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We are measuring 60Ohms between the 3V3 and GND connections. Where is this happening?
There is about 50-60mA of current being drawn from 12V supply.
Test Plan:
Remove R6 to check if current leaking through the 5V, Opamps, and through D5.
Result: 8mA current. This is the LED current (1mA), the opamp supply current (<1mA) and the Iq of the 5V regulator (5-6 mA ish from datasheet). No board heating
Also try removing D5 if this does not make any changes. (after replacing R6)
Result: 13mA current draw. This is due to the extra current from the resistors used as voltage dividers after the opamps. So everything still works properly.
If this fixes it, then the opamps should be fine (as the inputs are both at 0V thus there is no leaking through them), and it is something downstream causing the issues (MCP3515 or another board component).
If we are still having issues, then try removing the opamps and/or the MCP2515
Removing MCP2515 (after replacing other components)