help with buck converter

So i'm looking for a buck that will output 4v I have a few of http://www.dhgate.com/product/f85-3pcs-lot-standard-power-supply-module/260137309.html#sc1-7-1b;frsh|3826030675 I also have the same but its locked in at 3.2v All i can see between the two is the one locked in has the pot removed and added a resister to another part of the board. Only issue I do not know the layout or what size resister i need.

and added a resister to another part of the board.

Just one resistor? I would have expected two.

Remove them and replace with a pot. Normally these are not critical so I would try a 10K pot.

If you have that module, you should have told us what is written on the board and the chip.
The board says : “D-SUN”, and chip is MP1584 (see attached picture).
Searching with those two words, results into this schematic (scroll down) : http://www.sunrom.com/p/dc-dc-step-down-3a-mp1584

Compare that schematic with the schematic in the datasheet of the MP1584, and adjust the resistor.
I think the FeedBack is 0.8V. That means with R2=8k2, the variable resistor could be 32k8 for 4V (I’m not sure).

mp1584.png

It's not uncommon to see the pot in a buck converter wired as a rheostat, with a fixed resistor forming the other side of the voltage divider.

Hi,

For a fixed output i would have to recommend using fixed resistor(s). Pots can be troublesome unless you really need a variable output or some fine tune adjustment and then you should use a high quality pot.

Thats what I want is to swap the pot for a resistor. This is what i have now. just need to know what size will lock it in around 4v

Did you read my Reply #2 ?