bahstrike Posted October 19, 2011 Report Share Posted October 19, 2011 Hi all,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. Quote Link to comment Share on other sites More sharing options...
bahstrike Posted October 19, 2011 Author Report Share Posted October 19, 2011 I'm continuing some investigations and found that the external device ties its DC GND to the actual case Ground. Should I be doing the same? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted October 19, 2011 Report Share Posted October 19, 2011 You're generally better off not connecting the 0V rail to chassis because you don't know what the 0V rail of the device you're powering is connected to. Quote Link to comment Share on other sites More sharing options...
bahstrike Posted October 20, 2011 Author Report Share Posted October 20, 2011 This particular one is solved.Turns out the cable for another connected device was tying my -12v to chassis.On it's own, this was "fine" and didn't really cause a problem.The problem arose when connecting to the external device, which ties its (and my) 0v to chassis, thereby completing a short to -12v supply through mains GND prong.Crazy to have a silly DC short going through wiring in the wall.Unfortunately, I don't know whether this is the same scenario in a couple other reports of similar-sounding issue. Quote Link to comment Share on other sites More sharing options...
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