms
If the supply fluctuates and its lowest voltage is 3v - the above feeder resistor can be used.
If the supply rises 2v, the current will increase through the feeder and this current will be taken by the zener.
The new current through the feeder resistor is: (3v + “x”) / feeder resistance = _______Amps.
You need to work out if this current (minus 10mA) will be higher than say 400mW for a 400mW zener. The wattage is worked out by (______Amps – 10mA) x Vz = ______mW.
Answers to your questions:
The input, on the left of the circuit is assumed to be fluctuating. This could happen if a power source such as a solar panel were being used, which would fluctuate with the level of light - YES
The output, on the right of the circuit, will be fixed at the Zener voltage of the Zener diode - YES
The resistor is there to protect the diode, just as would happen with an LED
At the moment there is no load - YES, but the feed resistor is also included so the input voltage can fluctuate and the output of the circuit will remain constant.
Current will only flow when the voltage rises to the level of the Zener voltage. This is because the Zener blocks the current (like all diodes do) until it breaks down. YES/NO. The zener does not conduct until its rated “breakdown voltage” is reached.
As the input voltage increases, the current through the Zener will increase too. However, the voltage drop remains constant. YES
As there is no load, the Zener will be getting quite hot - YES
A load is now added. For this example, a resistor
Current begins to flow through the resistor. This current is taken out of the current flowing through the Zener - YES
The load (resistor in our case) will warm up, and since the current through the Zener has dropped, it will cool down a little - YES
Adding more load, such as another resistor, will mean that more of the current from the Zener will be diverted to the load, and the Zener gets cooler again - YES
Eventually, if more load is added, the current requirement will be more than can be provided by the Zener. This means no current flows over the zener - YES
When this happens, the Zener is, effectively, invisible to the circuit. It’s just a power source and a load like any other circuit - YES