In this opportunity, I would like to present a quick inspection of a cheap switch-mode power supply used as an Android phone charger. The initial objective was to see the charger can be modified for a different output voltage. However, in this quick review, we will only consider options of modifying it. We will start by inspecting the design of the smps.


The SMPS charger is shown below. It has a USB connector as the output port and a working input voltage of 220V.



After a brief inspection, it is found that its schematic is as follows:

Switch-Mode Power Supply Schematic
Switch-Mode Power Supply Schematic

It is interesting to note that the input voltage is rectified by a single diode (D1) to provide a negative supply (as circuit ground). The System supply voltage will present at C1. The system uses a fly back design around transistor Q1 (MJE 13001).

I am assuming that the following is the thinking behind the circuit design: R1 and R2 provide bias to the transistor making it look like an amplifier. I think the operating frequency is determined C4 and its interaction with the fly back winding of the transformer (as well as load current).

Again, it is interesting to note that fly back winding is used as a negative supply that is rectified by D2 and will present a negative voltage at C2. This design seem to be pretty common as compared to another SMPS as inspected by Rodericks in his blog (Fix a Cheap Switching Power Supply).

Voltage regulation is controlled by a zener diode D3, and its quenching action is transmitted via an optocoupler OK1 whose optotransistor will bring the base voltage of Q1 towards the negative supply at C2.


In the circuit, there is one thing puzzling. C5 seems to shunt from the high voltage side to the low voltage side. Depending on how the charger is plugged in, we will see high voltage induced in the output port. I am thinking, that if we tap the output with a test pen, we will see that the tiny neon bulb in the test pen will light up. I can’t think of any explanation for this capacitor.


Finally, if I was to change the output voltage of the SMPS, I think I will have to change the secondary winding of the transformer, as well as the quenching zener. However, I will leave it at this. Perhaps this can be done as an interesting small project in the future.

If you have any thoughts or suggestions on this brief presentation, I will be happy to hear them!

Thanks for reading.



  1. I wouldn’t be surprised if the supply still worked fine if you changed the output zener to something higher. This design is highly tolerant of input voltage, and a single product will often say “100 – 250 VAC” for the input. If it can do 5 volts at 110 volts input, it can probably easily put out 10 volts at 220 input.

    C5, the mystery cap, may be there for electrostatic discharge (ESD) protection. That way, the shell of the USB connector is sort of connected to ground (earth). Just a guess.


  2. Heheh C5 is the mystery cap. It will ground the USB connector if the plug is inserted in a way that C5 connects to neutral. In this part of the world (crazy Jakarta), you can plug the power connector either way (it is the same like a European power plug). So there is definitely a chance that C5 connects to line instead of neutral.

    However, thinking about it more, if this is designed in China (a country known to have different types of power plugs), perhaps the circuit is designed to have an American style power connector? That type of connector will have definite line and neutral pins and C5 can be placed conveniently to provide a path to ground (ESD? or a ground at a certain frequency threshold?) Who knows. The mystery continues.

    I am sure that this cannot be sold responsibly in the US.


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