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Unexplainable design fault

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Prehistoricman:

--- Quote from: Vovk_Z on January 16, 2020, 03:02:30 pm ---What does U1x do? (it has unconnected inputs, that's bad. Connect them ...

--- End quote ---
Now I understand!
U1X is showing the power supply connection for all U1 comparators.

Max common mode range doesn't seem to be violated, as long as VREF and the divider inputs are below B+ - 1.5

Schematic is a bit messy, as others have hinted to. The layout on the right side is particularly out-of-order.

Pentoad:
The LM239Dt has 4 comparators inside it which share the same power pins. U1x is just a placeholder name and is just there to show how the comparator is powered. The whole thing is powered by the battery cell apart from the optoisolator outputs which are powered up from an external 5v source. I have done a bit more fault finding and it seems to be to do with the reference voltage. It is meant to be 1.225 and on the faulty boards it is 1.07 when the battery is at 3.16. As I increase the voltage, the reference voltage slowly increases as I increase the battery voltage. When the battery reaches 3.8, the reference jumps to 1.225 and stays stable even if I reduce the battery voltage back down to 3.16 or below. It is like the voltage reference isn't starting up properly but it is just a diode so I don't see how this can happen. The worst thing is that replacing the voltage reference doesn't solve the problem.  As for power consumption, each module consumes 12ma. I have updated the diagram with K14 removed properly. It used to refer to a reed relay in an old version of the system which has now been replaced by Q2.

SiliconWizard:

--- Quote from: Pentoad on January 16, 2020, 03:47:07 pm ---I have done a bit more fault finding and it seems to be to do with the reference voltage. It is meant to be 1.225 and on the faulty boards it is 1.07 when the battery is at 3.16. As I increase the voltage, the reference voltage slowly increases as I increase the battery voltage. When the battery reaches 3.8, the reference jumps to 1.225 and stays stable even if I reduce the battery voltage back down to 3.16 or below. It is like the voltage reference isn't starting up properly but it is just a diode so I don't see how this can happen.

--- End quote ---

This is obviously where you should now focus your investigations.
Have you checked all resistors' values on the "faulty" boards?

Prehistoricman:
The LM4041 is not just a diode:

http://www.ti.com/lit/ds/symlink/lm4041-n.pdf

Check R1 on the faulty boards. Maybe break the VREF line and see how much current is flowing out of it. Maybe something is pulling it to ground.

splin:

--- Quote from: Prehistoricman on January 16, 2020, 03:57:54 pm ---Check R1 on the faulty boards. Maybe break the VREF line and see how much current is flowing out of it. Maybe something is pulling it to ground.

--- End quote ---

Beat me to it. The LM4041 needs 60uA minimum so if R1 happened to be 47K (or more)  instead of 470R that could explain the battery voltage needing to be at least 3.8V before Vref is valid.

Also check R13 isn't short or low resistance which could load Vref with C2. The LM4041 is supposed to be stable with a capacitive load but it may have a limit,
especially if U1A's output is oscillating.  A bit unlikely though.

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