I have been thinking of using LNK305P from Power Integrations to integrate an AC -DC power supply in my next project.

The application note containing the circuit is here.

What things do I need to be aware of safety wise?

I do realize that this is a non isolated power supply and should never be touched while in operation and it would be inside an enclosure to prevent any accidental touching.

I plan to use this to drive a 12v relay (coil current 75ma) and an avr circuit (max current 120ma)

  • \$\begingroup\$ Because it's not isolated the ground on your AVR circuit is going to be referenced to the neutral line in some way. If you use J3 as ground and J4 as power, regulated down to 5V, then your ground is going to be 12V below the neutral line. I once blew up an AVR programmer doing this. Try to get a circuit that provides a positive supply, so you can reference your ground to the neutral line. \$\endgroup\$
    – Austin
    Commented Mar 19, 2015 at 9:26

1 Answer 1


Do your self a favour and go to premier magnetics to get a ton of designs for off-line ISOLATED switchers such as this one: -

enter image description here

Adding a transformer just isn't that big of a deal. I know this is a 7V output but they will sell you a transformer suitable for 12 volt DC and give you a circuit. They have many off-the-shelf designs.

  • \$\begingroup\$ Thanks .. I was hung up on trying to save some space by going transformer less but I think I should stick to an isolated one until the point I learn the basics \$\endgroup\$
    – Ankit
    Commented Mar 20, 2015 at 15:09
  • \$\begingroup\$ I've been in the business longer than I care to remember and I'd always use isolation. \$\endgroup\$
    – Andy aka
    Commented Mar 20, 2015 at 15:17
  • \$\begingroup\$ I agree that using transformer is a safe practice but for applications where space is severely limited and you are use of no one touching the supply in operation, isn't transformer less supply the ideal option? \$\endgroup\$
    – Ankit
    Commented Mar 20, 2015 at 15:42
  • \$\begingroup\$ I can't think of a situation where this couldn't be avoided. \$\endgroup\$
    – Andy aka
    Commented Mar 20, 2015 at 15:55

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