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So I am trying to build a detector that will find the lowest voltage dip of a DC source.

So for example, if I attach a load I can see what the voltage drops to.

I have built this peak detector in LTSpice

enter image description here

But I can't manage to get it to work to stick at the lowest voltage reached.

Do you have any tips?

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1 Answer 1

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  1. With 0 and 9 V rails, LM741 will only work for input voltages between something like +4 and +5 V. Maybe try this with an ideal op-amp first, then find a better choice than LM741 to get a reasonable common mode input range.

  2. The way your diode is oriented, you've made a positive peak detector, not a negative peak detector. Try reversing the diode orientation.

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  • \$\begingroup\$ ... or add a voltage divider, 100 k resistors, to the input circuit so that you do not work close to the supply voltage. \$\endgroup\$
    – skvery
    Feb 16, 2017 at 18:41
  • \$\begingroup\$ Yes, I know this is a positive peak detector. Sorry should have been clearer. I would like to modify it to be a negative peak detector. Inverting the diode just gives me an output I've about 1.3v with no change. Completely flat. \$\endgroup\$
    – James
    Feb 16, 2017 at 18:52
  • \$\begingroup\$ Would you also be able to suggest a better op amp? Should I just try different ones from the library in LTSpice? What features should I be looking for to be closer to ideal? \$\endgroup\$
    – James
    Feb 16, 2017 at 20:22
  • \$\begingroup\$ @James, depends what input voltage range you need to deal with. If your input might cover the whole 0 to 9 V, then you need something with rail-to-rail on both input and output. And you need something that doesn't go too nuts when saturated...which often isn't mentioned on the datasheet. \$\endgroup\$
    – The Photon
    Feb 16, 2017 at 21:32
  • \$\begingroup\$ When you say "negative peak detector", do you mean finding the minimum voltage on a positive voltage, or do you mean (as I suspect) finding the minimum voltage on a negative? If the latter, since your op amps are configured with only ground as the - supply, of course they won't produce a usable output. Try running your op amps from +/- 15 volts. \$\endgroup\$ Feb 16, 2017 at 23:43

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