# Zener diode

Hi,
I'm trying to check some zener diodes I've found around, they are marked as 1N4728A, but looking on the net that code seems to represent an entire family of diodes. I've so decided to wire a small test rig and use my Arduino to measure their breakdown voltage, but I'm experiencing something weird, so I'm asking for your advice and verification of the attached diagram.

The readings at pin A0 don't seem inline with my expectations as I'm apparently reading 2.17V (540 over 1024, Arduino powered from laptop USB) but I was expecting to read 3.3V!

I've tried to add a resistive load in the form of a resistor and an LED, but that's just lowering the reading down to 1.87V (383 over 1024).

What have I done wrong?

I think you're over-complicating things by using an Arduino. Do you have a power supply and a multimeter? If so, set your power supply to something like 12V. Connect the anode of a diode to the power supply -ve and the cathode to +ve via a resistor (say, a 1k). Measure across the zener with a multimeter set to volts to see what the zener voltage is.

That diode is 3.3V 1.3W - look at page 2 of the datasheet: http://www.vishay.com/docs/85816/1n4728a.pdf

MarkT:
That diode is 3.3V 1.3W - look at page 2 of the datasheet: http://www.vishay.com/docs/85816/1n4728a.pdf

I agree with you and I believe it is, but how can I assert that? I mean, why am I not reading 3.3V on A0?

Billysugger:
I think you're over-complicating things by using an Arduino. Do you have a power supply and a multimeter? If so, set your power supply to something like 12V. Connect the anode of a diode to the power supply -ve and the cathode to +ve via a resistor (say, a 1k). Measure across the zener with a multimeter set to volts to see what the zener voltage is.

Isn't what I'm doing with the Arduino almost equivalent? I'm using the analog pin to read the voltage and sourcing 5V to the Zener with a resistor in series... Shouldn't be the same? If it isn't, where am I wrong?

Thank you both

I agree with you and I believe it is, but how can I assert that? I mean, why am I not reading 3.3V on A0?

This datasheet shows an interesting curve (fig 2).

Isn't what I'm doing with the Arduino almost equivalent? I'm using the analog pin to read the voltage and sourcing 5V to the Zener with a resistor in series... Shouldn't be the same? If it isn't, where am I wrong?

Probably, yes. But there's more uncertainty in the setup. If you're confident in your ADC reading, then that acts as the DMM reading volts.

An ideal zener diode has a 90° knee at the zener voltage, but a real zener tends to have both series and parallel resistive elements which cause its behaviour to depart from the ideal. With an LED and resistor, it's possible that the zener's passing so little current that you're not getting it up to the zener voltage. Typical LEDs are used at 5-10mA, but many will light at anything over 1mA. The datasheet reveals that the test current for this part is 76mA. I think that's your answer.

Use a 22 ohm pullup to 5V and see what you get...

This datasheet shows an interesting curve (fig 2).

That's the forward current function, the direction in which a zener is not normally used. Interestingly, there is no curve for reverse current, but that can be approximated from the figures given in Table 8.

dlloyd:

I agree with you and I believe it is, but how can I assert that? I mean, why am I not reading 3.3V on A0?

This datasheet shows an interesting curve (fig 2).

Ok, I was referring to a different datasheet. Table 8 on the datasheet you pointed says maximum working current for the 3.3V version is 276mA with teh test current was 76mA. Figure 2 on the other hand I'm not sure I understand it (sorry): that curve is about the forward voltage which, I belive, it's to be considered in case GND get's higher than Vcc... am I wrong?

Back to the data on Table 8, to provide those 76mA I need a 65 Ohm series resistor, I have a 47 Ohm resistor at hand for a 100mA current... That seems to produce an output of 3.7V

UPDATE I've just noticed the zener current should be calculated on the voltage drop, in my case on 1.7V, which seems to correspond to a nice value of 22 Ohm for a 76mA current, but I'm already above the 3.3V with a 47 Ohm resistor (36mA zener current). I'm not sure I understand what's going on here.

BTW, I'm measuring voltages with Arduino AND a multimeter, same readings.

Billysugger:

Isn't what I'm doing with the Arduino almost equivalent? I'm using the analog pin to read the voltage and sourcing 5V to the Zener with a resistor in series... Shouldn't be the same? If it isn't, where am I wrong?

Probably, yes. But there's more uncertainty in the setup. If you're confident in your ADC reading, then that acts as the DMM reading volts.

I concur, that's why I started double checking my readings with the multimeter.

Billysugger:
An ideal zener diode has a 90° knee at the zener voltage, but a real zener tends to have both series and parallel resistive elements which cause its behaviour to depart from the ideal. With an LED and resistor, it's possible that the zener's passing so little current that you're not getting it up to the zener voltage. Typical LEDs are used at 5-10mA, but many will light at anything over 1mA. The datasheet reveals that the test current for this part is 76mA. I think that's your answer.

Billysugger:
Use a 22 ohm pullup to 5V and see what you get...

That's confirm my understanding on how to calculate Iz, but my readings still stay weird...

On top of all my doubts I've managed to exclude zener diodes as being of any use in battery powered projects

Up to reply #6, you have not specified the zener current or provided the resistor value to calculate it. Some zener, passing some current will drop some voltage. That's all normal.

Then in reply #7 you say that with 100mA you get 3.7V. So with a test current higher than nominal you get a voltage higher than nominal. All is good there too. What do you not understand?

I've managed to exclude zener diodes as being of any use in battery powered projects

Indeed. A zener shunt regulator is horribly inefficient and unsuited to battery applications.

Low voltage zeners are so far from ideal its a laugh! High voltage "zeners" are
actually avalanche mode diodes and behave better, but have a poor tempco.

Before cheap voltage regulators and voltage references zeners were used a lot,
now they are rare.

Billysugger:

Up to reply #6, you have not specified the zener current or provided the resistor value to calculate it. Some zener, passing some current will drop some voltage. That's all normal.

Then in reply #7 you say that with 100mA you get 3.7V. So with a test current higher than nominal you get a voltage higher than nominal. All is good there too. What do you not understand?

Actually that was a 47 ohm load which I erroneously calculated as 100mA Iz referring to the source 5V, but using the 1.7V drop that corresponds to only 36mA... In other words I'm reading 3.7V with only half of the test current and that voltage gets higher if I increase the current... So the Zener is not regulating much....

Yes, as Billysugger pointed out, the datasheet shows the forward characteristics. However note how sensitive the zener is to both temperature and current. Although the characteristics are better in the reverse direction (normal mode of operation), it still makes a poor regulator due to it's internal series resistance. Take a look at the chart here near the bottom of the page, it shows 1 mA current giving 3.3V +/-5% for a 1W 1N4728 zener (not 1N4728A). This reverse current would probably be close to what you would need in order to get 3.3V drop across the zener.

I did some tinkering in the past with a 5.1v Zener diode and found that there has to be some amount of current flowing thru the zener diode for it to stabilize and start controlling at 5.1v. I think the value may be a couple of ma. In my setup I was using a pot as an adjustable voltage divider, and was going to use the zener on the pot wiper to prevent accidently adjusting a too high of a voltage into the arduino analog in pin. In this setup the zener did not limit pot wiper voltage to 5.1v. Did some research using Google and found out about the minimum current flow requirement.

zoomkat:
I did some tinkering in the past with a 5.1v Zener diode and found that there has to be some amount of current flowing thru the zener diode for it to stabilize and start controlling at 5.1v. I think the value may be a couple of ma. In my setup I was using a pot as an adjustable voltage divider, and was going to use the zener on the pot wiper to prevent accidently adjusting a too high of a voltage into the arduino analog in pin. In this setup the zener did not limit pot wiper voltage to 5.1v. Did some research using Google and found out about the minimum current flow requirement.

You are correct (once again), every zener diode has a minimum and maximum current limit it must operate in. This will be stated in it's datasheet. And the values change with the 'wattage rating' of the specific zener. Again the datasheet is your friend.