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One circuit I have seen used many times is an op-amp and a current sense resistor to implement overcurrent protection. I have seen this circuit implement both an overcurrent protection that turns off the circuit and something more along the lines of a constant current circuit.

I am interested in overcurrent detection, not trying to implement a constant current circuit. I constructed this circuit in LTspice to try and better understand how this works. I based this off my understanding from reading reference circuits and application notes from supplie

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V1 is an AC voltage source so the current in the circuit varies over time. R1 is the load and R2 is the current detection resistor. V2 is my reference voltage. I am expecting that when the current in R1 is above 1 amp the voltage of the LM308 swings high.

I had to add some feedback via R5 because LTSpice would never actually produce a simulation otherwise. I am expecting comparator behavior from this circuit, not a proportional output.

V3 is there just to run the op amp. R3 is the "load" of the op-amp, which is basically just there to source the output of the op-amp back to ground for simulation purposes. The op-amp I used is an LM308, this appears to be the single version of the extremely LM348 I see in many circuits.

Here is the simulation of the circuit

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The red line is the voltage coming out of R4 and driving the op-amp. The green line is the reference voltage from V2. The blue line is the output of the op-amp which is basically constant. Why is this happening? Is there some additional components I am missing which are commonly omitted from reference material?

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  • \$\begingroup\$ Hint: what is the input common mode voltage range of LM308 and what is the common mode voltage of your inputs? \$\endgroup\$
    – The Photon
    Oct 27, 2017 at 3:59

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You are operating the device way outside at least one of its specified parameters. Datasheet page 3, "Output voltage Swing" and "Input Voltage Range". Both inputs must be within the stated range. Separate from that, the output voltage is limited to the stated range even when the input and feedback conditions dictate that it be something else.

For example, if the inputs are +2 V and +3 V, the output will try to make either + or - 1001 V, depending on the input polarity. Obviously the part cannot make 1000 V out with only a 9 V power source. Not so obvious is that when the output tries to saturate against the positive or negative rail, it can get only so close. Worst case conditions, the output voltage range with a single +9 V power source is from +2 V to +7 V. The simulation shows the output at less than +40 mV; the datasheet makes it clear that a real part cannot do that.

The input voltage range imparts a similar but different set of operational restrictions.

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  • \$\begingroup\$ National Semiconductor's datasheet lists a minimum output voltage swing of 13 volts and a minimum input voltage of 14 volts. There is no stated maximum on the datasheet. The supply voltage to the op-amp is 15 volts DC. \$\endgroup\$
    – Eric Urban
    Oct 27, 2017 at 12:53
  • \$\begingroup\$ So I'm still not 100% sure on what you're trying to say, but from what I can tell the important fact here is that I need a minimum voltage difference between my amplifier inputs and the amplifier supply voltages. So I can't connect the negative rail to ground and expect this to work. It seems to operate as expected if I use a -5 VDC supply. \$\endgroup\$
    – Eric Urban
    Oct 27, 2017 at 13:47

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