DC converter

Discussion in 'The Projects Forum' started by Jimbo-m, Feb 18, 2009.

  1. Jimbo-m

    Thread Starter New Member

    Feb 17, 2009
    Hello there,

    With help from Bertus (thanks Bertus). I have re-designed my DC converter and now have a reasonably stable 12V at varying loads.

    I have attached the circuit diagram; any suggestions/improvements?

    I need to know where to find (or how to make) a 1mH coil.

    And how do I protect my light bulbs and fuses while the converter falls into balance?
  2. eblc1388

    Senior Member

    Nov 28, 2008
    Your control loop is open. How can the output voltage be stable at 12V with varying loads?

    Have you tried changing the lamp load from 3Ω to 5Ω and see what the output voltage becomes?

    Can you also attach the .asc file too as this will save others time and can then be imported directly into LTSpice IV. In this way, hopefully you will get more response.

    You can buy one with the rated current or make one using powered iron dust toroid or even using a ferrite EI core.
  3. thingmaker3

    Retired Moderator

    May 16, 2005
  4. Jimbo-m

    Thread Starter New Member

    Feb 17, 2009
    Right-E-O Here is the .asc file (quite small aren't they; 3.3kB)

    It isn't totally steady: At 0.5 ohm (22.5A) it is about 11.3V and at 10 ohm (1.2A) it is about 12v. That's ok for what I am using it for. With a smaller coil it fluctuates more (energy store?).

    Does it need a feed back loop? If so is there a nice easy way of making one?

    Great, thanks for the coil links.
  5. eblc1388

    Senior Member

    Nov 28, 2008
    Maybe not for bulb load, judging from the small voltage output fluctuation.

    However, please note that using 555 in astable mode, the first cycle could be twice as long as subsequent ones because it takes time to charge the uncharged capacitor from 0V to 2/3V after power ON instead of from 1/3V to 2/3V.

    If you use this time to turn ON the MOSFET, then the first turn ON period will be twice as long. This could cause a higher output voltage initially.
  6. Jimbo-m

    Thread Starter New Member

    Feb 17, 2009
    It starts at 72V then drops to <15V in 1-3ms depending on load.

    Could I use a zener diode or something like that to protect the bulbs? I will also be running a 5v reg from it as well which I would prefer not to smoke...
  7. bertus


    Apr 5, 2008

    Does the FET have a diode in it ?


    That will handle the inductive kickback.

  8. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    Sorry to resurrect a dead topic but I was searching the site for .asc files and this circuit I saw really looked like it had a problem.

    It had a high 72 Volts output for long enough to blow up the bulbs so I added a sense line and now it should work more or less. It sims at 13.8 Volts max for turn on.

    If somebody did happen to search and find this thread and circuit - it should be a little more useful.