Need a Very Cheap Power Supply or Wall Wart But Can't Find the Right Voltage?

Ever had a need for a replacement or project power supply that you just can't find anywhere or costs way to much?
This instructable will teach you to take junk wall warts from thrift stores and hack the circuit to give you what you need. Not the connector but the power itself. I never pay more than a buck for one using this mod.
I have been doing electronics experimentation for decades and often find a major stumbling block for my small projects a suitable power supply. 
We can very simply change a single part and "make" what we need.

Need 5 volts? 9 volts? 12.6? 13.5 or any number in between? Here is your answer.
Should cost about 1 dollar!


Here we go.

Step 1:

First a tiny background.
These little wall warts come in 2 styles. Switching and Linear.
We need to use switching type for this hack. 
How do I tell the difference?
Linear supplies, which we don't want are usually heavy in the hand and rated input is 110 - 120 VAC 60 HZ
Switching are rated 100-240 VAC 50-60 Hz, thats what we want.
NEVER WORK ON ANY LINE OPERATED EQUIPMENT WHILE PLUGGED IN. ELECTROCUTION IS POSSIBLE AND INJURY OR DEATH MAY RESULT. BE SMART, BE CAREFUL.

Step 2:

Now let's say it is a 12 volt DC output at 1000 ma. Problem is you need something different for you project or replacement purposes.
Let's say you need 13.5 volts DC.
OK.
Find the screw that opens this thing and you will be able to pry it open and find a small printed circuit board inside.
There are 4 connections to this PCB. 2 wires come from the wall AC, the other 2 usually black and red, usually black negative and red positive.
Near the output or black and red wire side you probably will find a small 3 legged transistor looking thing marked TL431 or something 431. If it's not on the board I will add directions later for that zener regulated type.
If you see it, that's good, that's the brain to the power supply. It regulates the output by monitoring the output and via a opto coupler tells the input to increase or decrease pulse width till the output is what is desired.

Step 3:

These devices work on a voltage divider that "programs" the correct voltage for the desired output.
The formula to program the regulator is:
V = 2.5V * (1 + RX / RY)
You can work it out or go to a site like http://www.lknorman.com/TL431/
The spec is there if interested.
Now look on the bottom of the PCB and you'll see 2 very small resistors under the TL431 part. They look like 1/4 a grain of rice and have numbers on them. Mine have a 1002 and a 393, that's a 10,000 ohm and 39,000 ohm (100 +2 zeros = 10,000) and (39+3 zeros= 39,000)
I put those numbers into the formula on the website and get 12.2 volts.

Step 4:

The 393 resistor must be removed and replaced with a new value we got from the formula or website.
Since they are very small and really not usable  just heat it with soldering iron and flick it off the board. make sure there are no solder webs or short where it was.

Now let's say I need a 13.5 volt output. Go to the website.
Just put 13.5 in the top box and add a ")" to the 1000 in the second box, then click calc. comes back 44,000 for bottom resistor.

*I did not have a 44,000 ohm but I did have 2-22,000 ohm ones. I just soldered them in series and they added up to 44,000 ohm. Resistors in series are additive, 22,000 plus 22,000 = 44,000. If you don't have 2- 22,000 ohm you can add any two you want to get the right values. ie.. 20,000 and 24,000 = 44,000!

The pins on the TL 431 are 1,2,3, from left to right with the flat side toward you.
The correct resistor to change is the one that connects pin 1 to the red wire.
that's it. Unsolder the "393" resistor and replace it with one to give you the right output.

My adapter allowed me to put them on the top side of the board using regular not surface mount resistors. I soldered one leg of my "homemade" 44,000 resistor to where the red wire connects and the other end I soldered to the little disc capacitor whose leg also connects to the pin 1 of the TL431.

One you make sure it's right, carefully close the cover, replace the screw and use for your project or replacement needs.


Lets say i need a 6 volt output i leave the 10,000 and put in 6 volt and get 14,400.
that means changing the value of that one part will allow me to set the output over a wide range.
The pins on the TL 431 are 1,2,3, from left to right with the flat side toward you.


Hint. If you put a small 20,000 ohm potentiometer in place of that resistor then you could have a adjustable output supply for next project you need to power. I would put a small value resistor like 1,000 ohm in series so you never go to zero ohms in the circuit.

Step 5:

In my example in the picture, I changed a 12.0 (measured 12.2 which agrees with formula) volt to a 13.5 volt to charge a sealed lead acid battery.

FYI, I would use Dave's, me,  inverse law of power. It sates power remains the same no matter what the voltage and current do. huh?
Example we stared with a 12 volt 1000ma (1 amp) supply. 12 x 1 = 12 watts. if we changed it to a 6 volt supply i would expect 2 amps out of it. 6x2 = 12 watts. same goes that if I went to 18 volts then 18 x .750=12 watts. so this little buck supply will give us, 2 amps at 6 v, 1 and at 12 v, and 750 ma or 3/4 amp at 18 volts.


more to come on zener type mods.