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

Intro: 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.

Comments

[Yonatan24]

Interesting

Make sure that the it doesn't exceed the maximum voltage of the electrolytic Capacitor that's on the output!

[fifty_ohms]

This is great. What a trick! I will try this once I can find a discarded wall wart.

[crazypj]

One of the most informative 'structables' I've seen in a while. Thanks

[banman11]

Great instructable. I have a very similar power supply to your example.

Is it possible to say increase the rated current output from say 1A to 1.5A by adjusting components or would components need to be upgraded?



Well done again

.

Thanks.

[lkndave]

i am afraid it is not just as easy to mess with the current as it is the voltage. i will say that for the most part the output of this can be thought of as watts. with that said if you adjust the voltage to lets say 6 volts then one could probably expect 2 amps. think of it as a wattage formula. original design is 12 volts at 1 amp. that is = to 12 X 1 or 12 watts. 12 watts being the real rating of the wart. now lets for argument sake you wanted to lower the voltage to 6 volts then because it is rated to 12 watts it should be able to put out 2 amps. 6x2=12 watts. 9 volts= 1.33 amps. so if you wanted to get 1.5 amps from this that number is 8 volts. 8 x 1.5=12 watts. if you took it too 15 volts then it would be 800 ma or 15x.8=12 watts. that is my answer

[banman11]

Just an update.

I managed to pull apart an old non working 1.5A power adapter I had. I compared it to the 1.0A version I have which is similar to yours. There were no screws on this one. I used a little solvent on the joint seam and was able to pry it open after few minutes of waiting (I found a small line of petrol/gasoline works without damaging the plastic casing).

As you can see it has a very similar layout to the 1.0A switching power supply in your example. There is an extra level of circuity added to the 1.5A version.

looks like an upgraded transformer. The power transistor has a heat sink added to it. Interestingly this transistor has the same model identification as the 1.0A model I have (13007).

There is a large transistor shaped object near the output end that has a 2 diodes facing each other symbol stamped on it. It looks like it's possibly a low voltage high current rectifier.

The AC bridge rectifier circuits are beefed up too.

The main electrolytic capacitor is badly bulging and the circuit board is heat damaged . Too far gone to replace all the components effectively.



I would like to see more instructables from you.


Thanks again.

[lkndave]

Great info, right back at you. I am not surprised things had to be beefed up. looks like maybe the 1.5 A may be a little conservative. some of those parts look like they can handle a lot more. The AC input Cap has a raised dome. that is evidence of gassing out due to dry electrolyte. that is common for caps that have been hot, or left unused for a long time. The 3 pin device you described is a half bridge rectifier, probably good for 8 amps. this time i thank you. this looks like a nice work horse supply bu putting a few different fixed resistors to a rotary switch, think universal supply. at 1.5 amps your are just leaving the weenydom of power. thanks. more instructables? about what? i am seeing if i can cobble together a small device right now that must fit inside a watertight camera case. my goal is to build an audible alarm to let my friend know if the case has water in it or a pressure leak. cr2032 battery, self driving piezo, Attiny85 digispark clone and BMP180 pressure sensor. all in a 1.75 inch x 1 inch x 3/8 in. space. fun

[banman11]

Thank you so much for that consice explaination. .

[Akh-WordP]

i am trying to charge a battery with a wall wort, but do i hook the hot (positive) lead to the positive or negative pole of the battery?

[Akh-WordP]

like as if it was being wired in series, or should be like parallel?

[Yard Sale Dale]

Cool. I'm going to look at this again later. I want to use a power adapter from a dead printer (16v DC output, wall wart for USA) to power a "12v" picnic cooler in the house, before loading it in the car for trips. I also would like to power a CB radio in the house sometimes. I could buy an adapter for that at $30. But I already have this printer power cord. I like the note about potentiometers. There are very common ones for guitars at 250,000 and 500,000 ohms, "linear taper" and "audio taper". I'm new to this, but would the "linear taper" work better? They're $2-4 at Radioshack or free on busted guitars.

[Yard Sale Dale]

My plug says input 100-240v /1A 50-60Hz. Output 40W max, and there are 2 "hot" wires, 32v DC 940mA LPS (what's LPS) and 16v DC 625ma. The cooler is a generic Walmart Igloo cooler with the peltier junction type cooling system and no switch, just a cig. lighter plug. What do you think? Can I just wire a cig. outlet in a box to the 16v and plug in the appliances (cb or cooler)?

[lkndave]

cig adapter to peltier will work fine. if your trying to use your 2 output adapter in your car it wont work it needs 100-240 VAC to work. cars are 12.6 to 14.5 DC. the adapter will just burn and cause a small fire.

[Yard Sale Dale]

No, I am intending to cap off the 32v wire, and use the 16v wire ending in a cigarette outlet from autozone, to use household power for 12v cooler in the house, overnight before the trip or as an office fridge. then I can just plug the fridge in the car as normal and it will already be cold.

[lkndave]

remember the power going to the fridge (peltier) is going to have to be from 5 to 10 amps. i would guess you printer supply is not anywhere near that. i think it will be weak to work.
everyone i know that used these for their car has gotten rid of them because it will kill your car battery in a few hours.
just sayin'

[Yard Sale Dale]

I like using them in the car. I usually use them bring home gourmet groceries from a distant city, or to carry picnic food 1 way to events. I have a large truck battery, but don't leave the fridge on without the car running for more than 1 hour. I used to use one in a diesel truck, with a deep cycle battery under the bunk. The fridge battery had a cigarette plug going to the truck system, and was interrupted by a voltage controlled shut off that cut the connection between the truck and fridge battery at 11.5 v, but the truck had its own shutoff at something higher. like 13.5, which cut all power use unless the truck was started again. I'll just buy a converter from Amazon. I got one for $20 last year that is made for the 12v coolers.

[Yard Sale Dale]

If you are charging batteries with the wall wart, how do you control it? Do you just use a cheap multimeter in the system and unplug it when it's "done" (unplug wall wart to check, right)?

[lkndave]

if you are using a fixed voltage wallwart to charge batteries the power must be controlled to avoid overcharging. discover circuits . com has plenty of simple circuits. if you over charge a litium battery they will explode. Nicads and NiMh's will stop holding a charge after a few cycles. it ruins batteries to keep them on charge all the time.

[lkndave]

linear pot is the way to go for controlling anything that is not sound or audio related. Peltier junction coolers and CB radios take a bit of current. peltiers should have 5 amps available and should be at least 3 amps. make sure your wall wart can handle it.

[lkndave]

PS the peltier is OK with 16 V the CB should be 14.5 volts.