This power supply components made entirely saved from the landfill delivers about 250 watt, the output voltage is 2×35 load current 3,5 A.
After rectification by diodes bridge BR1, the mains voltage is filtered by capacitors C1-C2. The NTC R1 limits the current of capacitor charging power. The filtered voltage is applied to a voltage converter carried out according to the scheme of an inverter half bridge on the transistors TR1, TR2. The converter is loaded with the primary winding (I) of transformer T1, which converts the voltage and galvanically decouples the drive output from the alternating current network. Not drawn in the diagram but in the present embodiment, the emi filter consisting of a double filter coil and a pair of capacitors from 400V 0,1uF, these elements prevent the penetration of RF interference on 230V. This half-bridge inverter converts the DC voltage into a rectangular variable with a frequency of 27 kHz. The transformer T1 is designed so that its magnetic core is never saturated. The self-oscillating operation mode is provided by a feedback circuit, whose voltage is tapped winding III of the transformer T1 and it is applied to the winding I the auxiliary transformer T2. The resistor R4 limits the voltage across the winding I of the transformer T2. The resistance of this resistor depends within certain limits by the frequency conversion.
To ensure reliable start-up of the converter and its stable operation, It is used a boot node, which it is a relaxation oscillator of a transistor TR3 which operates in avalanche mode. When the power is turned on through the resistor R5, the capacitor C5 starts to charge and when the voltage reaches 50 … 70 V, It triggers the avalanche effect that triggers TR3 and the capacitor discharges. The current pulse triggers the transistor TR2 on and start the converter autooscillazione.
I transistor TR1 e TR2, D2 and D2 as well as the bridge diodes are installed on a single heat sink taken from an old ATX. The winding characteristics of the transformers are summarized in table.
The two ferrite rings that constitute the transformer T1
originate from an ATX power supply output filter
The winding I (verde) It is positioned uniformly along the length of the ring. To facilitate the launch of the converter, the winding III (rosa) of transformer T1 it must be positioned in a location not occupied by the winding II (yellow). The insulation in transformers is carried out with a ribbon. Among the winding I and the other of transformer T1, the insulation is made with a double layer of tape.
It should be noted the trigger winding consists of two turns and a half (for implementation convenience I added half a turn) in the foreground in a free zone.
For the trigger transformer T2 we follow this schema
The final realization made on breadboard Having regard to the low number of components used.
Generally the power supply works correctly assembled at the first shot, however in some rare cases it may be necessary to replace the transistor VT3 due to a slightly marked snowball effect that generates. To test its functionality, temporarily disable the emitter output and connect it to the negative terminal of the mains rectifier. On the oscilloscope screen, observe the voltage across the capacitor C5 – a sawtooth signal with a field of 20 … 50 V with a frequency of several hertz. If supply voltage of the sawtooth, the transistor must be replaced.
A frequency of operation of the converter to the winding II is 27 kHz editable by varying the value of R4. The power supply with a load of 50 – 70% It maintains the practically cold power transistor.
WARNING! A PRIMARY CIRCUIT LINE VOLTAGE, so you need to take precautions during installation and operation.