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enter image description here

The Zener diode is an LM385-BZ 2.5. I could not find that SPICE model, so the circuit is running on standard values.

The first picture below with Vout = 0-3.3 V is what I am trying to get, but the second picture is what I got.

First pic with correct output

Does anyone know how to fix this?

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    \$\begingroup\$ I fixed the image links, but I don't think the "first" picture is in fact #1. In any case, we can't help you unless you also show us the circuit you're simulating. \$\endgroup\$
    – Dave Tweed
    Commented Dec 12, 2023 at 21:46
  • \$\begingroup\$ I have added my circuit. \$\endgroup\$
    – luffy
    Commented Dec 12, 2023 at 22:00
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    \$\begingroup\$ @luffy - Hi, We got flags about the mess which this question had become. It seems that other site members have kindly fixed broken links & pictures in the wrong place. I have just deleted the "non-answer" you posted. || Please note the text box at the bottom of the page is labeled "Your Answer". Since you asked the original question, it is unlikely you would use that box. Only if you have a new, unique answer to your question, which solves your original problem, and it is not just restating someone else's answer, then you would you use the "Your Answer" box to add the new answer. \$\endgroup\$
    – SamGibson ♦
    Commented Dec 12, 2023 at 22:18
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    \$\begingroup\$ (continued) As you're new here, please see the tour & help center, as the site rules differ from typical forums. And note that this site is not a forum. If you need to add missing info to the question which would be useful to everyone, then edit the question. However also note that now you have an answer to the original question, it is too late to change the question. However if the original question needs clarification, then edit it. Or use a comment to respond to another comment. Thanks. \$\endgroup\$
    – SamGibson ♦
    Commented Dec 12, 2023 at 22:19
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    \$\begingroup\$ A zener diode isn't a good substitute for an LMx85; the latter has a much sharper knee than a zener. \$\endgroup\$
    – Hearth
    Commented Dec 13, 2023 at 14:39

1 Answer 1

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There is a design error (at least one). You need to put some minimum current through the reference to get the expected constant voltage. The LM385 needs a minimum of 10uA up to 20mA.

You can add a resistor from the 'cathode' of the LM385 to your +12V supply voltage. In this case, 10kΩ will work but you can do some calculations if you like to optimize it.

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