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I'm trying to solve a problem which requires the rating current of this diode 1N702. In the book, the author uses 1N961 zener for the examples and states that the rating current \$I_z = 12.5\$mA. In the exercises, however, the author requires 1N702. The question is

Design a 7.5-V standard voltage source using a zener diode if the supply voltage is 20V. Use a 1N702 diode.

The solution is

$$ R = \frac{V_{dc}-Vz}{I_z} $$

I can also determine the dissipating power but I need the current first.

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  • \$\begingroup\$ What does the datasheet say? \$\endgroup\$ – The Photon Nov 30 '16 at 2:31
  • \$\begingroup\$ @ThePhoton, I couldn't find it. This is why I'm asking. The book is very old though. \$\endgroup\$ – CroCo Nov 30 '16 at 2:33
  • \$\begingroup\$ Try findchips.com or octopart.com or Digikey or Mouser to find what vendors make it. If they don't provide datasheet links, go to the manufacturer's website to get the datasheet. \$\endgroup\$ – The Photon Nov 30 '16 at 2:33
  • \$\begingroup\$ Also, you shouldn't design for the maximum current capability of the zener, you should design for the maximum requirement of the load. Unless you are using it as a hand-warmer. \$\endgroup\$ – The Photon Nov 30 '16 at 2:37
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The 1N702 was a 400mW zener diode. Izt is 5mA for 1N702~1N707.

enter image description here

This is a particularly horrible example, by the way. A 2.6V zener operated at 5mA will make a really lousy regulator.

Distinguish between Izt (the current at which the zener voltage is guaranteed) and the maximum zener current, which will be limited by power dissipation. It will be a bit hard to estimate for the 1N702 because the voltage will be much higher than 2.6V at the maximum current, so the latter will be well under the 80mA you might expect were the voltage to be constant with changing current.


P.S. I really have to wonder whether the part number is a typo and they really intended to specify a 1N711 7.5V zener, which would make a lot more sense.

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  • \$\begingroup\$ What is Izt? Is it forward current? \$\endgroup\$ – zack1544 Nov 30 '16 at 2:38
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    \$\begingroup\$ Izt is the reverse current at which the Zener voltage is guaranteed. Think I = current, z = Zener, t = test. If you run the zener at 5mA it should have a voltage between 2.0V and 3.2V for a 1N702, with 2.6V as the nominal voltage. \$\endgroup\$ – Spehro Pefhany Nov 30 '16 at 2:41
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    \$\begingroup\$ @SpehroPefhany, thank you so much. I guess Google failed me with the search. \$\endgroup\$ – CroCo Nov 30 '16 at 2:43
  • \$\begingroup\$ @SpehroPefhany, the book indeed has several typos. Some of them are major. I actually asked myself exactly same question. The link for 1N711 is broken by the way. \$\endgroup\$ – CroCo Nov 30 '16 at 2:50

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