My questions are:
1- Why the 12V zener?
( MAintains constant low resistance in MOSFET by Vgs=12 but controls Vs directly from high impedance on pot. better is use MOSFET with logic level threshold (1~1.2V) using 3~5V LEDs or zener
2- If i replace the IRF740 MOSFET by any 400v/10A BJT transistor, what would happen?
note: I know how to connect the BJT.
That wont help as BJT is current control current source and FET is (nonlinear) Voltage controlled resistor. But a Triac would help before bridge to lower heatloss and temp rise. to maintain Vdc Min above Vout But then you need 2 bridges, one for low current Bias on Pot, the other either a triac bridge or same bridge with triac front end to drop average voltage just above Vout. But then you need ripple current filter for big cap. like a tungsten bulb or ICL which can handle extreme temps.
3- why C2 is only 10uF dislike C1?
Big uF caps cause surge currents inverse to ripple voltage in percent. eg 10%V=10x current, bad for caps.
4- from:
https://avecircuits.blogspot.com/2012/07/0-300v-variable-high-voltage-power.html#.UI3WD2dQaoY
I found a similar circuit:
**broken link removed**
Author said:
How R3 value depends on HFE?