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This is the mystery circuit (the Op-Amp part):

Mystery Power Supply Circuit

It appears in the voltage regulation section of a stepper controller board (stepper driver and micro not pictured).

At a glance, it seems like it loads the 12v rail through Q1 if the rail starts to exceed PB12V, but why would the designer even want to do this?

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    \$\begingroup\$ It looks like it discharges the 12V rail capacitors if the input power is disconnected. I'd be worried about the fuse though! \$\endgroup\$ – Andy aka May 10 '17 at 14:19
  • \$\begingroup\$ Note that the 311 is a comparator with open collector/emitter output (pins 7 and 1) \$\endgroup\$ – PlasmaHH May 10 '17 at 14:21
  • \$\begingroup\$ That's not an op-amp, but a comparator (and the circuit topology gives it some hysteresis). It looks like it's monitoring the input voltage. \$\endgroup\$ – Enric Blanco May 10 '17 at 14:25
  • \$\begingroup\$ @Andyaka: not really, whats the capacitance, 2-3mF? Within tens of ms it will drop below the 1A and that is by far not enough time to blow it with this little over current. \$\endgroup\$ – PlasmaHH May 10 '17 at 14:27
  • \$\begingroup\$ @PlasmaHH I guess it survives but I'm not convinced. Do you agree that it discharges the output caps when power is removed? Why have the fuse at all? \$\endgroup\$ – Andy aka May 10 '17 at 14:40
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It looks like it discharges the 12 volt rail capacitors when the input power is disconnected. When input power reduces to 0 volts (or drops significantly) the comparator output switches to 0 volts. This turns off Q2 and biases on the clamping MOSFET. This discharges the 12 volt output capacitors. If the input supply voltage has only partially dropped, the circuit won't fully discharge the capacitors.

I have no idea what the function is, but that's what it appears to do.

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I think Andy and Enric got it with the supply cap discharge explanation.

Based on that I can think of 2 reasons this circuit might be necessary. One, like others have mentioned is that makes the system power down quickly when the power is removed. The other, is this: If the user manually turns the stepper motor, it will send voltage into the motor driver, and back-power the 12v rail through the protection diodes. I've witnessed this on other designs that don't have this circuit, (displays flicker, etc). The diode and mosfet arrangement means that the 12v rail effectively has a 6 ohm load whenever the power is unplugged, that would probably bleed of any back-power from the motor.

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LM311 is a comparator, not an op-amp. The designer has configured it to have some hysteresis, probably to avoid any false triggering of the circuit.

In that configuration, the output transistor of the LM311 will switch ON when the input voltage from the connector goes below a certain threshold (667 mV). The designer must have concluded that if the input voltage goes below that threshold then it is an actual power off, and not just a temporary droop.

Another thing that happens when the input voltage goes down, is that diode D2 will isolate the capacitor bank of the 12V rail from the input voltage line, but they will still be charged at 12V. More on this later.

Going back to the LM311, we find out that switching off its output will switch off Q2, which in turn will turn on Q1. When Q1 turns on it will switch a 6 Ohm load to ground, which will generate across it quite a high current from the 12V rail.

That current will quickly discharge all the capacitor connected to the 12V rail, provided the fuse doesn't blow in first place. If an adequate fuse is selected, the initial 2 A peak current will not last enough to blow it. This way Q1 and the resistor will be protected from a faulty operating condition (continuous 2A conduction) but still be able to do its work under normal conditions.

So, what's this circuit intended for? Probably for discharging quickly the 12V rail capacitor, as Andy aka says. Why such a thing is needed is a question that can only be answered by the designer of by careful analysis of the broader design.

Note that the designer could have opted for connecting a bleeding resistor between the 12V rail and ground, but that would have greatly increased the current consumption if the discharge time required is short. What the designer does here is connecting a strong bleeding resistor when it's required only (at the expense of a greatly increased complexity).

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There seem to be two 12V-ish rails, one the 12V in the circle, one the PB12V which makes me think "car battery" but according to D2 this is supposed to be above 12V, so the comparator senses if this is above the (capacitor buffered) 12V rail, and if not the comparator (powered by the 317 and the capacitors) stops driving Q2 which stops pulling the gate of Q1 to GND, making it conduct.

This will then discharge the capacitance of the 12V rail, putting the capacitors into a safe state.

The fuse will not blow as even if it exceeds the rated 1A, fuses usually don't blow unless their rated current is either way exceeded, or mildly exceeded for an extended amount of time, which certainly it is not for discharging the capacitors.

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