# Zener diode_reverse bias

#### BC547

Joined May 22, 2018
41
Hi,

I am learning about zener diodes and set up a simple experiment to study the reverse bias characteristics. Below is the circuit implement with two different voltage sources (5V and 3.3V)

With 5V supply i measured a reverse voltage at zener to be 3.15V whereas with 3.3V it measured 2.76V
The zener diode has a reverse breakdown of 3.6V

I dont understand why the zener does not drop full voltage of 3.3V in case 2 (with 3.3 V supply)?
How does the zener adjust the voltage as seen in both cases ? Can we calculate this value ?

#### ericgibbs

Joined Jan 29, 2010
12,883
hi BC,
At which two points on those circuits are you measuring the Vzener voltage and what Volt meter type are you using.?
E

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#### Audioguru again

Joined Oct 21, 2019
3,171
Your currents are very low. The part number of the zener diode links to its datasheet that says its rated current which might be 40 times the current you have.

#### crutschow

Joined Mar 14, 2008
27,170
A Zener diode breakdown voltage varies some with current, with the low voltage Zeners having more variation (softer knee).
Below is a set of example reverse-bias breakdown voltage versus current curves for various voltage Zeners.
Notice how the breakdown knee gets sharper as the Zener breakdown voltage increases.
The rated Zener breakdown voltage is only achieved at it's rated current per the data sheet.

#### Audioguru again

Joined Oct 21, 2019
3,171
Why don't you tell us the part number of your zener diode so we can see its rated current???

#### BC547

Joined May 22, 2018
41
hi BC,
At one which two points on those circuits are you measuring the Vzener voltage and what Volt meter type are you using.?
E
Hi,
I am measuring the voltage across the terminals of the diode in both of the above cases. It is a generic zener with 3.6V breakdown.
Regarding the voltmeter, I am using a not so precise HTC multimeter.

#### BC547

Joined May 22, 2018
41
Why don't you tell us the part number of your zener diode so we can see its rated current???
Sorry, I dont have the part number as it is a generic zener diode. It has a 3.6V reverse breakdown.

#### Audioguru again

Joined Oct 21, 2019
3,171
Since you have a garbage part then you do not know the amount of current it was designed to operate at. With a 5V supply its voltage was 3.15V (if your meter uses a very low current) when the resistance was 940 ohms. Then its current was inly (5V - 3.15V)/940 ohms= 2mA. Try using 100 ohms which might let the zener work near 3.6V with a current of (5V - 3.6V)/100 ohms= 14mA.

#### BC547

Joined May 22, 2018
41
A Zener diode breakdown voltage varies some with current, with the low voltage Zeners having more variation (softer knee).
Below is a set of example reverse-bias breakdown voltage versus current curves for various voltage Zeners.
Notice how the breakdown knee gets sharper as the Zener breakdown voltage increases.
The rated Zener breakdown voltage is only achieved at it's rated current per the data sheet.

View attachment 227710
Hi,

I found that the leakage current is of the order of mA (perhaps its not a standard diode!).

#### crutschow

Joined Mar 14, 2008
27,170
I found that the leakage current is of the order of mA
At what voltage?

#### BC547

Joined May 22, 2018
41
At what voltage?

I measured the reverse voltage(Vr) and reverse leakage current(Ir) tabulated above.

#### Sensacell

Joined Jun 19, 2012
2,819
Zener diodes tend to have a "soft knee" not a sharp transition.

#### bertus

Joined Apr 5, 2008
21,361
Hello,

It will mention the knee current and maximum current.

Bertus

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#### Ian0

Joined Aug 7, 2020
2,162
A Zener diode breakdown voltage varies some with current, with the low voltage Zeners having more variation (softer knee).
Below is a set of example reverse-bias breakdown voltage versus current curves for various voltage Zeners.
Notice how the breakdown knee gets sharper as the Zener breakdown voltage increases.
The rated Zener breakdown voltage is only achieved at it's rated current per the data sheet.

View attachment 227710
Whose datasheet is that from? I've never seen such a good graph of the "soft knee" characteristics of low-voltage zeners.

#### MrAl

Joined Jun 17, 2014
8,231
Hi,

I am learning about zener diodes and set up a simple experiment to study the reverse bias characteristics. Below is the circuit implement with two different voltage sources (5V and 3.3V)

View attachment 227679View attachment 227680
With 5V supply i measured a reverse voltage at zener to be 3.15V whereas with 3.3V it measured 2.76V
The zener diode has a reverse breakdown of 3.6V

I dont understand why the zener does not drop full voltage of 3.3V in case 2 (with 3.3 V supply)?
How does the zener adjust the voltage as seen in both cases ? Can we calculate this value ?
Hello,

Many zener diodes are not very precise components in that they dont act as very good voltage regulators compared to modern devices. That is why components called voltage reference diodes and plain voltage reference IC's came into being. Voltage reference IC's have very precise specifications while zeners do not and they have a somewhat poor regulation capability. This means you could see a wide variation in the zener voltage with current and so many times wont even appear to be the right value for a given part number.

What is more is the voltage variatoin with temperature is also somewhat poor. In the old days before cheap voltage reference IC's were developed, we used to place a standard silicon diode in series with the zener in order to get somewhat of a voltage stabilization with temperature. It worked good with some voltage zeners and not very well with others. That was the old days. Today when a precise voltage reference is needed a voltage reference IC is used which is very stable with current and with temperature. We still see zeners being used though in devices that do not expect to see a wide variation in temperature and are relatively inexpensive. Some regulated AC wall warts use zeners, unfortunately, even though they are labeled as "regulated".

To get an intuitive idea how the zener diode behaves in a circuit like yours, you can picture a perfect voltage reference (like 3.6v for example) in series with a small value resistor like 20 Ohms. As you increase the current through the device the perfect voltage reference holds it's voltage at 3.6v exactly, but because of the added series resistance the total voltage drop is also dependent on the current through the resistor. At 10ma the extra voltage is 0.1 volt, and at 20ma the extra voltage is 0.2v, making the entire device voltage rise from 3.6v to 3.7v to 3.8v. So you can see the voltage gets higher with increased current.
That is not by any means an exact analogy because the zener voltage has a curve to it but it makes it easier to visualize what is happening. Something inside the device is causing a change in voltage drop with current and it goes up with increased current.

So the zener is not a perfect device in fact it is really very "unperfect" and nowhere near as precise as a voltage reference IC.

In the 1970's and into the 80's really good voltage reference IC's could cost as much as 50 dollars (USD) but today you can get 1mv stabilized IC's for less than 5 bucks. Even less dollars for ones that are not that good but still very very good for many many applications (less than a dollar).

#### crutschow

Joined Mar 14, 2008
27,170
I measured the reverse voltage(Vr) and reverse leakage current(Ir) tabulated above.
That corresponds to the curves I posted in post #4.

#### BC547

Joined May 22, 2018
41
Hello,

Many zener diodes are not very precise components in that they dont act as very good voltage regulators compared to modern devices. That is why components called voltage reference diodes and plain voltage reference IC's came into being. Voltage reference IC's have very precise specifications while zeners do not and they have a somewhat poor regulation capability. This means you could see a wide variation in the zener voltage with current and so many times wont even appear to be the right value for a given part number.

What is more is the voltage variatoin with temperature is also somewhat poor. In the old days before cheap voltage reference IC's were developed, we used to place a standard silicon diode in series with the zener in order to get somewhat of a voltage stabilization with temperature. It worked good with some voltage zeners and not very well with others. That was the old days. Today when a precise voltage reference is needed a voltage reference IC is used which is very stable with current and with temperature. We still see zeners being used though in devices that do not expect to see a wide variation in temperature and are relatively inexpensive. Some regulated AC wall warts use zeners, unfortunately, even though they are labeled as "regulated".

To get an intuitive idea how the zener diode behaves in a circuit like yours, you can picture a perfect voltage reference (like 3.6v for example) in series with a small value resistor like 20 Ohms. As you increase the current through the device the perfect voltage reference holds it's voltage at 3.6v exactly, but because of the added series resistance the total voltage drop is also dependent on the current through the resistor. At 10ma the extra voltage is 0.1 volt, and at 20ma the extra voltage is 0.2v, making the entire device voltage rise from 3.6v to 3.7v to 3.8v. So you can see the voltage gets higher with increased current.
That is not by any means an exact analogy because the zener voltage has a curve to it but it makes it easier to visualize what is happening. Something inside the device is causing a change in voltage drop with current and it goes up with increased current.

So the zener is not a perfect device in fact it is really very "unperfect" and nowhere near as precise as a voltage reference IC.

In the 1970's and into the 80's really good voltage reference IC's could cost as much as 50 dollars (USD) but today you can get 1mv stabilized IC's for less than 5 bucks. Even less dollars for ones that are not that good but still very very good for many many applications (less than a dollar).
Thanks for the articulated explanation. I agree with all the points.
In my observations, firstly the change in current can cause the zener voltage to oscillate and secondly temperature gives rise to fluctuation of the breakdown voltage. Perhaps zener that operates at higher voltages and extremely small leakage current may function better than low voltage zeners.
Important learning for me here. Thanks.

#### Audioguru again

Joined Oct 21, 2019
3,171
The datasheet of a zener diode shows its change of voltage with temperature change and with current change.
All the specs together show that a zener diode with a voltage between 5V to 8V has the best voltage regulation.
Here are some specs from datasheets:

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#### crutschow

Joined Mar 14, 2008
27,170
All the specs together show that a zener diode with a voltage between 5V to 8V has the best voltage regulation.
And the 5.1V ones (or so) have the best temperature coefficient (least change).

#### MrAl

Joined Jun 17, 2014
8,231
The datasheet of a zener diode shows its change of voltage with temperature change and with current change.
All the specs together show that a zener diode with a voltage between 5V to 8V has the best voltage regulation.
Here are some specs from datasheets:
If we look at the specs we can see why the ua723 voltage regulator IC used a nearly 7 volt zener.
However, that curve is somewhat misleading it makes it look like the 7 volt (or near that) zener is perfect when it is also not. I'd watch out when using any superlatives or similar also, because a zener only qualifies for use in a given circuit based on what the circuit specs have to be so a broad judgement is not very useful. For example, a 7v zener voltage can easily be 7 percent off which is not acceptable in applications like Li-ion battery charging except in the cheapest circuits.

The ua723 is also a very old part being used as far back as the 1970's. Back then that's all we had. Compare that to even some of the low cost reference voltage IC's and we can quickly see the difference. This doesnt mean we can be sloppy about how we implement the new devices either though because there is usually at least some temperature sensitively ... for example the LM317 (and related) has an unexpected temperature sensitivity when used at medium to high currents due to the fact that the voltage reference is so thermally close to the power handling power pass element. As the power pass element heats up, the reference also heats up.

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