Introduction: Changing the Output Voltage of a Cheap Power Supply

By

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

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.

Step 2: 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

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 ...