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Project Notes

#341 Power/UsbPowerSupplyKit

Building a USB Wall Wart and taking a look at how they work .. instead of taking them apart or having them blow up.



I’ve taken apart and had enough “wall wart” power supplies blow up on me .. so perhaps it is about time to put one together instead!

Love them or hate them, “wall warts” represent amazing innovation upon which ubiquitous low voltage regulated power is built. I found an irresistable deal on a kit version of a standard power supply, so time to take a peek inside a “common design”.



Part Description
R2 10kΩ
R3 15Ω
R4 100kΩ
R5 27kΩ
R6 1kΩ
R7 2.2MΩ
R9 560Ω
R10 100Ω
R11 9.1kΩ
R12 10kΩ
R13 1kΩ
C1 2.2µF
C2 103, 10nF
C3 682, 6.8nF
C4 104, 100nF
C5 222, 2.2nF
C6 104, 100nF
C7 470µF
D1 1N4007
D2 1N4007
D3 1N4148
D5 1N5819
Q1 MJE13003
Q2 S9013
U1 PC817
U2 TL431
LED red 3mm
T1 trifilar transformer
USB female USB socket

The single-sided PCB is well designed with decent separation between high and low voltage sides. It also has a detailed silk-screen, which is just as well, as the schematics I’ve found online for the circuit appear to be incorrect.




The schematic available from sellers doesn’t appear to be correct - it does not match the PCB in a coupled of important ways.

Here is my transcription of the schematic .. available here in EasyEDA). I’ve verified this matches the PCB..


How it Works

The kit specifies: AC 220V to DC 5V 800mA. This circuit is similar and typical to a whole variety of switching power supplies.

Breakdown of the circuit:

  • D2, C1, R1 comprise a half-wave recitfier turning 220V AC into 300V DC unstable
  • R1 cater to the insurance function .. ??
  • start-up resistor R7 for the switch Q1 start current
  • D1, C3, R4 composite to switch collector peak voltage absorption circuit
  • switching transformer T1 primary winding 1-2, feedback winding 4-5, the positive feedback network R6, C2 and switches compose to a self-oscillating circuit
  • Q2 is shunt, pulse width modulation and variety of its protection through the switch base current shunt to achieve;
  • R2, R3 composition overcurrent protection circuit;
  • D3, C4 composite to rectifier filter circuit, power supply for the optocoupler;
  • C5 of the pulse width modulated signal from the smoothing effect, eliminate the shunt shunt in advance;
  • the secondary winding of the transformer switch 6-10 induced voltage generated through the freewheeling rectifier tube D5, C7 filter output DC;
  • R13 and composition of output LED power indicator circuit;
  • R11,12 composition sampling circuit;
  • a reference voltage generated by the TL431;
  • error signal after isolation optocoupler U1 transmitted to the pulse width modulation circuit accurate control of the switch, thereby stabilizing the output DC voltage.

Build and Test

It’s some enjoyable Sunday afternoon soldering therapy. Throw on your favourite music and while away an hour or two…





Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.