Author Topic: What does it mean "to leak VAC" for a PSU?  (Read 881 times)

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Offline analogoTopic starter

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What does it mean "to leak VAC" for a PSU?
« on: May 13, 2018, 01:11:34 pm »
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

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This brick PSU is certainly not mains earth referenced and leaks plenty of AC like most SMPS units

or

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I must have like a hundred devices here with cheap SMPS that each have ~50% mains VAC between their ground and mains earth.

or

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the output floats to half mains voltage due to the EMC filter caps.

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?
 

Online madires

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Re: What does it mean "to leak VAC" for a PSU?
« Reply #1 on: May 13, 2018, 01:24:45 pm »
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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