I see some phone chargers using safety capacitor (after some research I found its safety capacitor)But not optocoupler to isolate in and out . How safety capacitors replace the optocouplerenter image description here

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    \$\begingroup\$ How safety capacitors replace the optocoupler Explain why you think that this capacitor replaces the optocoupler. They're completely different types of components. Many SMPSs have an optocoupler and a Y-rated capacitor. \$\endgroup\$ Commented Sep 2, 2019 at 6:51
  • \$\begingroup\$ I am new to these components and I just thought it as a isolating device between input and output ground \$\endgroup\$ Commented Sep 2, 2019 at 8:28

1 Answer 1


It generally does not replace the function of an optocoupler, it's to control EMI by capacitively coupling the output ground with the input DC ground (after filter and rectifier etc.).

It should be a "Y" type safety capacitor. Usually it's around 1nF. For example, this one (from a Power Integrations datasheet):

enter image description here

The optocoupler function (when present) is to isolate the output voltage error (as a current) and send it to the controller which runs from the mains. If there is no optoisolator it may be using a transformer winding as feedback, which can work, but it's not as accurate.

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    \$\begingroup\$ Some phones chargers seem to do away with feedback, using expectations about their load as sole means of regulation :) \$\endgroup\$ Commented Sep 2, 2019 at 12:19

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