re: zener diode

lemon

Joined Jan 28, 2010
125
Hi:
I have been given a question to answer in a weeks time, but don't have a clue how to answer it.

Question:
Determine the minimum and maximum loads currents for which the zener diode on the figure below will maintain regulation. What is the minimum RL that can be used.

Vz=12v, Izk=3mA and Izm=90mA

Please see attached image file.

I know a zener diode allows current and voltage in both directions. The way this power source is connected is forward biased. Izk appears to be going in reverse bias.
If anyone is up to a challenge, please help guide me through this.
thank you

StayatHomeElectronics

Joined Sep 25, 2008
1,020
First of all, what happens if the current through the zener is below 3mA or above 90mA?

If there is no load resistor, how much current is going through the zener?

What happens to the current going through the zener when a load is added?

lemon

Joined Jan 28, 2010
125
Hi StayatHomeElectronics:
Thanks for your reply. And Beenthere of course.
I will try to answer your first three questions, but before that, could you tell me what is meant by the terms Vz, Izk and Izm. Of course they are current and voltage but I have never seen the zkm before?

Duane P Wetick

Joined Apr 23, 2009
420
The general formula is:

Iz(max) = Vin(max) - Vz(min) / Rs - Iload (min)

Pz(max) = Iz(max) x Vz(max)

Fairly straight forward notation...

Cheers, DPW [ Everything has limitations...and I hate limitations.]

StayatHomeElectronics

Joined Sep 25, 2008
1,020
I was hoping that would have been defined in the class.

I would think the Izk is the current (I) of the zener (z) at the knee (k) of the chart where the diode starts to regulate correctly.

I would also think the Izm is probably the current (I) in the zener (z) at it maximum (m) value that it can sustain.

Vz is the voltage (V) on the zener (z) when it is maintaining good regulation.

Last edited:

lemon

Joined Jan 28, 2010
125
yes. It would have been nice to have been introduced to the terms and been able to become familiar with them before teach raced off at 180mph. This is why I accepted the challenge to answer his question, therefore forcing myself to understand it before next class tomorrow.
Looking at the diagram I can see Izk is Iknee. Izh is Imax. Vz is the voltage supply. I'm sorry, I should have been able to work that out before asking the question.

lemon

Joined Jan 28, 2010
125
sorry - Vz is as you guys said of course - voltage supply across the diode

lemon

Joined Jan 28, 2010
125
sorry sorry. its Izm as you said also, not Izh. I understand them all from your explanation now.
so, if I understand so far. A Zener diode can function in both forward and reverse bias. It is used to supply a stable voltage to a circuit when there is an unstable source. This is possible because of doping in the depletion region. R is required to drop the voltage difference between the Zener voltage and the unstable supply.
If we remove Rl from the diagram, we have 24v supply and 470ohm resistor.
V=IR - I=0.05A=50mA
We are given Iknee=3mA
For minimum current to maintain Vz at 12v, the resistance required is
12/3mA=4kohms
hmm. If we add Rl(load), then isn't that going to force the current through the diode?

Wendy

Joined Mar 24, 2008
21,840
Think of the zener as a variable diode, that will adjust itself to whatever value is needed to create it's zener voltage. This is a math analogy, but it mostly works.

If the voltage drops below the zener value, it no longer conducts, it is out of circuit. This sets the minimum current value, where the voltage drop of the resistors formed by the load and the limiting resistor cross the zener voltage.

The maximum current a zener regulator is basically the wattage of the zener. With no load the zener will absorb all the current, and heat up using the power formula...

P=EI, Watts = Voltage X Current

Adding a load "steals" the zeners current, the zener has to absorb less current as the load absorbs more.

So not having a load forces current through the zener diode, not the other way around.

lemon

Joined Jan 28, 2010
125
so, should I consider the limiting resistor and the load resistor as resistors in series?

Wendy

Joined Mar 24, 2008
21,840
Definately. The zener is a variable component next to the load, equivalent to a parallel resistor.

lemon

Joined Jan 28, 2010
125
So actually, calculating the minimum Rl is easy (he says, brimming with confidence yet unaware he is just about to slide on his face)

24v=3mA(Rl+470)
24v=0.003+1.41
24v-1.41=0.003Rl
22.59/0.003=7530 or 7.53kohms

How am I doing?

sgardner025

Joined Nov 5, 2009
79
The way I do it is to look at Imax, which will be (Vsource-Vz)\Rs. This is what you have to work with. The maximum load resistance will be what affects Izm and the minimum load resistance will affect Izk.

lemon

Joined Jan 28, 2010
125
What is Rs? Resistors in series?

Wendy

Joined Mar 24, 2008
21,840
Actually the zener will absorb all the load (no load resistor) just fine. Your problem stated 3ma as the minimum load? Is this for the 24V power supply or just the load resistor?

With no load, 24VDC, and a 12V zener your limiting resistor will drop 12V. From there you can figure how much current the circuit will pull with no load resistor.

The only time this circuit really stops regulating is if the load resistor is too low, and pulls too much current. It is pretty equivalent to current foldback on a power supply. I like zener regulators (not everyone does), for such a simple circuit they work really well.

Was the R of 470Ω a condition of your homework, or did you calculate that?

When dealing with real world zener regulators wattage is as important as the other parameters.

sgardner025

Joined Nov 5, 2009
79
What is Rs? Resistors in series?
Yes I was talking about the series dropping resistor. This resistor will always drop Vsource-Vz, or as long as the zener is conducting anyway. If the zener falls out of conduction then things will change.

lemon

Joined Jan 28, 2010
125
Bill - It was written on the diagram (see attached file)
R=470ohms

Attachments

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lemon

Joined Jan 28, 2010
125
So, I'm getting really confused now. I have no idea what I need to consider or calculate in order to answer these two questions from the diagram. I believe I basically understand how the zener works but get confused calculating it all in a circuit as I'm not sure how each thing affects the other. I have a basic understanding of circuits - like V=IR and P=IV, series and parallel circuits etc.

Wendy

Joined Mar 24, 2008
21,840
Figure the circuit without the load. This will tell you how much the circuit will draw from the 24V power supply. It will still be regulated.

Figure out what the load has to be to match the zener voltage if the zener isn't there. This will tell you the maximum current it will provide.