Zener Diode Testing Circuit Question

[penmarker]

Hi good day everyone.
Sorry if this topic had been beaten to death.

I followed a simple zener diode testing circuit from this youtube video
https://youtu.be/of86B18H4c8

For context, I'm using a 19V DC power adapter from an old laptop I had lying around so all calculations were made based on my supply voltage.

For testing I tested on a spare 12V BZX55C zener I had lying around.

The datasheet indicates a V_nom of 12V and I_F 200mA

From the equation

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(Vin-Vout)/R = 1

I got

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(19v-12v)/R=200mA
7/R=200mA
7v/0.2V = 35 ohm

I soldered multiple resistors for a measured value of 40 ohm.

However when put in the test jig the measurement rose up to roughly 10V (am I supposed to get a ballpark of ~11V based on 40 ohm?)
It got hot and melted the solder joint between diode and resistors.

Was my power adapter delivering too much current? Should I have used a pair of 9V battery (and adjust calculations accordingly) instead?

[wasedadoc]

You need to understand data sheets better.

https://www.tme.eu/Document/20e2dd1b121226feb9921d78e0ec5f19/bzx55c.pdf

The sheet says that for a forward current of 200 mA the voltage drop is 1.2 Volts.  That means the power dissipation is 240 mW which is within the 500 mW figure in the top right hand corner.

However a Zener is normally operated in reverse mode and at a much lower current.  When you try to pass 200 mA in reverse mode at 12 volts the diode is dissipating 2400 mW.  No wonder it got hot.

Do your resistor calculation again with a current between 5 mA and 10 mA.

[penmarker]

Amazing! Got it just right.
Thank you very much!

[RoGeorge]

Congrats! 

I_F is the maximum Forward current.  The one you need is the I_Z, the Zener current, which for a 12V BZX55C is I_Z=5mA, much smaller than I_F=200mA.

The current limiting resistor should be 7V/5mA=1k4, maybe smaller if you consider the current drawn by whatever circuits is powered from the voltage stabilized by that Zener diode.

Beware that the range of current variations in the load should be much smaller than those 5mA of the I_Z, or else the load will influence the functioning of the Zener.  If the load needs more current, then you'll have to add a simple voltage follower with a transistor, or something else that can stabilize bigger currents.

[penmarker]

Definitely. I tack soldered a 1.42k from my resistor stash for that value. Looking great at 12.1V.

I'm currently rejuvenating an old amplifier, and found a couple of dead zeners.
The OK zeners measure 1V off their specced min/max values.
The zeners are RD13E and RD15E.

[TimFox]

As discussed above, Zeners are normally operated in the reverse direction, where they "break down".
This also applies to the drawing, where the Zener current is in the opposite direction from the "arrow" that indicates the direction of forward current.
Therefore, it also applies to the location of the bar on the package, that indicates the position of the "non-arrow" on the schematic symbol.
When replacing Zeners, be sure to get the direction of the replacement diode correct.

[inse]

Usually Zeners fail short (or open if completely burned off).
I haven’t found derated ones yet.
There is also a wide span at which current the Zener voltage is defined ranging from 50uA to 1mA (or maybe even higher).
So your improvised setup is likely to give deviating values.

[Terry Bites]

You might want to build a very basic current source to set Iz with a little more certainty. [not brilliant, but good enough]
Q1 can be any small pnp.