Instructables

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.
 
Remove these adsRemove these ads by Signing Up

Step 1: Reverse engineering

Picture of Reverse engineering
BE CAREFUL! THE PARTS MAY STILL CONTAIN HIGH VOLTAGES IF TEARING APART SHORT AFTER USAGE!

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.
tinkernaut5 years ago
Great!
ReCreate5 years ago
that looks like some sort of car connector
johanncnc6 years ago
looks like a euro style power strip...
hotpadrino6 years ago
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. hotpadrino@yahoo.com
frickelkram (author)  hotpadrino6 years ago
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 ...
sineSurfer6 years ago
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)  sineSurfer6 years ago
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.
yes, explained like you just did, makes more sense to go shopping for the right psu. Thanks! ;)
you can pull 1a from a 500 ma adapter BUT the adapter will be hot
Derin Derin6 years ago
for that be sure to use a unregulated one tho
enigma1236 years ago
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)  enigma1236 years ago
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!
no problem,just try to stay away from large increases as you don't want burnout
pcapelo6 years ago
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)  pcapelo6 years ago
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 pcapelo6 years ago
non switching power supplies will usually show a higher voltage with no load. Once they are loaded down, the voltage will drop.
marcward866 years ago
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.
Bongmaster6 years ago
useful instructable :) thnanks