#341 USB Power Supply Kit

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

Build

Notes

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”.

Parts

kit_parts

Part	Description
R1	1Ω
R2	10kΩ
R3	15Ω
R4	100kΩ
R5	27kΩ
R6	1kΩ
R7	2.2MΩ
R8	1Ω
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.

kit_pcb_front

kit_pcb_rear

Construction

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..

Schematic

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.