Introduction: Changing the Output Voltage of a Cheap Power Supply

Picture of Changing the Output Voltage of a Cheap Power Supply

This instructable show how to change parts inside a small power supply to chnage the output voltage to suite your needs.
For DIY project I needed a stabilized voltage of exactly 7V dc and about 100 mA. Looking around my parts collection I found a small dc power supply from an old mobile phone that was unused.
The power supply had written 5,2V and 150mA written on it. That looked fine only the voltage needed to be pushed up a little bit until it was 7V.

Step 1: Reverse Engineering

Picture of Reverse Engineering


It was easy to tear the power supply a part. It only had one screw that kept the case together.
After opening the case a small circuit board fell out ... containing just a few parts.
It is a simple switching power supply. The stabilization of the output voltage is done using a TL431. This is a shunt regulator with a refence voltage and an input pin to adjust the output voltage. The data sheet of this device can be found on the internet. I located the resistors that are responsible to set the output voltage. They are named R10 and R14 on the pcb. I took the values of them and put them in the calculation formula that is written in the data sheet.
Vo=Vref*(1+R10/R14) . Using R10=5.1kOhm and R14=4.7kOhm the Result is exactly 5.2V as it is written on the power supply.

Step 2: Calculating New Parts and Modifying the Device

Picture of Calculating New Parts and Modifying the Device

I wanted to keep the sum of R10 and R14 about the same as it was in the original circuit. That is round about 10kOhm. To get a higher output value I needed to modify the resistors according to the data sheet. I also needed to replace the protecting zener diode.
For the protective zener I choosed a 10V type because I found it in my parts collection. This voltage protects the output capacitor.
Calculating the new resistor values I started with R10 using the formula of the TL431 data sheet and kept the 10kOhm in mind. The calculated resistor would be 6.5kOhm. That is not a resistor value that is common. I selected a near value of 6.8kOhm.
Now I calculated the value of R14 using the choosen value for R10. The calculation leads to a value of 3.777kOhm for R14. I choosed a value of 3.3kOhm and added a 500Ohm trimmer potentiometer.
Because of the tolerance of the circuits it seems to be a good idea to insert a trimmer to adjust the output voltage.
After removing the original parts from the soldering side of the pcb I added the new parts on the components side because I did not use smd parts.

Step 3: Results

Picture of Results

The voltage meter shows exactly 7V (ok .. it's 7.02V). That's what I wanted :-)
Now I can use the power supply for my beetle bot project ... coming soon ...


BoraD1 (author)2014-09-24

I was planning to convert a good quality laptop adapter into a 5v charge center but this is too "hardcore" for me ^^

BasilM (author)BoraD12014-11-28

just add a regulator LM7805 on the output and you will get 5volts on 1A stabilized

krishg1973 (author)BasilM2017-07-26

Sorry to say but I tried the same with my Philips 9V/1.56A adapter, but the output though was constant 5V, not able to charge my smartphone with 3000mAH battery, where as an old samsung adapter with 5V/0.7A was able to charge at 700mA, sometimes as as high 1019mA. I checked that using "ampere app" from. As per my understanding the displayed current would is -400mA used by the display. Hence the charge current might be as high as 1400mA. But 7805 was showing 800mA in the app, but the charge percent never increased even after waiting for 10-15minutes.

IC5 (author)BasilM2015-07-02

Don't do that. The LM7805 is a linear regulator, and given the common laptop power supply provides about 19 V, the efficiency would be 5V/19V * 100 which is about 26.3%. That means 73.7% of the power is lost into heat, so unless you have a liquid nitrogen cooling setup, I wouldn't do that. If the case is regulating the output you can use a switching step-down DC-DC converter for about $3 on eBay.

I3ordo (author)IC52015-07-16


IC5, how do i learn if it is a regulated out? besides, will the amperage scale well?

for example if it is 19v 2A, once stepped-down: > 5v 7A

IC5 (author)I3ordo2015-07-19

The regulated modules usually have a potentiometer which you can use to adjust the output voltage. The output amperage limit is set by the input power and the efficiency. For example; for the case you put above, and assuming an efficiency of 80%: 19V * 2A * 0.8 = 5V * xA thus theoretically the maximum output current would be 6.08A. You should take a look into the specs of the module, as switching regulators also have current limits.

ChristianS145 (author)BoraD12017-01-11

The simplest way to do this and get full power aka watts/amps out of it, is to use a PMW ( Pulse Width Modulator ) controller, these are very inexpensive and some can control from 0 to 100 volts so if you put in 19v the maximum that can come out in its regulation is a bit short of 19v if you need a pointer, just contact me

krishg1973 (author)2017-07-26

Is there a way to reduce the voltage, I have Philips adapter with 9.02V/1.56A.

I want to use it as high power mobile charger. The circuit does have the secondary side of the transformer a TL431 to regulate voltage through opto-coupler. Same as R10 and R14, here in this circuit some other numbering, the value of R10 is 11K and R14 is 4K, providing a voltage of 9V. Now to bring down the voltage to 5.0V I tried to change R10 and R14 as per the formula, but the voltage never drops below 6.7V. Any idea why is it so.

illumation_ (author)2016-09-06

Nice! Any ideas how to increase amperage?

plesperance (author)2016-08-24

I want to raise the outgoing power to around 30 volts can it be done and if so what would I be looking for? I want to add more input power to my Variable Bench Top Lab Power Supply.

jonaking (author)2016-07-31

Hi. I got here following TL431. Your article was quite interesting. I'm looking for technical ideas to cheat our dishwasher's temperature sensing. It's highest available temperature is 50°C (and in reality it's even lower, around 46°C), which happened to be not enough, grease accumulates in the corners, bad washing performance. I believe that the temperature is controlled by a TL431. The question is, how could I raise the temp from 50 to 65°C? One technical detail, it might help (this is written on the panel): WQP8-9239H S-S(9239H). Every help appreciated. Thanks in advance.

guruji1 (author)2016-07-09

Hi thanks for sharing. I have a power supply and would like to modify it from 21.30v to 9v. There are two 470ohm resistors with a bdx zener. What resistors should I change to for this output? Can you please help me?


spekis182 (author)2016-02-11

Hi! Do you know which components I need to change in this one?

TejasG1 (author)2016-02-08

Great Job... was looking for this tut since last found one.


Leong (author)2015-11-26

Great instructable :) thanks, managed to convert a 12V switching power supply to 5V for my set-top box.

AlexS141 (author)2015-11-07

In my case my PS output was 5.25v and I wanted to reduce it to no more than 5.05v. You mentioned the sum of R1 plus R2 being 10Kohm and I'm a bit worried about it because R1 was 5,1K and R2 was 4,7K originally and I simply put a 56K resistor in paralell with R1 to achieve 4,67K and get the 5.05v. The problem is that 4,67k + 4,7k ~ 9,3k and the lower resistor is R1 instead of R2. Does it make any difference? The datasheet sheds no light upon it.

tinkernaut (author)2009-02-03


ReCreate (author)2009-01-28

that looks like some sort of car connector

johanncnc (author)2008-08-24

looks like a euro style power strip...

hotpadrino (author)2008-08-02

Hi, is it possible to do the inverse? change the input voltage to the source? I have a pair of bateries and would like to use my switch with this, the input is 110 Vac and output 5 Vdc, but i see this works good with input 48 Vdc, but with 24 Vdc the output is 0. Inside is a IC regulator wich says work at 30 Vdc, however, i measure it Vcc pin and shows just 16 Vdc. Or if someone know an easy way to step down 24 to 12 at 1 Amp without linear regulators (heating) ill be glad. Thanks for the help.

frickelkram (author)hotpadrino2008-08-02

Most of these switching supplies work in a wide varity of input voltages. The often work from 100V input up to 240V input. If you go as low as 24V I think only a very few of them might work. As you found out most of the switching regulator ic's used in these supplies do work from 30V up. The whole device needs more than the30V to work properly. There need to be a starting voltage to make the switching regulator start it's internal switching frequency generator. Those circuits measure the input voltage and start operation only at a certain voltage. If you tested a supply and it startet at 48V ... you are lucky :-) If you get it working at this voltage. I am very sure that that it does not work well. I think you lose much power inside the device. I suggest you buy a special dc-dc supply that can handle the 24V input and give you a constant 5V and that is build for this scenario. You can also build such a device on your own using linear power regulators (7805 and such). Nowadays you can also buy switching regulators that are as easy to use as linear voltage regulators. I think that using a power supply für 110V input voltage doesn't make much sense for the things you like to do with it ...

sineSurfer (author)2007-11-16

great instructable!!! How about doing the opposite, let's say 12v down to 9v? Or, is it possible to increase the amperage for example from 500mA to 600 mA? I have 1 one power supply that could use in a synth by changing the mA.

frickelkram (author)sineSurfer2007-11-17

Generally spoken ... everything is possible but not everything make sense. Changing the output voltage from 12V to 9V should'nt be a problem. While changing output voltages the overall power of the supply in watts should remain the same. There could be some drops in overall efficency but that would be minimal. Adding more output current while keeping the voltage the same would mean that the over all power needs to be increased, If you want to do that it is likely that you need to change many parts of the power supply. The hardest would be the transfomer, but also the primary capacitors and the primary switching devices might be changed. This would be rather difficult and expensive. If you need a bigger power supply just buy one, that would be much easier and cheaper.

sineSurfer (author)frickelkram2007-11-17

yes, explained like you just did, makes more sense to go shopping for the right psu. Thanks! ;)

Derin (author)sineSurfer2008-06-13

you can pull 1a from a 500 ma adapter BUT the adapter will be hot

Derin (author)Derin2008-06-28

for that be sure to use a unregulated one tho

enigma123 (author)2008-04-17

Can I use an adaptor that has an output of 6 volts for a cordless phone that has an input of 6.5volts? Does 0.5 volts make any difference? Thanks in advance

frickelkram (author)enigma1232008-04-19

I don't think that it makes a difference, just give it a try. But check if the low power plugs are wired correctly. There are many different plugs on the market using different polarity!

fegundez1 (author)enigma1232008-04-18

no problem,just try to stay away from large increases as you don't want burnout

pcapelo (author)2007-11-15

I have a question for you. Did you measure the voltage before the modification??? I never see a power adapter the have the voltage show. For example I have a 9V adapter that always give me 12V. A 12V that gives me 14V-15V, always.

frickelkram (author)pcapelo2007-11-16

Yes I did ... it was 5.2V exactly. But "5hales" wrote it ... switching supplies are quite exact with their output voltage. I think that you are talking about an unregulated power supply containing a transformer because the voltage in your example drops about 2-3V. The voltage on those power supplies will drop if you put load on them. You could also have a regulated power supply containing a transformer. The drop will be not as big as the unregulated ones, normally not more than 0.5V.

5hales (author)pcapelo2007-11-15

non switching power supplies will usually show a higher voltage with no load. Once they are loaded down, the voltage will drop.

marcward86 (author)2007-11-15

excellent! i've always wondered about this...i've been needing a power supply for an old record player. my wife cordially thanks you as well, because now i'll have less nonworking electronic junk laying around.

Bongmaster (author)2007-11-15

useful instructable :) thnanks

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