A signal generator or other separately powered device connected to any inpu= ts?=20 Best regards, SP On 2013-04-06, at 9:36 PM, veegee wrote: >> Occam's Razor says that the preferred explanation is that the voltage >> regulator is oscillating. >>=20 >> Whether this is the actual problem is another matter. >> (Occam only tells you the preferred answer, not necessarily the >> correct one :-) ). >>=20 >> In this case I'd say there is a very high probability that Occam's >> simplistic reductionist dangerous approach has actually supplied the >> correct answer :-). >>=20 >> What regulator are you using? >> Be CERTAIN that the regulator capacitor in and out specs are known and >> met. On some regulators Cout may not have too low or too high ESR. >>=20 >> A look at the power rail with an oscilloscope would show what is >> really happening. >=20 > I tried several different regulators, including the LT1086-3.3V and > L7833. I'll make sure to try the specified capacitors shortly, but I > checked with an oscilloscope and didn't notice any oscillation at all. > The signal was extremely clean with no ripple I could see, even when > switching a small load with a power MOSFET on the same power supply > rail. I'm using a 0.1uF ceramic and a 100uF electrolytic on each supply > rail at the regulator, and a 0.1uF ceramic at each pair of IC power > supply pins. >=20 >=20 >> Another possibility is that something on board is introducing bad >> supply rail swings. >> Oscilloscope finds that too. >=20 > Checked again, and removed the switching power MOSFET and the switched > load. No change as far as I could tell to the quality of the power signal= .. >=20 >=20 >> Then there is possibly excess voltage on a non psu ic-ic path. >>=20 >> If you have an external energy source whose voltage is higher than >> Vpsu it may be externally sourcing energy via some connection to it. >> This may pump up Vpsu (maybe via body diodes on some IC as a bonus) or >> inroduce AC energy. >=20 > Just a ~4.8-5.5V battery pack and one 3.3V linear regulator. Double > checked connections - all ICs are powered by the 3.3V regulated line > only. I also always use an appropriate Schottky diode with acceptable > reverse leakage (but perhaps a silicon diode would be more appropriate) > pointing into the regulator's Vin. Everything seems to be pretty well > isolated. >=20 > Just a note that these "hot IC" events are not common. They're rare > (maybe once or twice per experimentation session), but mysterious. >=20 >=20 >> As always a circuit would be a really really good idea and a >> description of intended functionality and connections may help. eg if >> this is an RFID transmitter that drives an inductor then what you are >> seeing would not be too unexpected. If this controls and connects to >> an RF plastic welder then what you are seeing would almost be more >> expected than not. >=20 > No RF or inductors involved anywhere. Mostly digital logic like PIC > micros and shift registers, with I2C and SPI interconnects, some > 4000-series CMOS ICs, and small power MOSFET's like Olin's favourite > IRLML2502, and some of its P-Ch brothers. >=20 > --=20 > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .