Electronic – Isolation of the transformerless power supply

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One of the disadvantages of transformerless power supply is the isolation problem because of the absence of the transformer. My question here is : Doesn't the capacitor in series at the input do the trick? the capacitor is 2 parallel plates isolated from each other and the current flows through them by the electric field.
isn't that enough?

Best Answer

First, you should have a capacitor for both line and neutral. Otherwise, depending on the installation/country/plug orientation/..., you'll end up with a non-isolated circuit.

Then, you should use X or Y rated capacitors for this application, to ensure safe failure mode (if it fails short, you end up with a non-isolated circuit). And I think 250V is not enough. A few thousand volts would seem better to me.

Now, honestly, even with those two requirements fulfilled, I wouldn't do that for an AC supply.

However, it can be interesting when you want to isolate two low-voltage supplies: there is even an app note from maxim.

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