# Max a zener can clamp

#### MikeJacobs

Joined Dec 7, 2019
201
So this is almost a data sheet question

I want to use a 12V zener to clamp a voltage to a max of 12V pretty standard.
This is hypothetical by the way.

Say i apply 13V to a 12V zener and it clamps to 12. Ok no problem The zener doesn't work so hard and does have to dissipate a lot of heat or current.
If i do the same thing but apply say 17V to a 12V zener it has to work much harder to clamp it. It sinks more current and dissipates more heat.

What is the value in a data sheet that tells me what a Zener is good for as far as clamping goes? How do i calculate it?

Im just not familiar with the terms used in the data sheet. Thanks

#### Dodgydave

Joined Jun 22, 2012
9,951
The main thing is don't overdo the Max Wattage rating, each Zener will have its own voltage and wattage , use a series resistor to limit the current, any more than the rating will increase temperature and will eventually blow the Zener.

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#### MikeJacobs

Joined Dec 7, 2019
201
Thanks for the thoughts but i want to know how to calculate that and what the parameter is in the data sheet

#### ElectricSpidey

Joined Dec 2, 2017
1,653
That would be wattage, and would depend on the resistor used with the zener.

#### BobaMosfet

Joined Jul 1, 2009
1,767
So this is almost a data sheet question

I want to use a 12V zener to clamp a voltage to a max of 12V pretty standard.
This is hypothetical by the way.

Say i apply 13V to a 12V zener and it clamps to 12. Ok no problem The zener doesn't work so hard and does have to dissipate a lot of heat or current.
If i do the same thing but apply say 17V to a 12V zener it has to work much harder to clamp it. It sinks more current and dissipates more heat.

What is the value in a data sheet that tells me what a Zener is good for as far as clamping goes? How do i calculate it?

Im just not familiar with the terms used in the data sheet. Thanks
Good question. It's more complex than the wattage rating. When you read a datasheet, you should be aware that it is written in favor of the part in question to make it look good saleswise in comparison to competitor parts.

Look very carefully at the TEST CONDITIONS- these usually tell you the actual and true region in which the manufacturer intends the part to be run at.

Zeners operate by dumping current. Whether or not your zener fries is a question of Thermal Junction Temperature (the zener is a diode with a junction. That little wire strand will fry if exposed to too much energy.) The Thermal Junction Temp is what you want to be aware of- how quickly can it dissipate heat; how much heating are you doing based on current you're trying to run through it, etc. How much can a heatsink help (in this case tied to a leg of the diode).

If it is impractical to use a zener as your clamp itself, then use a zener to control a transistor, which will allow you to clamp much greater loads. Be aware of the diode drop of the transistor as well, so if you want 12V, then you probably want a 12.6V zener as an example.

This can help introduce you to the subject:

http://www.bristolwatch.com/ele/zener_power_supply.htm

#### WBahn

Joined Mar 31, 2012
26,398
So this is almost a data sheet question

I want to use a 12V zener to clamp a voltage to a max of 12V pretty standard.
This is hypothetical by the way.

Say i apply 13V to a 12V zener and it clamps to 12. Ok no problem The zener doesn't work so hard and does have to dissipate a lot of heat or current.
If i do the same thing but apply say 17V to a 12V zener it has to work much harder to clamp it. It sinks more current and dissipates more heat.

What is the value in a data sheet that tells me what a Zener is good for as far as clamping goes? How do i calculate it?

Im just not familiar with the terms used in the data sheet. Thanks
If you apply 13 V directly to a 12 V zener, you will almost certainly let the magic smoke out. Whether you do or not will depend on the internal resistance of your 13 V source.

Put a resistor between the zener and the source. The voltage across the resistor will be approximately the difference between the two, so 1 V in this case. That will let you determine the current in the resistor which is also the current in the zener. If you know the voltage across the zener and the current through the zener, can you find the power dissipated by it?

#### dl324

Joined Mar 30, 2015
12,871
What is the value in a data sheet that tells me what a Zener is good for as far as clamping goes? How do i calculate it?
It's going to be related to maximum current (wattage) and heat dissipation.

For over voltage protection, you should be considering TVS diodes. They're like very high power zener diodes. They're designed to take 10's or 100's of amps. IIRC, their failure mode is short, so a fuse in series would prevent bad things like fires.

#### AlbertHall

Joined Jun 4, 2014
11,306
The zerner voltage and maximum power dissipation will be found in the datasheet.
A 12V zener will clamp the voltage at 12V. You must provide some resistance to limit the current when that happens.
Let's say the voltage source may get up to 17V. Let's say it is a 400mW zener (datasheet number). 400mW @12V is 33.3mA. So we must limit the current through the zener to a maximum of 33.3mA. The voltage across the series resistor will them be 17V - 12V = 5V. The resistor value must then be 5V / 33.3mA = 150Ω (minimum value to keep dissipation within the diode power rating).

#### MikeJacobs

Joined Dec 7, 2019
201
ok maybe i should clarify
Something more like this.
Say the zener is driving something that requires a 12V but no more. For the sake of the argument lets say its like a switch that can only accept 12V max

So i setup a BJT then make a votlage divider and us a 12V zener to clamp
How do i calculate how much current is in the zener and how do i know if the divider is not dividing enough to let the zener work?

#### BobaMosfet

Joined Jul 1, 2009
1,767
Is this a homework question?

#### ElectricSpidey

Joined Dec 2, 2017
1,653
In this example the voltage across the resistor is 5 volts, so 5 divided by 100 is .05 amps

So the flow in the zener is .05 amps minus the flow in the 1k which is 12 divided by 1000 = .012 amps.

So the flow in the zener is .038 amps, so the wattage is .038 x 12 = .456 watts

#### Delta Prime

Joined Nov 15, 2019
991
What is the value in a data sheet that tells me what a Zener is good for as far as clamping goes? How do i calculate it
Hello there
You're all wrong!
Good. Clamping is a rather ambiguous term
I see excellent examples voltage references and voltage regulators & over voltage protection I'll give you that one in the spirit of clamping. These are all DC circuits what of AC. Oh and by the way for the TS please do not use the word clamping. You are unwittingly confusing yourself.

#### MrChips

Joined Oct 2, 2009
23,507
You cannot install a zener diode across a power source or a load and hope that it will regulate the voltage. The zener diode will blow. Think of a zener diode as part of a voltage divider circuit.

Here are some numerical examples.

Suppose zener voltage Vz of D1 is 12V and supply voltage V1 is 24V (just to simplify the numbers in this example).

Suppose,
R1 = 12Ω
R2 = 12Ω

Voltage in V1 = 24V.
Voltage out = 12V
Current through R1 and R2 = 1A

At this point you don't need the zener diode.
However, if V1 goes higher, or R2 goes higher, the output voltage also rises.

With the zener diode in place, and V1 = 24V
current through R1 = 1A

If the current through R2 = 1A, current through zener diode = 0A
If the current through R2 = 0A, current through zener diode = 1A

In other words, the zener diode and the load R2 will share the load current determined by
I = (Vs - Vz) / R1

where Vs = V1

Usually, we need to calculate R1 = (Vs - Vz) / I

and just as important, we need to calculate the wattage dissipated by R1 and D1

power in R1 = I * I * R1
power in D1 = I * Vz

With the values used in this example,
Power dissipated by R1 = 12W
Power dissipated by D1 = 12W

You want to specify components that are rated >150% of the wattage, i.e. 18 to 25W for both R1 and D1.

#### MisterBill2

Joined Jan 23, 2018
8,727
There are a lot of motorcycles that use a BIG zener diode to hold the alternator voltage to about 12 volts so that the lights do not burn out when it rises much higher, which it will if the zener fails or if the one poorly crimped connection in the bike wiring fails. When that fails, usually on a dark night going fast on the way home from a bar, with the OL sitting in back, all the lights get real bright just before they all burn out. The the biker is rolling down the road, about 60mph, in the suddenly very dark. That is what happens with shunt zener diode regulation. Not good, by the way. Replacing the shunt zener regulator wit a simple series transistor regulator using the replacement zener as a reference make it work very well and gets an incredible amount of gratitude in addition to the payment for the fix. The rest of this story is not suitable for publication here.

So the answer is that shunt zener clamping is limited both by the maximum zener current and the maximum zener dissipation, and also by the max operating temperature. It is usually fine for clamping transients and spikes though.

#### ChasNC

Joined Jul 14, 2020
3
I was reviewing your BJT circuit with the zener and you might reverse things a bit (cart in front of the horse). You could bias the zener with whatever is driving the BJT's base and let the BJT source your load directly. You would get one diode drop from the BJT base-emitter junction. Search for "BJT Zener supply and you will get tons of Google images to check out. This configuration puts the power dissipation on the BJT.