130VAC coming out of 12VDC PSU... can this damage electronics? - Page 1
Forum section: electronics / beginners

author: Maurice Chávez
author_rank: Newbie
timestamp: January 17, 2024, 09:46:09 pm
content: Firstly I am a noob, but don't go easy on me.I was in the process of installing some shielded Ethernet cable. Shielded Ethernet cables require that the Ethernet ports on both ends are grounded).I know my Ethernet switch is grounded, but wasn't sure if the cable reciever was grounded. So I get out the cheap multimeter, set it to continuity, shove one probe into the earth pin on a kettle lead, and start poking the other probe behind my Cable box. *beeeeep*. Great, the chassis is grounded, but does it extend to the ethernet port? So now I put the conductive part of the probe on my index finger to help guide the probe... it actually took several seconds before I realised I could feel electricity. It was that low power.So I switched the meter to AC Volts and there's 130V AC between the jack plug's Negative DC and the house's ground. There is also 130V AC between Postive DC and house ground.There is also weird AC voltage between the -DC and +DC. It alternates between 75VAC and 6VAC every second. Alternating Alternating Current?I wouldnt care much about the Cable box, it's rented from the cable company... But the issue is that the chassis is at 130V AC, and this is going through the HDMI cable shielding and causing my other components to be used as a path to ground. And another issue is, once grounded, the 130V goes away... but the 75VAC and 6VAC alternating alternating voltage is still there between chassis and house ground.This might be a stupid question, but is this normal? and will this not damage my other electricals? If I've said anything silly please do correct me. I am very confused right now.Thanks.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 17, 2024, 09:51:27 pm
content: Are you measuring it with a high-Z DMM? If so you’re measuring leakage current, most likely from an EMI filter.

author: electr_peter
author_rank: Unknown
timestamp: January 17, 2024, 10:39:28 pm
content: Hi,Quote from: Maurice Chávez on January 17, 2024, 09:46:09 pmI assume you measured AC->DC adapter plug used to power Ethernet switch. Adapter provides isolated DC power and it is normal for it to float at around half (or less) mains voltage w.r.t. earth ground. Actual AC leakage current should be very small (few mA max, hopefully <1 mA) - this can be felt as a slight tingling feeling. DMM is able to measure 130VAC because of high impedance (i.e. almost no load) - this is misleading and is called "ghost" voltage.It is recommended to use DMM with low-Z feature (impedance of ~3kOhm) for this measurement - DMM should then show ~0ACV (if not, something is not right).QuoteIt is difficult to say without knowing more details. Likely DMM just got confused with some noise from SMPS. Better measurement with oscilloscope could answer this, but I would not worry about this.What DMM are you using?

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2024, 11:12:49 pm
content: It sounds like it's capacitive coupling through the Y capacitors, which are part of the EMI (Electro Magnetic Interference) filter has hinted above.Here's a simplified schematic of a switched mode power supply, note the capacitor bridging the mains and DC side?

author: radiolistener
author_rank: Super Contributor
timestamp: January 18, 2024, 01:20:18 pm
content: Quote from: Maurice Chávez on January 17, 2024, 09:46:09 pmThis is because your devices are designed for mains connector with ground pin, but your mains connector don't have ground pin or have it unconnected. Such devices have mains filter which requires ground wire from mains, if it's disconnected it leads to a capacitive voltage divider and you get half mains voltage AC potential on your device metal surfaces. It's current is limited with capacitors, but still can make damage to your electronics or even can do electric shock to you.It can be fixed by providing ground wire on mains connector, if it's impossible it can be fixed by removing internal Y-capacitor filter from devices

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 18, 2024, 09:59:51 pm
content: If you can feel it I would get another PSU. I know you have 130VAC because your meter has high impedance but still if you feel it so the voltage is perhaps 50VAC or so with you as the input impedance.