On 2023-May-07, at 1:54 AM, Rob Jarratt via cctalk wrote: > The comments about the tolerance of the 7812 were right, it doesn’t appear to > be an issue with the replacement 7812 regulator because when I tried using > the bench PSU to feed exactly 12V to the circuit from the output of the 7812 > the comparator still gave the wrong result. It was still wrong if I applied > only 11V > > I then looked at the value of Vz on the good and bad PSUs, when applying 12V > to the 7812 output. That was 5.4V in both the good and bad PSUs. Where I saw > a difference was on the -12V output, it was +0.4V on the good PSU and 0.56V > on the bad one (the voltage varied so this was an average). I checked the > voltage drop across the current sense resistor. It is 0.01V on the good PSU > and 0.08V on the bad PSU, which would explain the higher positive voltage on > the -12V output and the comparator being turned on.
From earlier measurements and the 45uA calc of current through the 51Ω sense resistor, the V across the 51Ω ISense-12 resistor should be only 0.002V. So a question is where is this 0.08V coming from? An unfulfilled -12V supply for the E3 power pin might have been an explanation, as extra current might be drawn out of the E3d.+in input due to the +in being pulled below (negative to) the E3 -power pin. But you say that pin is connected to GND, so the source of the 0.08V should be sought, some more comprehensive measurements around the E3d inputs / ISenseR might help. From calculations from design/schematic: - E3d.+in should = 1/2 the voltage across the 51Ω ISense-12 resistor. - E3d.-in should = 0.033 V. - The trip condition to send the E3d output high (shutdown) is: V(ISR-12) > 0.066V (V across ISense-12 Resistor). Also: Is that ISense-12 resistor really a 51Ω resistor? Or is it 5.1Ω or 0.51Ω? Again from calculation, 51Ω would imply a current limit of only 1.3mA, which is rather low.
