This 300V variable power supply circuit diagram is designed with high voltage transistors. This 300V variable power supply circuit diagram can provide an output voltage between 10 and 300 volts which can be adjusted with the potentiometer P1.
Transformers typically used in such sources have several windings of high voltage and low voltage winding. In this case we used a winding high voltage 300 V, while a winding 9 V supply was used for circuit stabilization. This circuit is very simple and need not explain.
Redressing the secondary voltage (300 V) with diodes D1 ... D4 are obtained 420 V. filter it, must be connected in series with two electrolytic capacitors rated voltage of 350 V. Two resistors (R1, R2) connected in parallel ensure equal voltages on the two capacitors. R5 resistance reduces power dissipation of Q1 and, together with a second RC network (R3, R4, C3, C4), feed collector of Q1 with a filtered input voltage of 350 V. The transistors T2 and T3 can be BC547 type, because they have nothing to do with voltage and high current. Diode D7 protects the base of Q3 against excessive voltage peaks negative.
You can set four different thresholds of current through the second switch keyboard with 6 poles: 1 mA (no switch pressed), 5 mA (S2 down), 15 mA (S2 and S3 hold) and 50 mA (only S3 down ).
S4 switch lets you connect instrument to the measuring voltage indicator.
The current is limited when selected exceeds the 10%. Output voltage. Fine tuning calibration can be achieved with P2 potentiometer.