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I wonder if it is possible to replace the second figure below with the first one? Would both Vo output be the same on the figures below? If they are different, could anyone explain it by numbers? I don't have a zener diode. I wonder would it be possible to replace the zener diode with resistors/capacitor/inductor/transistor?

Schematic 1 and 2

Datasheet of the comparator.
Datasheet of the zener diode.

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    \$\begingroup\$ Please draw real schematics using real opamp and comparator symbols. It is impossible to "see" the circuit from these diagrams without first figuring out the schematic. \$\endgroup\$ – Olin Lathrop May 3 '13 at 20:14
  • \$\begingroup\$ The main reason to limit the ability to post pictures and multiple links is to fight spam, of which we get quite a lot. Users with more reputation points can edit questions and answers. \$\endgroup\$ – AndrejaKo May 3 '13 at 20:15
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    \$\begingroup\$ @ Olin Lathrop: it might be a problem for you. But I don't think it is a problem for one who use datasheets, and the figure clearly says that it is a comparator(om amp) the terminals are well defined (in A-; inA+; out A). Lastly, this attitude is not constructive. \$\endgroup\$ – sven May 3 '13 at 20:28
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    \$\begingroup\$ @sven - EVERY diagram in that datasheet except the one intended to show the physical pinout (which is not part of a schematic) is not drawn the way your diagram is drawn. Saying you're using the same figure as the datasheet is disingenuous, as the figure in the datasheet that resembles yours is only drawn that way to properly show the mechanical-electrical correspondence, not as part of a circuit. \$\endgroup\$ – Connor Wolf May 3 '13 at 20:58
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    \$\begingroup\$ @ Connor Wolf: I am sure you have seen million times more diagrams than I saw how such ICs are represented on diagram.and I believe the diagram is very obvious to understand, especially as I link to the datasheet. My reaction is minusing a question because of one does not do what other wants. As I said, I will bear it on mind how I should draw the diagram next time. I think keeping arguing on how I should have drawn the figure is waste of effort and no use for anyone. \$\endgroup\$ – sven May 3 '13 at 21:23
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The Zener, in combination with the 1K resistor, is forming a stable voltage source for the comparator to compare against. If your 12V supply is stable enough, you can divide that down and use that for reference instead.

But not the way you have shown. The Thévenin impedance of the original circuit is ~0 + 10k, or 10k ohms, but the Thévenin of yours is 10k || 1k, or 909 ohms. So the original circuit will have much more hysteresis than yours.

The CircuitLab simulator should be sufficient to show you the difference between the two circuits. In addition, you can use it to get a reasonably-drawn schematic.

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    \$\begingroup\$ Even worse, the Thevenin voltage of the zener circuit is 10V, while the Thevenin voltage of the replacement circuit is only 1.09V. \$\endgroup\$ – Dave Tweed May 3 '13 at 21:38
  • \$\begingroup\$ thank you for the clear answers. how can I replace the zener diode on the diagram without changing the 10k and 10V Thevenin impedance and voltage? \$\endgroup\$ – sven May 3 '13 at 21:53

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