Hello!
I will try to explain this and why does it matter to me - I need your help.
So, I live in Slovenia, Europe, where we got 230VAC 50Hz. Socket is properly earthed. All AC voltage readings are done between Earth PE (main socket) and barrel connectors.
- For project I have Meanwell GST60A12-P1J 60W 12V AC-DC power supply via "3-prong cable", which has L, N and PE. On output I get 12V DC on the barrel connector, but also I also get 118VAC on both + and - of the barrel connector in reference to Earth. This seems to be exactly 1/2 of VAC of the socket (236V).
- When I compare this with 10 other wall plug power supplies (I've tested all I have on hand), they mostly are 80-90VAC, some are 35VAC, some around 17VAC.
- DELL laptop power supply brick has 0VAC, as it seems it has DC GND internally connected to Earth PE.
- HP laptop power supply brick also has 0VAC. Same as DELL.
Now why this matters to me? I've found out that my PCB that I power with 12V through this PSU has problems. It has long wires to sensors and sensors are really sensitive. I've found correlation with this VAC reading and problems with sensors. PCB works with ANY other PSU, except the one I've bought for it
(Meanwell GST60A12-P1J), it also starts working if I touch the PCB with my finger, decreasing the VAC to around 70-80VAC. It also works if I have USB connected to it, as it's connected to PE ground via PC, it also works if I try to probe it, as scope has GND connected to ground PE, ofcourse it's also working with lab power supply etc.
Obivousely, choosing different PSU solves the problem instantly for me, but I want to know what is happening, why and what I can do on the PCB itself to reduce this. I've replaced the cable to the sensor with shielded one, which has shown a little improvement, but the sensor is still twitching (i've tried connecting shield to DC GND or float and it's the same). So shielded cable doesn't help with this issue. Is there anything else I could do to the PCB or the sensor, apart from the obvious (choosing different PSU or connecting PCBs DC GND to Earth PE)? I mean like some chokes, ferrites, capacitors...?
Please, if possible, explain why this is happening and why I'm having probelms because of it. This is first time seeing this phenomena and I want to understand it. I'm also "too short" to know what exactly to look for, but I've found this youtube and I think this is actually what I'm seeing here.
I'm also curious on how to know when buying, if barrel GND is tied to PE Earth or not.
Best regards,
Jure