Zener diode maximum forward current

I’m wanting to use your zener diodes, either 3SMAJ5918B or SMBJ5338B, as voltage limiters in an intrinsically safe product. The data sheets are fine for the zener characteristics which are the important numbers for the application, but I can’t find anything on the maximum forward current. Without this data I can’t satisfy the Certified Body who I need to approve the design and consequently can’t use your product.

Can you help? Maybe there’s a document somewhere that I haven’t located?


Roger Fell

Welcome to the forum.

The maximum permissible current is calculated from the power rating and the voltage drop across the zener.

Amps = Watts/Volts (I = P/E)

So if the zener is dropping 10V it can only handle 1/10th the current it can handle when it’s dropping 1V.

The power rating of a semiconductor is temperature dependant so you may need to derate the maximum power level based on the maximum temperature it will be exposed to.

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Dear Paul

Thank you for your early response.

Is this information available on a datasheet? I am responding on behalf of Dr Roger Fell who is on holiday. He may well have other questions and so is it acceptable that he contacts you direct?



Only the power rating, zener voltage and power derating per °C are given on a typical zener diode data sheet. The rest is learned in an electronics technician or EE class.

I’m just a volunteer participant in this forum and at my age am no longer doing paid consulting work. If other questions are posted in the thread I may be able to help, free time permitting.

You should probably hire a consulting EE with experience in the type of certification you need to get through the certification process.


Dare I ask how old you are Paul?

I’m well over 60

Me too as is Dr Fell for whom I was registering this message. It’s good to see we are still continuing.

It’s unlikely that one will find this in zener datasheets, for a few main reasons:

  1. Zeners are typically used in the reverse direction
  2. The actual value depends on the application; since a manufacturer can’t know that up front or characterize it for all test cases, it’s left to the user to evaluate in context of their particular circumstances.

When mentioned, semiconductor power ratings are typically example figures based on an assumed test case (which may or may not bear resemblance to practical use cases) with the “real” number one cares about being the maximum permissible junction temperature. Using the datasheet for the 3SMAJ5918B-TP as an example, it can be seen that the 3W power rating is derived as the estimated power dissipation that will cause the junction to reach the 150°C maximum temperature when the leads are at 75°C.

The math involved isn’t rocket surgery by any stretch, but the ministrations of a qualified consultant may indeed be the shortest path to satisfying a certifying body. There’s a listing here of design firms that might be useful if you don’t have an existing connection for such services.