Zener Diode Help

Thread Starter


Joined Jun 22, 2004
I have a question about Zeners. I'll attach a circuit. In the circuit it doesn't matter what the Vin is or the actual values of the Resistors. The Resistor what is in parallel with the Zener will be the Load Resistor.

My questions are:

If your Load Resistor Voltage increases then what happens to your Zener Current?

If The load resistor increases then what's the problem if any?

If your load resistor voltage decreases then your Resistor current does what? (this is refering to the first resistor in series)

If Voltage in increases then the current through the 1st resistor will do what?



Joined Jan 22, 2004

am referring to the series resistor. it is a current limiter resistor to ensure that the zener diode does not operate beyond its knee region. that knee region is set by the manufacturer of that device and it a standard practice that it is set at 5ma.

now that you know what the current limiter does, i guess you can put things 2 & 2 together to answer your various question.


Joined Nov 17, 2003
When you say load resistor, do you mean R2?

If so you must be aware that the zener circuit you have drawn works to maintain the volatge at a constant voltage, irrespective of changes in Vin and values of R2. Further increases in Vin will appear across R1. This is known as a Zener Shunt Regulator and is often employed at the output of a AC to DC convertor and AC to DC power supply (Ok, that come in slightly more complex form than shown!)

Just a little infomation on the current limiting resistor that mozikluv talks about. The series resistor must be capable of supplying current IR1:

IR1 = Izmin + ILmax

Where Izmin is your minimum zener current required to cause avalanche breakdown to occur in the zener diode (and thus allow it to operate as a voltage regulating element), and ILmax is the maximum current that your load can handle. All this must be achieved whilst:

Vout = Vz

Even at the lowest value of Vin.

Buy applying simple circuit analysis rules we can design these regulators to achieve pretty much any (reasonable) specification we require.

I've tried to cover your questions without giving you the answers. :D If theres anything you still do not understand, post back and I'll try to explain it better.


Joined Jan 22, 2004

heres some useful formulas you can use whenever you wish to apply this kind of circuit.

1. Vn(min) > 2 + Vo(max)

2. Load voltage = Nominal Zener voltage

3. R1 = Vn(min) - Vz(max) / Iz(min) + Il(max)
Iz(min) > Izk = 0.25ma * 2 = 0.5ma
Vz(min) = Vz(nom) - %tolerance
Vz(max) = Vz(nom) + %tolerance

4. Iz(max) = [Vn(max) - Vz(min) / R1(min)] - Il(min)
R1(min) = R1(nom) - %tolerance

5. Pz(max) = Iz(max) * Vz(max)

6. Pr1(max) = (Vn(max) - Vz(min))^2 / R1(min)

i would like to correct the knee region of 5ma. it should be 0.5ma

"If The load resistor increases then what's the problem if any?"

first and foremost when you use this kind of circuitry the spec of the zener diode that you will use is in conformity of the load. i believe what you are trying to imply is this, if you change the load but is not of the same spec as the original. these are the problem that you will meet, 1. voltage need of the replacement is not the same as the original load 2. current requirement of the replacement is not again the same as the original. 3. the voltage need of the replacement is the same as the original but the current need is higher than the original (you'll burn your parts)
4. the current need of the replacement is lower than the minimum current you have designed.


Joined Nov 17, 2003
In addition to what has been said before:

Izmin corresponds to Vin(min) and IL(max)
Izmax corresponds to Vin(max) and IL(min)

Just to also say, that when you obtain a practical design specification for one of these Zener Shunt Regulators, you will find that the value of Vz is generally constant, i.e. you require a fixed, stable output voltage, with no strict maximum and minimum, therefore you can use the equations that mozikluv has stated before but use your value of Vz for Vz(min) and Vz(max).


Joined Jan 22, 2004

i think this the kind of reply you want:

. "as a voltage regulator, smoothing out any voltages variations occuring in the supply voltage across the load.
When being used a voltage regulator, if the voltage across the load tries to rise then the Zener takes more current.
The increase in current through the resistor causes an increase in voltage dropped across the resistor.
This increase in voltage across the resistor causes the voltage across the load to remain at its correct value.
In a similar manner, if the voltage across the load tries to fall, then the Zener takes less current.
The current through the resistor and the voltage across the resistor both fall.
The voltage across the load remains at its correct value."

this was extracted from a hyperphysics project from Georgia State Univ.