| Electronics > Beginners |
| What does it mean "to leak VAC" for a PSU? |
| (1/1) |
| analogo:
In the thread "TS100 doesn't function properly when earthed" users Rooster Cogburn and Gyro discuss the implication of a non-earthed PSU connected with a 3-prong cable. They use expressions like --- Quote ---This brick PSU is certainly not mains earth referenced and leaks plenty of AC like most SMPS units --- End quote --- or --- Quote ---I must have like a hundred devices here with cheap SMPS that each have ~50% mains VAC between their ground and mains earth. --- End quote --- or --- Quote ---the output floats to half mains voltage due to the EMC filter caps. --- End quote --- Could someone explain what is this AC leakage and how it can be detected? If I had an oscilloscope (I don't), could I measure the voltage between mains earth and the -V of the 12V PSU output and see an oscillating wave with amplitude 120V (240V/2)? What about probing earth and +V? Would I get an oscillating 108V wave (240V - 12V)? If so, isn't that extremely dangerous? How could it be that Rooster has hundreds of such PSUs in his house? |
| madires:
The last quote is the answer ;) SMPSUs usually have an EMI suppression cap between primary and secondary side (typically 2.2nF class Y2). That cap passes a mains referenced low current from the primary side to the secondary side. It's not dangerous but can give you a tingle. To check for the cap simply take a DMM and measure the AC voltage between output (ground in most cases) and mains earth. |
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