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I have here a voltage regulator circuit using a Zener diode.

We were tasked to find the maximum allowable \$i_L\$ given the varying voltage input so the the load voltage will remain at 8 V.

The way I understood the problem, we are asked to looked for \$I_{z(max)}\$ since past that, the Zener diode would no longer be able to maintain the voltage at 8 V. But then, I thought \$I_{z(max)}\$ is supposed to be given in a Zener diode's data sheet and also depends on the power rating of the diode (which was not given,) so I am unsure how to proceed with this problem.

  • 1
    \$\begingroup\$ So, the zener is 8V but what is the power rating ??? \$\endgroup\$ Commented Oct 9, 2020 at 8:03
  • \$\begingroup\$ @GrahamStevenson It wasn't given. The quantities I mentioned in the post are the only ones given. \$\endgroup\$
    – user263783
    Commented Oct 9, 2020 at 8:09
  • \$\begingroup\$ Lord Above ! Is there no end to daft question asked by lecturers. Would a classic 1.3W zener be OK ? \$\endgroup\$ Commented Oct 9, 2020 at 8:11
  • \$\begingroup\$ @GrahamStevenson I don't think so lol. They don't like us making assumptions with that stuff. \$\endgroup\$
    – user263783
    Commented Oct 9, 2020 at 8:15
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    \$\begingroup\$ @Batt max zener power dissipation is at max input voltage 33V and zero load I.e. all current is flowing through zener \$\endgroup\$
    – mhaselup
    Commented Oct 9, 2020 at 9:45

1 Answer 1


The maximum load current is when then zener no longer has sufficient current to regulate. Since we don't have any information about the Zener diode itself, we can assume it to be perfect so it stops regulating when the zener current drops to zero. That's actually a reasonable assumption for an 8V zener.

The worst-case conditions on the low side are minimum input voltage, maximum resistor value within tolerance (we're not given tolerance so assume it to be perfect) and maximum load current.

You should be able to calculate this, as this is a homework problem we don't give complete answers, but this should be enough of a hint.

  • \$\begingroup\$ I got the answer right based on what you said. So then the maximum power dissipated by the zener would be when the input voltage is at maximum? \$\endgroup\$
    – user263783
    Commented Oct 9, 2020 at 8:53
  • 3
    \$\begingroup\$ When the input voltage is at maximum and the load is at minimum. \$\endgroup\$ Commented Oct 9, 2020 at 9:42

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