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I am trying to figure out the zener voltage of 1n4742a zener diode. I constructed the following circuit:

enter image description here

The data sheet of the diode can be found here: http://download.siliconexpert.com/pdfs/2016/8/24/2/35/11/334/gda_/manual/21n47xx.pdf

I measured the voltage across the diode and the current through it, and I stopped at the point where the current going through it was 67.5 mA and the voltage was 13.453 V.

enter image description here

The data sheet states that the zener voltage is 12 v however here I have about 12% discrepancy. Is that usual? or is there something wrong with the way I am doing it. I did not further increase the voltage source, because I am afraid it will destroy the diode, so is there a better way to measure the zener voltage?

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    \$\begingroup\$ The data sheet says for 12 volt zener , the test current is 21 mA. \$\endgroup\$ – Marla Feb 17 '18 at 4:02
  • \$\begingroup\$ could you explain what test current is? I am new to this. \$\endgroup\$ – NegativeTension Feb 17 '18 at 4:04
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    \$\begingroup\$ You tested at a zener current of 67.5 mA, instead of 21 mA \$\endgroup\$ – Marla Feb 17 '18 at 4:06
  • \$\begingroup\$ one of my data points is (-12.446,-20.33) which is within range of the uncertainty reported. However, I still don't understand what so special about 21 mA to be the test current. \$\endgroup\$ – NegativeTension Feb 17 '18 at 4:10
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    \$\begingroup\$ What is so special is that the data sheet says what the specification is : 21 mA. \$\endgroup\$ – Marla Feb 17 '18 at 4:12
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The data sheet that you linked shows that the "Test Current" for the 12 volt zener is 21 mA.

You tested at a "Test Current" of 67.5 mA. You should re-test at 21 mA (zener current).

EDIT : A comment asked if the "breakdown voltage is not constant".
The graph below shows that the breakdown voltage is not constant, but varies with the test current (near where VZ is marked in red).

enter image description here

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