# A 3.3V power supply with LNK3202

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I want to convert the schematics of the design note DER-507 (https://ac-dc.power.com/design-support/reference-designs/design-examples/der-507-175-mw-constant-voltage-universal/) to a 3.3V power supply. As far as I understand, the only thing I need to do is to replace the voltage divider R3 and R4 to an appropriate value. I am planning to keep R3 equal to 2K and changing R4 to 1.3K. With this change, the voltage divider should give me a ratio of 0.606. This number should be sufficient since 0.606 * (5V) = 2V. Are these calculations correct? Would these changes convert the power supply in the design note to a 3.3V power supply?

Thanks!
Victor Oliveira

Hello,
Thank you for considering Power Integrations for your power supply application.

That is correct. R3 and R4 are the parameters that need to be changed in order to get a FB pin voltage of 2V at your desired operating voltage. The value of resistors that you have calculated would give you a 3.3 V operating voltage.

Best Regards,
PI-Kryten

Thank Kryten!

Hi again (:
I've just built the power supply using the design note DER-507 and the appropriate resistor divider to have a 3V3 output voltage.
While I was using it in my project, an issue keeps happening again and again. The input resistors R1 and R2 are burning out even though I am using equivalent resistors as recommended in the design note. The part number I've chosen for R1 and R2 is ERJ-P06J221V.
I am also sending the schematics of my project for inspection. In this case, GNDA is connected to LINE. AC input is 220V/60Hz.
Do you think I need to use resistors with greater power rating?

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lnk3202d-3v3power_supply.png39.72 KB

I created a new topic because it is a different issue.