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Tripple limited bench PSU ( digital ) from wall adapter

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Jan Audio:

--- Quote from: nctnico on March 21, 2019, 12:44:25 pm ---If you use wall-warts make sure they have a transformer. The switching ones very likely have filter capacitors which bias the output to half the mains voltage. This will kill your circuit.

--- End quote ---

This i dont understand, how can a 12v switching adapter give 110volt ?, that is not safe.

Cliff Matthews:
Although output (-) isn't DC-coupled to mains, blue Y cap beside transformer pulls its avg to about half (w/ resect to GND).

Jan Audio:
So when does this happen with switching adapters ?

Cliff Matthews:
Most are like this. The isolated output side of the adapter can be like a radio transmitter with square wave pulses from 30 to 300khz broadcasting switching harmonics. The Y cap effectively shunts this to meet EMI requirements (keeping your neighbors radio's happy..). If your project uses an SMPS psu, clip out the Y cap and encase it in a shielded box with isolated binding posts at front.

Jan Audio:
I mean when.

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