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I'm looking at the datasheet for the BZX55 Zener diode. The given Zener IV characteristic are in page 5, Figure 8 and 9, (one copied below):

IV characteristic of Zener diode

I'm confused on what this is saying. I understand that the hyperbolic curve is showing the power limit i.e. VI < 500 mW. But why are there multiple IV contours? What decides which contour I should be looking at? All the diagrams of Zener diode curves I have seen have a single line. I'm sure it's simple but I don't understand.

Nick_2440
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2 Answers2

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The multiple curves are for zeners of different voltages. They usually label them. The spec sheet should tell you at what current the zener voltage is specified, probably 5 mA for these parts.

The upper "hyperbolic" curve defines the safe operating area at 25 degrees C. Above that and the part will overheat. I would make it a dashed line so it isn't as confusing.

enter image description here

Mattman944
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  • Slight addendum that can help clarify: the upper curve is hyperbolic as it is a power limit, and P = IV. You draw that curve by setting P = P_tot; if you go above that you are dissipating more than P_tot, and you are operating outside of the specification for this particular part. – Vladimir Cravero Oct 28 '22 at 11:56
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This is a set of curves for all higher voltage zener diodes in the series. It's not just for your specific zener voltage. There is a separate set of curves for the lower voltage zeners. They have just overlayed them onto one graph to save space.

So pick the one that is closest to your Vz at IzT1 and you'll have some rough idea of how your zener will behave at different currents than the one test current. Of course this is just a general idea, any guarantees are only the min/max numbers in the datasheet specifications.

enter image description here

Spehro Pefhany
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