help needed with smp using 555 and 2n3055

Discussion in 'The Projects Forum' started by drspastic, Jan 1, 2013.

  1. drspastic

    Thread Starter New Member

    Jan 1, 2013
    hi, im not making much progress on this so i ask for help.

    i dont have access to many parts so please go with what i have!

    i need to urgently build a laptop charger to supply 9.5 volts @2-5 Amp from 11-15 volt input.
    it must be stable through this range of input.
    i have only one 555 timer, 2x 2n3055 a bag of small npn transistors, resisters capacitors and a 741 op amp, icl7612, max660, and a few basic cmos logic chips.

    the circuit needs to be simple as possible, and tough as nails. i tried a design using a lm317t driving the 3055's in paralel but the 317 cant handle the dropout and output voltage drops on load.

    i think the best way is to use the 555as a pwm chopper but i dont know how to realise that with feedback and some kind of reference.

    a linear dropper might do, but pwm would be more efficient, my power comes from small solar panels and old car batteries and i dont want to waste a watt as heat if i can avoid it.

    so i invite suggestions of circuit diagrams to do this job with the parts on hand, bear in mind that i cant get other stuff and dont live in a place its easy to get stuff. i only have a basic test meter.

    thanks i look forward to hearing ideas.
  2. crutschow


    Mar 14, 2008
    Here's (page 104) a 555 circuit that may do what you want but it uses a MOSFET for a switch. Is possibly could be adapted to using an NPN BJT in a Darlington or Sziklai pair configuration instead but the efficiency would may suffer. You would likely want to operate at a lower switching frequency (with a larger inductor) to reduce switching losses in the BJT.
    absf likes this.
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Where are you going to get the 5A inductor you need to make the buck converter?

    You will also need a 5A diode for the free wheeling diode.

    Understand that you asked:

    If you build a converter using a 555, it won't have any current limiting. It's life will probably be short and unhappy.
  4. takao21203

    AAC Fanatic!

    Apr 28, 2012
    people see your username and many will just think WTF.