23 Apr
2023
23 Apr
'23
9:11 p.m.
On 2023-Apr-23, at 8:04 AM, Peter Coghlan via cctalk wrote:
...
This should result in a current
of about 45 microamps flowing to ground through those components mentioned.
...
It ought to be possible to measure the same 0.6V
across the diode to confirm
this is where it is being dropped (and to measure the remaining 3.4V of the
4V at E1b across the 75k resistor). As to why it is only 0.4V on the working
power supply, I haven't thought that far ahead yet :-)
The 0.4V across the working-supply diode in contrast to 0.6V is because the current is way
down in the microAmps. This is well on the low side of the curvy knee of typical Si diode
switching, so lower voltage drop and high per-component in-circuit variability are not a
surprise.
I think the +0.6V on the -12V line is explainable and
to be expected under the
test conditions described. It looks like there could be something wrong in
the control circuitry which is preventing the power supply from starting up.
This might also account for the difference between the 0.4V and 0.6V.
Brent's suggestions for checking the condition around the comparators and how
they are supplied with power are good ones. I haven't made any further
suggestions because I don't have any right now :-)
In addition to the IC power pins:
The voltage value of Vz (V of zener by 390 ohm R) is not presented in the schematic.
Calculating from the measurement of 4V suggests it will be ~ 5V.
It tends to be helpful if the voltages of zener diodes are presented in a schematic.
Also note:
The ISense+/Isense- labeling in the schematic is the reverse of the polarities which will
actually be present/expected on those lines (for all three outputs). That is, the lines
labeled ISense- will be more + than the lines labeled ISense+ under normal operation as
well as at times of over-current shutdown.