I have a device that outputs a regulated +12, -12, +5v supply for it's own use.
The current design ties GND to the center-tap of the transformer.
This device detects a low voltage DC signal from an external source.
To provide a common reference, I tie my GND to external source's GND with essentially no impedence.
Usually this works fine. However in a couple instances, it causes what appears to be a dead short in my device- the transformer/regulators get hot and the ICs are not powered up. When I disconnect the external source, the device works fine.
The external "active" signal is detected through 50k or higher ohms of resistance, so I cannot imagine that is the problem. So the fact that the GND are connected directly together indicates to me that either my device, or the external device, has a rapidly changing GND potential and is sinking/sourcing substantial current- appearing as a short.
Attached is the schematic of the power supply, with a representative model of the external signal detector.
Does anything here look obviously misguided?
Should I reduce the 100k input resistor down to 50k, and add the other 50k to the external GND?
I'm not sure if this would destabilize up my A/D converter.