The post explains a simple transformerless zero to 300 volts variable voltage and current power supply circuit which can be used for many different applications.
To protect against my higher voltage research projects from shooting up in smoke permanently, I developed an easy circuit which is able to render a variable voltage supply of 0 to 330 Volt.
But please be cautioned, the circuit is not isolated from mains potential, and therefore can inflict a lethal shock.
The supply is short-circuit proof: the current is restricted to approximately 100mA.
The design does not require a transformer, rather a 100 watt bulb is introduced at the input in order to provide ultimate safety in case of a short circuit or a component failure.
The mains voltage from after passing through the lamp is rectified with bridge D1 (1Amp / 500V) and C1.
T1 is configured as a source follower: the source of T1 complies with the voltage of the wiper of R3. D2 is insured to safeguard the gate of T1.
T2 and shunt resistor R2 establish the current limiter. Whenever the output current results in being excessive, T2 quickly discharges the gate of T1.
This stops the current from increasing any further. The value of R3 was basically identified experimentally; however it actually depends on the Hfe of T2 which means you may need to adjust the value of R2 appropriately.
Keep in mind T1 requires a large heatsink: in nastiest situation T1 would probably disperse 330V x 100mA = 33Watt!
You may try mosfets such as a BUZ 326 (400V/10.5Amp) or you may likewise use an IRF740 (400V/10Amp).
The output impedance of the power supply varies according to the beta of T1, therefore the bigger the MOSFET, the lesser the output impedance!
WARNING: THE CIRCUIT CARRIES LETHAL MAINS VOLTAGE AND THEREFORE IS EXTREMELY DANGEROUS. IT CAN KILL ANYBODY, IF TOUCHED ANYWHERE ON THE CIRCUIT IN POWERED CONDITION. OBSERVE APPROPRIATE PRECAUTIONS TO AVOID ANY MISHAP.