I was looking for an Op Amp input protection circuit and the most common I found looks like this.

When the voltage at Input Signal goes above/below Op-Amp's supply rails schottkey diodes start to conduct.

Let's assume a +100V is applied at Vin. A current equal to I = (100V - (+15V) - 0.7V) / 5kΩ = 16.86mA will flow from the upper diode to positive supply rail.

The question is, how dangerous is that current flowing to supply rails therefore to the circuit power supply? Would it cause damage to power supply if that amount of voltage is permanent and not transient?

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  • \$\begingroup\$ This circuit is meant to dump ESD to power capacitors (not shown in the schematic). It is not meant to protect against prolonged over voltage on input signal. There is already an answer to the question with explanation why. \$\endgroup\$ – Jurkstas Feb 16 '19 at 11:25
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    \$\begingroup\$ Just to be sure, do you only want to know how how dangerous this circuit is, or, would you like to know a better way to protect your circuit? There are many different ways to do this. Depends on whether you want DC or not and what do you want to do with the excessive energy (block it or dump it) and other factors. \$\endgroup\$ – Jurkstas Feb 17 '19 at 18:45
  • \$\begingroup\$ I'd like to know both! Yes I can think of few other ways but as you said it depends on what you're after. I'd like to know a way to deal with excessive energy when a big DC voltage applied to input and how dangerous is that energy to a circuit like that I posted in question \$\endgroup\$ – MrBit Feb 20 '19 at 16:55

100Vdc could fuse open Rovp if it is not rated for 2W.

100Vdc could damage LDO's.

  • Most LDO's are emitter followers, so they source but not sink or pull down.
    Thus it could try to use the 100Vdc input to pullup Vcc to 85V or damage the LDO, so a CA type TVS is recommended on input or as OVP protection to protect supply.

This Op Amp has input pins with clamp diodes to the power supply pins and to each other. Limit the input current to 10 mA or less whenever input signals exceed the power supply rail by 0.3 V.

This Op Amp is CMOS so all precautions to prevent sensor or input/output voltage pullup before Op Amp power is applied must be done.

It is somewhat rugged with 6kV finger ESD protection as long as above precautions are added if expected.

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  • \$\begingroup\$ If input voltage was significant less than 100Vdc (say 5Vdc above/below supply rails) would still cause damage to an LDO. So what's the purpose of clamp diodes if other parts of the circuit get damaged? \$\endgroup\$ – MrBit Feb 16 '19 at 9:35
  • \$\begingroup\$ Would an extra zener diode on supply rail prevent voltage to rise above/fall below supply limits? \$\endgroup\$ – MrBit Feb 16 '19 at 10:11
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    \$\begingroup\$ Of course every Clamp diode must have a power limiting source impedance to perform as an attenuator, which is my 1st answer ( your fuse) \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 16 '19 at 15:28
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    \$\begingroup\$ Yes just increase it to 50k and consider Iin offset or use on both inputs Rfb,Rovp, to null input current offset voltage, but this also lowers immunity to stray B field noise ( current loop transients) \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 16 '19 at 15:29
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    \$\begingroup\$ OK then you want differential R-DAC with specs on immunity, noise reduction, CM impedance and Diff input impedance like any DMM \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 16 '19 at 15:48

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