# Interesting Zener Diode Observation/Question

## Is the knee intersected by line: 1. Green, 2. Red, 3 Violet

• ### 3. Violet

• Total voters
12
• Poll closed .

#### MrAl

Joined Jun 17, 2014
7,508
Hi there,

Shown in the attachment is a curve for a particular zener diode.
I is the current in amps, V is the voltage in volts.

Is the knee of the curve at the intersection of the blue curve and either:
1. The green line
2. The red line
3. The violet line

Mod note:
.Wendy

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#### OBW0549

Joined Mar 2, 2015
3,309
It's a rather arbitrary judgement call but in most cases I'd probably opt for something in the vicinity of the red line, roughly where the increase in conductivity per applied volt is greatest. But there are situations where I'd be most interested in what the Zener is doing around the green line or even below, and situations where I'd want to know what it's doing around the violet line or even higher.

So what I'd call the "knee" kinda depends.

#### Jony130

Joined Feb 17, 2009
5,157
Is there any formal definition of a knee voltage? Because I don't know any.

#### BobTPH

Joined Jun 5, 2013
2,349
Think of the curve as the top of your bent leg. The knee is clearly where the red line is.

Bob

#### MrChips

Joined Oct 2, 2009
20,912
The zener voltage is the voltage at your desired current.
Looks like it is a 6.8V zener to me.

#### SamR

Joined Mar 19, 2019
2,047
One of the definitions is to take the straight part of the plot and extend it to where it intersects the V axis as Vz. Where I/Vz is constant. Violet. The rest is leakage.

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#### kubeek

Joined Sep 20, 2005
5,718
I would say close to the red line. For me knee means the point where it bends the most, so where the peak of the second derivation is (or is it differentiation in english?) would be my choice.

#### Wolframore

Joined Jan 21, 2019
1,743
At point a reverse breakdown of the diode occurs and current increase sharply damaging the diode . this point is called knee of the reverse characteristics.

#### eetech00

Joined Jun 8, 2013
1,869
I would use the green line since that line indicates where the diode begins to "rapidly conduct".
(rapidly conduct = current "very quickly and increasingly" begins to flow) .

I agree with Mr Chips...looks like a 6.8 volt Zener.

eT

#### MrAl

Joined Jun 17, 2014
7,508
Hi,

Thanks for the replies this is even more interesting as we will see.

Well the question was to pick one of those lines or at the least point to a place on the curve between two of the lines, or even pick your own point but it must be on the curve because that is the example curve.

Before saying anything else, take a look at this next graph of the same zener but with the horizontal compressed about 10 times.
The voltage scale is the same and the current still runs from 0 to 0.1 amps, but it is just draw with the current (amps) axis shorter now.

Now again try to pick either of the three lines, the green, red, or violet, or choose a different point but please mention that point or at least keep your choice in your head as we continue the discussion. This all surprised me a bit too as i must have forgotten how this works.

opps, did it again.
Wendy

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#### eetech00

Joined Jun 8, 2013
1,869
Hi,

Thanks for the replies this is even more interesting as we will see.

Well the question was to pick one of those lines or at the least point to a place on the curve between two of the lines, or even pick your own point but it must be on the curve because that is the example curve.

Before saying anything else, take a look at this next graph of the same zener but with the horizontal compressed about 10 times.
The voltage scale is the same and the current still runs from 0 to 0.1 amps, but it is just draw with the current (amps) axis shorter now.

Now again try to pick either of the three lines, the green, red, or violet, or choose a different point but please mention that point or at least keep your choice in your head as we continue the discussion. This all surprised me a bit too as i must have forgotten how this works.
Hi

So...What are you REALLY wanting to know?

eT

#### Audioguru

Joined Dec 20, 2007
11,249
I think the knee voltage of a Zener diode is where its current is low and it only begins regulating the voltage.
The datasheet for some Zener diodes rated at 5mA shows horrible voltage regulation at only 1mA.

#### SamR

Joined Mar 19, 2019
2,047
The IEC & IEEE standards are different for it. Working from memory one is the point where a 45° angle from the x-axis that touches the curve and the other is a 90° line from the x-axis to where the plot straightens and become a straight line with a constant slope. For both forward bias Knee and the reverse bias Breakdown. I've also seen it ilustrated as the point where a line extended from the straight part of the plot intersects the x-axis. So apparently there is no overall consensus and often it is merely an arrow pointing in the general direction of the "Knee". Take your pick they are all "correct" answers.

#### MrAl

Joined Jun 17, 2014
7,508
Hello again,

Thanks for the additional replies i am going to try to respond to individual replies now so we can sort some things out. The order may be somewhat random.
I was basically opening a discussion where we could look into this and hear different opinions and hopefully get some solid math behind this.

#### MrAl

Joined Jun 17, 2014
7,508
The IEC & IEEE standards are different for it. Working from memory one is the point where a 45° angle from the x-axis that touches the curve and the other is a 90° line from the x-axis to where the plot straightens and become a straight line with a constant slope. For both forward bias Knee and the reverse bias Breakdown. I've also seen it ilustrated as the point where a line extended from the straight part of the plot intersects the x-axis. So apparently there is no overall consensus and often it is merely an arrow pointing in the general direction of the "Knee". Take your pick they are all "correct" answers.
Hello again,

Thanks, and it would be very helpful if you could look up that information and report back here and we can compare notes.

#### MrAl

Joined Jun 17, 2014
7,508
I think the knee voltage of a Zener diode is where its current is low and it only begins regulating the voltage.
The datasheet for some Zener diodes rated at 5mA shows horrible voltage regulation at only 1mA.
Hi,

Ok so i guess you are going with a definition that involves how well the regulation gets. Maybe in percent regulation over the flatter portion of the curve. That could be measured by the deviation from some lower current to some higher current. We'd have to be able to choose these two currents wisely.

#### MrAl

Joined Jun 17, 2014
7,508
Hi

So...What are you REALLY wanting to know?

eT
Hi,

Actually i wanted to start a discussion on this to see what other people thought. You might present your definition unless you did it already i am going backwards through the posts right now. I'll present some notes lastly.

#### MrAl

Joined Jun 17, 2014
7,508
At point a reverse breakdown of the diode occurs and current increase sharply damaging the diode . this point is called knee of the reverse characteristics.
Hi,

Well that is nice but it is a qualitative definition i was hoping for a more quantitative approach if you care to come up with something.

#### MrAl

Joined Jun 17, 2014
7,508
I would say close to the red line. For me knee means the point where it bends the most, so where the peak of the second derivation is (or is it differentiation in english?) would be my choice.
Hi,

Well what do you think now after seeing the second plot?
Isnt it interesting that changing the scale makes it look different?