I built a double DC multimeter comprising 2 completely independent boards with MCU + ICL7135. Power supply for each one is as follows:
- small mains EI transformer (a couple of watts)
- bridge rectifier + capacitors
- linear voltage regulators
After final assembly into a grounded metal housing, I found that they are very sensitive to disturbances: unit A can detect false key presses or its MCU can even reset when I turn on/off the unit B.
I separated one unit and was able to reproduce the misbehaviour by just touching the mains live line by a screwdriver at the entry into the housing. The screwdriver wasn't connected anywhere else, I was holding its isolated handle in my hand.
I measured the 3.3V rail from the regulator when the screwdriver's metal part gets into contact with the mains live. The circuit's GND was grounded via the scope's probe during this time; it is intended to be floating during normal operation. Here is the spike I get:
Similar signal can be observed on several GPIO lines of the MCU. While false key presses could be filtered out in FW, this makes me worry about component damage. In fact, I am surprised that everything is still working.
Given the ns time scale of the spike, I imagine it cannot come via the transformer but must be injected by radiation. So I put some metal foil between the mains and the low voltage sections of the system, connected to the ground. No improvement. With RC snubber on mains neither.
Any explanation & ideas how to get rid of this strange behaviour?