# Zener diode maximum reverse current

#### raziell122

Joined Mar 28, 2023
52
Hello!
I have Zener diode 4733A and I want to know what is the minimum & maximum reverse current I can supply through it.
I found the following table in the Zener datasheet but I'm not sure which column represents the max reverse current because IR seems to me very low value (10Micro Ampere).

#### Ian0

Joined Aug 7, 2020
8,386
1N4733 is a 1Watt zener. The maximum continuous reverse current is 1W divided by the zener voltage.
Pulsed reverse current can be higher, until you reach the "Non-repetitive peak reverse current"

#### crutschow

Joined Mar 14, 2008
32,899
The "Working Current" is the maximum steady reverse current, which is determined by the 1W power dissipation, and must be derated above 25°C ambient.
There is no minimum current, but the voltage will drop below its nominal value as the current gets to sub mA values.

#### Pyrex

Joined Feb 16, 2022
181
Hi,
if reverse voltage of 1V is applied, 10 uA current is expected. Or less, than 10 uA.
Ideal Zenner current is equal to 0, as applied voltage is significantly lower than breakdown value (5.1V)

#### Audioguru again

Joined Oct 21, 2019
6,155
Your 1N4733A zener diode is 5.1V (plus or minus 5%) only when its current is the "test " current of 49mA and the zener diode is cooled somehow to 25 degrees C. Less current reduces its voltage and more current increases its voltage. Higher temperature increases its voltage.

Then why do you want to spoil its voltage regulation by using low and high current? Use a voltage regulator IC instead.

#### crutschow

Joined Mar 14, 2008
32,899
Here's the LTspice simulation of a typical 5.1V, 500mW Zener for both a linear and a log horizonal-axis current scale:

The log-scale, straight line at low currents shows a log relation between voltage and current.

The linear-scale straight-line rise in voltage with current above about 10mA is from the intrinsic resistance of the Zener, (about 8Ω from the graph).
This compares to the 7Ω Idr @ Iztest for the 1N4733.

#### MrChips

Joined Oct 2, 2009
29,217
You are looking at different pieces of data in the spec sheet.

Firstly, IR is 10μA max at VR = 1V.
Notice that the reverse voltage is specified. This means that with VR @ 1V the reverse current will not exceed 10μA. This is a guaranteed specification of the device.

The second part is IZM = 178 mA continuous and IZSM = 890mA surge. That is, do not exceed 178mA when forward biased or 890mA when reversed biased.

The question is, why are you causing the zener to be reverse biased?

#### Audioguru again

Joined Oct 21, 2019
6,155
Do not exceed 178mA when forward biased.
No. Its maximum allowed current is 178mA when working (reverse biased).

The question is, why are you causing the zener to be reverse biased?
Because a zener diode is usually used with a reverse bias voltage.

#### WBahn

Joined Mar 31, 2012
29,163
Note that two of the columns in your table call out footnotes -- Footnote #1 for the V_Z and footnote #2 for I_VZM. Read those. They often have important information.

Also note that the entire table only applies when the junction temperature is 25°C. Elsewhere in the data sheet you can find information to at least estimate the behavior at other temperatures.

#### raziell122

Joined Mar 28, 2023
52
Real soon I'm going to assemble the following simple circuit on a breadboard:

I connected the Zener diode in this polarity so that if the 741 output is HIGH, then the Zener diode will break down and determine its Vz voltage in the input of the OR gate so that +VCC=15V won't destroy it.
I just wanted to know the current limitation of the Zener in reverse bias to know which resistor I should use and if 1Kohm is enough

#### Ian0

Joined Aug 7, 2020
8,386
It's the LOW output voltage you should be thinking about. The output of a 741 will not get close enough to the negative supply to produce a TTL logic LOW - so your circuit won't work - the output will always be HIGH.
Get a modern rail to rail 5V comparator (even an old LM393 will work if you add a pull-up resistor), then you won't have to worry about the logic HIGH either. (And don't use an op-amp when you need a comparator - because some op-amps have clamp diodes between their inputs and won't work as comparators. Using op-amps as comparators is a bad habit to get into)

#### raziell122

Joined Mar 28, 2023
52
It's the LOW output voltage you should be thinking about. The output of a 741 will not get close enough to the negative supply to produce a TTL logic LOW - so your circuit won't work - the output will always be HIGH.
Get a modern rail to rail 5V comparator (even an old LM393 will work if you add a pull-up resistor), then you won't have to worry about the logic HIGH either. (And don't use an op-amp when you need a comparator - because some op-amps have clamp diodes between their inputs and won't work as comparators. Using op-amps as comparators is a bad habit to get into)
Is it a sure thing that 741 won't work here?
I'm limited with the components I can use as it's a college task, but I think I can use LM393 if it is a better option.

#### WBahn

Joined Mar 31, 2012
29,163
The 1 kΩ resistors will limit the Zener current to ~10 mA, which should be safe. Your voltage probably won't quite be up to the rated Zener voltage, but it should be more than enough to be recognized as a logic HI by the 74xx32 (what family are you using)?

I'm a bit concerned about powering a 741 with 15 V and then expecting it to work well with an input that's at 13.5 V. You generally want to avoid signals that are within about 3 V of either rail.

For similar reasons, I'm concerned that with the negative rail of the amps being at 0 V, that you will see a low enough output to be recognized as a logic LO.

It would be best if you used a comparator instead of an opamp, but if you are going to use an opamp, use one that is rail-to-rail or at least one whose common-mode input range includes the signals you are going to apply and whose outputs are guaranteed to get close enough to the rails to meet the input voltage specs of your logic chips.

#### WBahn

Joined Mar 31, 2012
29,163
Is it a sure thing that 741 won't work here?
I'm limited with the components I can use as it's a college task, but I think I can use LM393 if it is a better option.
It's not guaranteed that it won't work, but that's not how circuits should be designed.

It might work when you build it, and then not work when you show your instructor. Do you really want to risk that?

The LM393 is a MUCH better choice. In fact, if you have both chips available and you use the 741 instead of the 393, you should lose points.

#### raziell122

Joined Mar 28, 2023
52
It's not guaranteed that it won't work, but that's not how circuits should be designed.

It might work when you build it, and then not work when you show your instructor. Do you really want to risk that?

The LM393 is a MUCH better choice. In fact, if you have both chips available and you use the 741 instead of the 393, you should lose points.
Thanks for bringing this to my attention. That's an interesting point because my instructor has seen (briefly) my Multisim circuit and did not say anything about the 741, but it will be really nice if I use LM393 instead of it because I never tried it before and just saw one or two theoretic examples in lecture.

#### MrChips

Joined Oct 2, 2009
29,217
No. Its maximum allowed current is 178mA when working (reverse biased).

Because a zener diode is usually used with a reverse bias voltage.
Got it!
Does anyone use zener diodes under forward bias?

#### Ian0

Joined Aug 7, 2020
8,386
Thanks for bringing this to my attention. That's an interesting point because my instructor has seen (briefly) my Multisim circuit and did not say anything about the 741, but it will be really nice if I use LM393 instead of it because I never tried it before and just saw one or two theoretic examples in lecture.
Don't forget the pull-up resistor. The LM393/LM339 was designed with an open-collector output so that it could operate with voltages up to 30V, but interface easily to 5V logic.

#### Ian0

Joined Aug 7, 2020
8,386
Got it!
Does anyone use zener diodes under forward bias?
Only when they have been put in the 1N4148 box by mistake, and nobody realises whilst they are being used on 5V logic.

#### WBahn

Joined Mar 31, 2012
29,163
Thanks for bringing this to my attention. That's an interesting point because my instructor has seen (briefly) my Multisim circuit and did not say anything about the 741, but it will be really nice if I use LM393 instead of it because I never tried it before and just saw one or two theoretic examples in lecture.
It will also be nice if you can explain to your instructor why you are using the LM339 and not the 741. May get some brownie points.

#### raziell122

Joined Mar 28, 2023
52
Don't forget the pull-up resistor. The LM393/LM339 was designed with an open-collector output so that it could operate with voltages up to 30V, but interface easily to 5V logic.
Yeah, thanks! does it matter which value of Pull-up resistor I use?
Also, all the other values I used here can stay as is? I mean the voltage supplies I used to operate the 741 and the voltage inputs, it's important for my circuit's goal.