Analyzing a power source often starts with input. 220V AC input, one end through a 4007 half wave rectifier, and the other end through a 10 ohm resistor, filtered by the 10uF capacitor. This 10 ohm resistor is used for protection, and if the rear fault causes excessive flow, the resistor will be blown off to avoid further trouble. On the right side of the 4007 4700pF capacitor, resistor, 82K, to form a high pressure absorption circuit, when the switch 13003 is turned off, the induction voltage on the coil for absorption, so as to prevent the high voltage is applied to switch 13003 and cause breakdown. 13003 for the switch tube (complete name should be MJE13003), voltage 400V, the maximum collector current 1.5A, the maximum collector power consumption of 14W, used to control the original side winding and power supply between the break. When the primary winding is switched on and off, a varying magnetic field is formed in the switching transformer, thereby generating an induced voltage in the secondary winding. Since the same ends of the windings are not shown in the diagram, it is not possible to see whether it is positive or negative.

However, from the structure of the circuit, it can be inferred that the power supply should be flyback. The 510K on the left is the start resistor and provides the base current for the switch to start. 13003, the lower 10 resistor is the current sampling resistor, the current becomes voltage after sampling (whose value is 10*I). This voltage is added to the base of the triode C945 after diode 4148. When the sampling voltage greater than about 1.4V, the switch current is greater than 0.14A, a triode C945 is conducted, which will switch the base voltage of 13003 low, thus the collector current decreases, thus limiting the switching current, prevent excessive current and burned (in fact, it is a constant current structure, will limit the maximum switch current is about 140mA). The inductive voltage of the winding (sampling winding) at the lower left of the transformer is rectified by the rectifier diode 4148, and the sampling voltage is formed after the 22uF capacitor is filtered. In order to facilitate the analysis, we take the triode C945 emitter at one end to ground.

Then this sampling voltage is negative (about -4V), and the higher the output voltage, the more negative the sampling voltage. The sampling voltage is added to the base of the switch tube 13003 through the 6.2V diode. I said, when the output voltage is higher, so the sampling voltage is negative when negative, to a certain extent, 6.2V zener diode breakdown, which will switch base potential 13003 low, this will lead to switch off or delay switch, thereby controlling the energy input to the transformer. We can control the increase of the output voltage, the voltage output function. The lower 1K resistor, in series with the 2700pF capacitor, is the positive feedback branch, which draws the induced voltage from the sampling winding and is applied to the base of the switch to maintain oscillation.

Then this sampling voltage is negative (about -4V), and the higher the output voltage, the more negative the sampling voltage. The sampling voltage is added to the base of the switch tube 13003 through the 6.2V diode. I said, when the output voltage is higher, so the sampling voltage is negative when negative, to a certain extent, 6.2V zener diode breakdown, which will switch base potential 13003 low, this will lead to switch off or delay switch, thereby controlling the energy input to the transformer. We can control the increase of the output voltage, the voltage output function. The lower 1K resistor, in series with the 2700pF capacitor, is the positive feedback branch, which draws the induced voltage from the sampling winding and is applied to the base of the switch to maintain oscillation.