2 PIN POWER SUPPLIES
@pythoncoder @gratefulfrog The leakage current from unearthed power supplies does kill semiconductors but it should be safe for people.
Almost all power supplies have a 4n7 cap from the 200V / 400V DC negative to the output negative. That's about 600k to 700k at 50/60Hz. That's actually about 170 uA from a 240V 50Hz power supply. For a double insulated Class II power supply it must be a Class Y1 cap. Class Y1 caps are tested and certified by labs such as TUV and UL to be safe for use like this. If you use Class Y2 caps you must put two in series to be considered safe. I have seen some "generic" or "no name brand" powers supplies with one Class Y2 cap. Ouch!
I recently had to investigate a case where a product I designed was blowing up one brand of equipment. My design had a free relay contact output that could control other equipment. It was running from a 240V wall wart and floating at about 90VAC. I found that this was being capacitively coupled from the relay coil to it's contacts and leaking about 10 uA though the relay contacts to whatever they were connected. The system that was dying (not mine) had an input connector connected directly to an STM32 input with no protection, not even a pullup resistor. It turns out 10 uA at 70VAC is enough to zap an STM32 input. It didn't kill the whole MCU or any other pin.
I'e started using larger power bricks on the test bench instead of wall warts so I know they are earthed. If you run a pyboard off a phone charger and USB cable and connect any GPIO to something that is earthed you risk blowing up the STM32 pin. Likewise from a floating Laptop like Bob's ASUS. Many Tablets and low power Ultrabooks including Microsoft Surface use 2 pin mains plugs or wall warts. This will only get worse as more Laptops start using USB C wall warts.
Bob, All two pin and some 3 pin power supplies don't earth the output. I have found some HP and Dell power supplies with 3 pin plugs and unearthed outputs. They use the Earth pin for the input EMC filter but don't earth the output. You could earth your Laptop, say with a mains plug to USB adapter. Run a mains rated green/yellow wire from the EARTH pin out of the plug and connect it to the shield of a USB cable. NEVER run a USB cable into a mains plug. The insulation isn't good enough. If you cut the end off a USB cable make sure the 4 internal wire ends are insulated. WARNING, only do this if you understand mains wiring and know which is the EARTH pin.
These power supplies are causing me a lot of grief. In most Australian workplaces mains powered equipment has to be tested for safety. Because they have a mains earth pin they can't be marked with a double insulated symbol (two concentric boxes) or a CLASS II marking. But there is no way to know if they have an intentionally isolated output with double insulation and a Class Y1 cap or if the're a CLASS I supply with a broken earth connection on the PCB. These power supplies often have brittle paper/phenolic PCBs with single sided non plated through holes. Its quite common to see cracks in the PCB and copper after they are dropped. Because they aren't marked CLASS II they have to be tested for earth resistance (must be less than 1 ohm between mains plug and output plug) and they all fail.
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