I recently came across this relatively cheap power supply for my projects. It features a single chip switch mode IC which has an integrated high voltage NPN transistor for the switching.
The design was curious because of the lack of filtering etc.
The Design
The power supply is built around the IC DK124. Essentially its quite a convenient package which requires a minimal number of support components.
The pinout of the IC is shown below.
The datasheet also shows you an application circuit with values and also how to calculate the transformer for your application.
Youtube’s bigclivedotcom has a nice video analysing/explaining the operation of the circuit so I won’t bore you with the details. However his version was a clone according to his comment(s).
The datasheet for this IC can be found here:
http://grupoautcomp.com.br/wp-content/uploads/2016/11/Specification-IC-DK124.pdf
If its not available you can download a summary here: Condensed-DK124.pdf
The Problem
Unfortunately the seller accidentally sent me a mix of 24V and 12V versions when I specifically ordered all 12V@2A versions.
Add to that the impending Chinese New Year Hiatus, I was left to figure out how to turn the 24V into a 12V.
Turns out 90% of the components inside are identical except for the feedback circuit…
I discovered that resistors R5, R6 and VR1 are the only differences between the two boards.
Below is a table of the values:
| Part | 12V version | 24V version |
|---|---|---|
| R5 | 20K | 75K |
| R6 | 4K7 | 8K2 |
| VR1 | 2K | 200R |
The quickest conversion is to replace those parts verbatim. However i decided I would try to retain the pot and attempt to change the voltage divider to suit.
The feedback circuit
First I set the 12V version to output a stable 12.10V. At that point I measured the 2K potentiometer to see what value it read. It read 562R.
The simplified feedback circuit is as follows:
With VR1 set to 562R, the equivalent series resistance with R6 is 4700R+562R = 5262R or approx 5K3. The voltage at the TL431 ref pin would be 2.5V as expected.
So to retain the pot on the 24V version, I just need to figure out the voltage divider values that will bring the same 2.5V vref.
After some experimenting around I figured out to retain the 200R pot, I can use the following R5 and R6 values.
R5 = 18K and R6 = 4.7K while VR1 remains at 200R.
This will give a nice adjustable range of 10.5V to 12.1V
Purchase
If you are considering buying this power supply. May I offer my affiliate link which while it helps me out, costs you nothing extra. In fact I can vet the quality of the unit as shown in the photos.