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.
