7 May
2023
7 May
'23
10:53 p.m.
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.