How to ged rid of the transformer of power supply

Discussion in 'The Projects Forum' started by djb, May 11, 2011.

Thread Status:
Not open for further replies.
  1. djb

    Thread Starter Member

    May 17, 2008
    Hello to everyone!

    My problem is how to step down the voltage to about 10-20V from the mains (250V AC) without transformer. I saw that flyback converter can do the job and provide isolation from the grid AC voltage, with a switching transistor , a flyback transformer and a timer to drive the power transistor. (very generally)

    I bought the full wave bridge 400V 2A and electrolytic capacitor 400V for smoothing. Now i have a high voltage DC as input like flybacks AC to DC.
    i have check the timer 555 for creating the pulses to drive the power transistor. the supply for this timer IC is one of the problems. i get some power transistors with heatsinks for the application and now i'm facing the following problems:
    a) what kind of transformer i have to use in this kind of application and where can i find those? The number of turns primary/secondary affects the output voltage? How i choose all it's properties?
    b) how i have to set the 555 timer? does this affect output voltage? What is the difference of classic 555 and cmos 555? And the most important, how i can supply the 555 timer since the input voltage is MAX 18V and my source is over 350V ? This is weird for me :) i will build a power supply, and i need power to supply the timer for the power supply :confused:.

    i make some thoughts about it and i would like you to comment on. Is it ok to half rectify the AC input so i will get lower DC input? problem not solved anyway because will be about 150V DC, but maybe is usefull.
    Voltage regulators have limited input range. i can't use them.
    What about making an amplified zener regulator with a bypass transistor to get some 100mA for supplying the timer 555 from high voltage DC? is possible instead of using a linear regulator?
    How about using a resettable fuse to supply the 555 timer for the startup and then take power from the output and feed it back on 555 so it can continue to pulse (like feedback). resettable fuse will go off as it reaches a temperature.

    Notes: No regulation needed for this, no optocouplers, no feedback control. Regulation will be done with linear regulators and heatsinks. I just want to step down from the mains somehow without a transformer. 0,5-1A ,10-15V is enough. And of course all recommendations and opinions are well acceptable.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    No such mains connected power supply can be discussed here - Terms of Service
Thread Status:
Not open for further replies.