# Zener Diode Voltage Regulator

Discussion in 'General Electronics Chat' started by mark_3094, Feb 27, 2009.

1. ### mark_3094New Member

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I'm trying to understand how zener diodes work, and what they're used for.

I found a basic voltage regulator on this site (towards the bottom) :
What is a Zener Diode - Electric Circuit

I am having difficulty understanding how this makes a stable output.

Thanks

2. ### colin55Well-Known Member

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The way the zener diode works is this:
We increase the voltage on the left side of the circuit and current flows through the resistor ONLY when the voltage on the right hand side of the resistor is equal to the zener voltage.
We now increase the supply voltage and the current will increase through the resistor and into the zener but the voltage on the top of the zener will not increase. This is the characteristic of the zener. That's why we use a zener diode.
We now increase the voltage to the circuit slightly more, and you will see the reason for this later.
Again, the current through the resistor increases and this current flows into (or through) the zener. By now the zener will begetting fairly hot.
This load is placed across the zener.
For now we will make this load a 220R resistor.
Since there is a voltage across the resistor, a current will flow in it.
This current is taken directly from the zener. The zener will get slightly cooler and the load will get slightly warm.
If we put another 220R resistor across the zener, current will also flow in this LOAD and the zener will get cooler and the two load resistors will get equally warm.
Now we come to a technical part.
If we add another 220R across the output, we find the current needed by this resistor is more than the zener can supply, in other words the zener has run out of current, just a bank running out of money.
What happens?
The output voltage drops and the zener effectively comes out of circuit and you can remove it as far as the next set of calculations are concerned.
The circuit becomes a set of resistors and the voltage across the 3 LOAD resistors can be worked out by a simple equation.

Just re-tracing our steps slightly.
A zener diode will remain in breakdown (that is: it will have its "rated voltage" across it) when a small current flows through it and also when this current increases until the wattage of the zener is reached.
When this current is less than a few milliamps (for small zener diodes) it will not produce the "zener voltage" across it.
There is also the separate issue of the resistance of the "feed resistor" and its wattage and the voltage of the "supply."

Last edited: Feb 27, 2009
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3. ### mark_3094New Member

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Thanks for that. Took me a while to let it all sink in, but I think I understand a lot better.
I just have a question about it:

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?

I have also written up some note about it in my own words. Could you let me know if I've understood it correctly?

 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
 The output, on the right of the circuit, will be fixed at the Zener voltage of the Zener diode
 The resistor is there to protect the diode, just as would happen with an LED
 At the moment there is no load
 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
 As the input voltage increases, the current through the Zener will increase too. However, the voltage drop remains constant
 As there is no load, the Zener will be getting quite hot
 Current begins to flow through the resistor. This current is taken out of the current flowing through the Zener
 The load (resistor in our case) will warm up, and since the current through the Zener has dropped, it will cool down a little
 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
 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
 When this happens, the Zener is, effectively, invisible to the circuit. It’s just a power source and a load like any other circuit

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5. ### ericgibbsWell-Known MemberMost Helpful Member

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hi,
A zener diode is used to provide a lower stable voltage from a higher voltage.

The series resistor is calculated to allow sufficient current to flow in the zener and load under the known conditions for the circuit.

Also the wattage rating of the zener is considered.

EDIT: did this help with your questions.?

I would suggest that you consider the zener circuit for a particular application you may have or say for a specific example circuit.

Last edited: Feb 28, 2009
6. ### colin55Well-Known Member

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Now we come to the point of stability.

"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.

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.

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
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

Last edited: Feb 28, 2009
7. ### ericgibbsWell-Known MemberMost Helpful Member

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Which is.?

8. ### flat5Member

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If stability means the circuit performs as expected (desired):
Then the power source, series resistor, zener current handling ability, and load are properly accounted for.

9. ### ericgibbsWell-Known MemberMost Helpful Member

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Have you considered the temperature stability of the zener diode.?

10. ### flat5Member

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Poof! Well I did say "zener current handling ability" but that would depend on the temperature environment.
It also helps if the site is not bombed that day

11. ### mark_3094New Member

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Thanks very much.
That's helped a lot

12. ### ericgibbsWell-Known MemberMost Helpful Member

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How would the zener voltage stability be effected by the different zener voltage ratings.. thats what I am trying to get across to you.

Look at what the effect of temperature has on say, a 3V3, 5V6 and 12V zener.

13. ### flat5Member

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ok. Thanks. "It's all clear to me now." ~ H. Bogart

14. ### ericgibbsWell-Known MemberMost Helpful Member

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Great guy, was a damn good actor, old Humpty.

15. ### brandonhillNew Member

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Hi there,

Sorry if this is not the right thread.

I'm trying to make a simple, efficient solar USB charger. The zener doesn't seem to be behaving as all the guides I've seen describe. The circuit doesn't regulate at 5.1V as expected, but at at 4.2V.

- solar array puts out >7.1V under a halogen pot lamp
- 1N4733A 5.1V 1W 5% zener diode
- 22 ohm resistor (also tried 100 and 330 ohm)

The resistor value was calculated here based on a (guess) charge load of 100 mA:
Zener Diode Voltage Regulator - Electric Circuit

I was looking for the most efficient setup, as well as cheap and simple. I'm going to go with a 7805 because that's pretty foolproof, but I'd still like to understand what I'm doing wrong with the zener approach.

Any assistance is greatly appreciated.

PS. input voltage varies and the load will vary depending on what's plugged into the USB, so I can't fill out a formula.