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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?
Your light dimmer circuit can adjust the voltage but with NO regulation. If the AC input voltage changes or if the load current changes then the DC voltage also changes.
The light dimmer circuit can be turned off then it requires the pot to be turned up fairly high before it starts working again.