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I have been testing a Zener diode I have using following this video. One thing I noticed and could not understand is that the Zener starts conducting when it has not reached it breakdown voltage yet.

I had a 9.1 V Zener diode and used my power supply and from 1 V upwards my multimeter was showing the zener was conducting. I was expecting it to reach around 9 V and then start conducting. Even in the video the zener starts conducting from a very low voltage. What am I missing here?

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  • \$\begingroup\$ If you connected the polarity of the zener diode backwards then it will act like an ordinary forward-biased silicon diode, beginning to conduct at 0.6V and max voltage of 0.7V or 0.8V. The datasheet for the zener diode will say its "test" current, some are high and others are low. \$\endgroup\$
    – Audioguru
    Commented Apr 8, 2023 at 23:56
  • \$\begingroup\$ No i connected vcc to cathode and negative to anode for testing . \$\endgroup\$
    – Amir
    Commented Apr 9, 2023 at 7:48

2 Answers 2

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The video is showing that you can find the approximate Zener voltage by carefully applying a limited current through the Zener diode. At a certain reverse current the voltage across the Zener will become somewhat constant.

Like most other components a Zener diode is not perfect in actual use. It will allow some current flow in either direction below the so called knee voltage, but at a certain point (positive and negative) the current will increase at a faster rate while the voltage changes much less. The datasheet of the Zener will give the range where the Zener voltage is specified and the current at that point.

Below is a diagram from a prior SE question that included a Zener diode I/V curve. Note that the curve is not perfectly flat between the knee curves. This is the current/voltage you are measuring.
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enter image description here

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That is how Zeners work.

Contrary to ideal model, real world Zeners are not ideal devices that are on or off based on their Zener voltage.

They are simply rated to have some rated voltage over them when some rated current flows through them.

The voltage will be less with less current and more with more current.

The slope may be very soft so it starts conducting at voltage much below the rated voltage.

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  • \$\begingroup\$ So what i have read so for that they should reach the break down voltage and then they start conducting is not correct ?In my test 9.1 volts zener was conducting even when voltage was 1 volt and it was conducting 1 volt over . \$\endgroup\$
    – Amir
    Commented Apr 9, 2023 at 7:46
  • \$\begingroup\$ That may be what you have read but it is not true. It is only a very simple model for explaining Zeners. Just like you may have been taught that wires have no resistance but that is not true either. \$\endgroup\$
    – Justme
    Commented Apr 9, 2023 at 7:49
  • \$\begingroup\$ Nice,even chatgpt was insisting that they should reach breakdown voltage (with some tolerance ) and then conduct otherwise the zener is defective ;) \$\endgroup\$
    – Amir
    Commented Apr 9, 2023 at 8:08
  • \$\begingroup\$ @Amir Don't use ChatGPT. It can't know what is true or false. \$\endgroup\$
    – Justme
    Commented Apr 9, 2023 at 8:22
  • \$\begingroup\$ Thank you @Justme, i should do some more testing and reading to fully understand the concept .I tought they are useful for voltage regulation but the way they are working and conducting even at very low voltage they can not be a good solution , but how are they used in voltage regilation so much ? \$\endgroup\$
    – Amir
    Commented Apr 9, 2023 at 9:23

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