Basics of using zener diodes

[diy didi]

This is a back to basics question. More specifically how to calculate the series resistor to for the zener diode if one is for example making a series pass regulator with a transistor or a CCS.
Experimenting with zeners led me to the conclusion that a resistor value calculated for a 15V 1W zener will differ to one calculated for a 15V 400mW zener.
The 1W needs a higher min Iz to keep regulation. But what is the min Iz needed? I know datasheets sometimes state this, but sometimes one has an unknown brand zener. What then?
Anyone have a quick fail safe method?

 

[Pommie]

If you are just using a zener to get a lower voltage then the series resistor must drop the excess voltage at the required current.

For example, if you have a 12V supply and need 5V as 20mA then the series resistor needs to drop 7V at 20mA - R=7/0.02 = 350Ω. On no load the zener will conduct the full 20mA and will dissipate 5*0.02 (0.1) Watts. The resistor will dissipate 7*0.02 = 0.14W.

HTH

Mike.

 

[Nigel Goodwin]

It really depends on the load conditions as well, and if they are constant or varying.

 

[ronsimpson]

The 1W needs a higher min Iz to keep regulation.

A 1W Zener does not need 1 watt. That is the maximum wattage. It will work at a much lower level.

I have always used current (like Ponnie) in post #2. You can get the same answer by using watts.
-Stay away from the 1W level. 0.5 is safe.
-If your load always is 0.5w then this is simple. But life is not simple. Many loads vary. For an example: 0.2 to 0.5 watts.
-Push 0.6W at the Zener and the load will take 0.2 to 0.5 of that leaving 0.4 to 0.1 watts for the Zener. If the load is unplugged the Zener can easy take 0.6W.

You should also do the math for changes in supply. From post #2. (12V to 5V) But the 12V supplies might be 11 to 13V. So you need to through enough power at the (Zener//load) so it works at 11V and does not burn out the resistor or Zener at 13V.

I make a table of 4 states. Each state must have some Zener current or wattage. Every state must keep the watts in the resistor below safe levels. The watts in the Zener must not get above rated levels.
1) Min load, Min supply
2) Min load, Max supply
3) Max load, Min supply
4) Max load, Max supply

 

[Colin]

"For example, if you have a 12V supply and need 5V as 20mA then the series resistor needs to drop 7V at 20mA - R=7/0.02 = 350Ω. On no load the zener will conduct the full 20mA and will dissipate 5*0.02 (0.1) Watts. The resistor will dissipate 7*0.02 = 0.14W."

This is the very reasoning to avoid.
With this reasoning you have allowed the circuit to drop out of regulation.

 

[Pommie]

"For example, if you have a 12V supply and need 5V as 20mA then the series resistor needs to drop 7V at 20mA - R=7/0.02 = 350Ω. On no load the zener will conduct the full 20mA and will dissipate 5*0.02 (0.1) Watts. The resistor will dissipate 7*0.02 = 0.14W."

This is the very reasoning to avoid.
With this reasoning you have allowed the circuit to drop out of regulation.

OK, I should have said max 20 mA. However, unless you want very rough regulation, you would not use a zener shunt regulator.

Mike.

 

[ronsimpson]

However, unless you want very rough regulation, you would not use a zener shunt regulator.

C10=10V Zener. You can see the "regulation" or voltage for a 10V Zener. "Good or rough" regulation is not well defined.

It is true that Zeners below 5V are not so good.

"BZT52C"

 

[Pommie]

That is a lot better than I remembered it.

Mike.

 

[Colin]

"That is a lot better than I remembered it. "

Things have improved in the past 40 years.

 

[Pommie]

"That is a lot better than I remembered it. "

Things have improved in the past 40 years.

However, the basic principal is still the same - see post #2.

Mike.

 

[Colin]

It is not a basic principal

 

[Pommie]

What is it then Colin?

 

[Colin]

the basic principle is still the same

 

[Pommie]

Would you care to enlighten me on "The basic principal"?

 

[Colin]

I have no idea what "The basic principal" is.

 

[Pommie]

It is not a basic principal

You seem to know what it isn't but not what it is!!

Or are we differentiating between A basic principle and THE basic principle.

 

[jpanhalt]

I read it initially as a lighted hearted attempt at a spelling correction, i.e, principal to principle.

 

[MikeMl]

...

I make a table of 4 states. Each state must have some Zener current or wattage. Every state must keep the watts in the resistor below safe levels. The watts in the Zener must not get above rated levels.
1) Min load, Min supply
2) Min load, Max supply
3) Max load, Min supply
4) Max load, Max supply

To answer diy didi's specific question, to design a basic shunt Zener voltage regulator, then start with case #3 (Max load, Min Supply). To provide the rated regulation, the minimum current through the Zener must be > Iz (test current) = 5mA for most of the "BZT" Zener series as shown on the plots in post #7.

 

[ronsimpson]

the minimum current through the Zener must be > Iz (test current) = 5mA

I am going to be the "donkey's behind again" and disagree.
I think the "test current" is only the current they test at. Not the minimum current.
From the BZX84 series data sheet. Note the voltage remains very good down well below uA. This type of diode was chose to show what I want to show. There are other diode families where the high voltage Zeners do not work well at 1uA.

On the right side, the 7.5V part is good to 2uA. Depending on what good means.

As long as I am being contrary; You can now get many types of Zeners at 1% and some at 0.5%.
Next item; temperature stability. This depends on which series of diode and what voltage with in a series. Example: most (certainly not all) families of diodes are best for temperature near 5V.

 

[MikeMl]

...
I think the "test current" is only the current they test at. Not the minimum current.....

I agree, but designing for Iz as a minimum Zener current makes for a reliable design. Usually, you do not know the max current drawn by the load that precisely.