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I am trying to build a voltage generator 0-10V to feed the input of a PLC ADC card (for simulation purposes). The PLC card has an input impedance of 133k. I am thinking to use the circuit from the picture below:

enter image description here

Since the signal will be applied with a pair of probes I want to implement short circuit protection. The protection is a classic one with a sense resistor (R8) and transistor (Q2).

When a short will happen (or the current will exceed the limit of 15mA) Q2 will open, limiting the base voltage of Q1 which in turn will limit the output voltage protecting the circuit.

Now... The voltage of feedback network (R6,R7) will go low. Because the op amp wants to have both inputs at same voltage will try to increase it's output voltage until the output will be close to it's positive power supply (LM358 it's not rail to rail).

My question is: an op amp with it's inputs at different voltages and an output close to positive power supply risks to be damaged?

Thank you in advance for your answers.

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  • \$\begingroup\$ This is the same condition as using an op amp as a comparator, which is usually not harmful to the op amp. I'm not sure I'm understanding your question? \$\endgroup\$
    – Hearth
    Oct 12 '19 at 14:30
  • \$\begingroup\$ This is not a good design. What are your desired output power specs? V,I max.. No damage if it is less and Absolute Max Specs \$\endgroup\$
    – Tony Stewart EE75
    Oct 12 '19 at 14:33
  • \$\begingroup\$ @Hearth Thanks for your answer. To be clear, my question was: when inputs are at different voltages (below positive power supply in common mode) is there any risk to damage the op amp? \$\endgroup\$
    – Petrescu Cristian
    Oct 12 '19 at 14:35
  • \$\begingroup\$ @SunnyskyguyEE75 The output voltage is between 0-10V and Imax 15mA. \$\endgroup\$
    – Petrescu Cristian
    Oct 12 '19 at 14:37
  • \$\begingroup\$ @PetrescuCristian I'd defer to the datasheet; are you within the maximum operating conditions? Then it should be fine. \$\endgroup\$
    – Hearth
    Oct 12 '19 at 14:38
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This is completely unnecessary, because the LM358 is already rated for continuous short-circuit to ground of any one output.

Specifically, Note 5 under "Absolute Maximum Ratings" states:

Short circuits from the output to V+ can cause excessive heating and eventual destruction. When considering short circuits to ground, the maximum output current is approximately 40 mA independent of the magnitude of V+. At values of supply voltage in excess of +15V, continuous short-circuits can exceed the power dissipation ratings and cause eventual destruction. Destructive dissipation can result from simultaneous shorts on all amplifiers.

If you think you might run into problems, a simpler solution would be to put additional resistance in series with the opamp output, inside the feedback loop. For example, 2400 Ω would limit the current to 10 mA under all possible conditions, but have negligible effect in normal operation (133 kΩ load in parallel with 20 kΩ feedback network).

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ This circuit provides current limiting, but capacitive loading will cause instability. Adding a suitably sized cap from amp output to the minus input will cure this. \$\endgroup\$
    – EinarA
    Oct 16 '19 at 0:10

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