No announcement yet.

Something wrong, help need! : making -8V and 5V PS,

  • Filter
  • Time
  • Show
Clear All
new posts

  • Something wrong, help need! : making -8V and 5V PS,

    Something wrong, help need! : making -8V and 5V PS,

    Hello guys..
    i am now making this 12VDC input, 5V and -8V output PS. (only building power suply, not


    but, mine doesn't produce -8V.
    but it produce -1V.. please, check this and tell me weather i got wrong components or wrong


    here is what i got and what it says on the surface. Mostly i got from and including their part number and link

    No. Fron schematic Actual component shows(on face shows) Part No.
    1. 555 timer IC, XRL555 (L555CP 9309) 34930
    2. 7805 5V reg IC, 7805T (L7805CV) 51262
    3. 220 uF cap CAP, Radial, 220uf 25V, 105C 158262
    4. 1K resistor Resistor, 1/4W 5%, 1.0K Ohm
    5. 3.3K resistor Resistor, 1/4W 5%, 3.3K Ohm
    6. Diode 1N4002 Diode, Sil Rec, 1N4002, 1A, 76961

    And, following are from local electronic shop.
    7. 7908 -8 Reg (7908 UC8352)
    8. 0.047 uF cap (A E 473)

    Let's assume that i wired all correctly.. (that's why it produce -1V rirhg?)

    here is some clue that i don't know.

    1. for diod, it looks like this, ====||
    and i matchiwired them as this,
    let me know whether i did opposite way or not

    2. Every compnent seems correct but 0.047uf Cap seems wrong or, maybe right..
    If it says A E 473, then is that right?
    also, schematic shows |( <----- like that, but it seems there is no + or - on it..

    3. Probably my timer is wrong???

    Please check the this and advise me why i am getting -1V not -8V..

    thank you
    <A HREF="" TARGET=_blank></A>

  • #2
    Moved to "Power Supplies".
    Player: Pentium 166MMX, Amptron 598LMR MB w/onboard Sound, Video, LAN, 10.2 Gig Fujitsu Laptop HD, Arise 865 DC-DC Converter, Lexan Case, Custom Software w/Voice Interface, MS Access Based Playlists
    Car: 1986 Mazda RX-7 Turbo (highly modded), 1978 RX-7 Beater (Dead, parting out), 2001 Honda Insight
    "If one more body-kitted, cut-spring-lowered, farty-exhausted Civic revs on me at an intersection, I swear I'm going to get out of my car and cram their ridiculous double-decker aluminium wing firmly up their rump."


    • #3
      The 555 timer circuits I've played with will give you a negative voltage 1V different than your input voltage. For example if you supply 5V to the 555 timer your can get a MAXIMUM of -4V out. Also this type of "power supply" cannot supply a lot of current. If there is too much of a current draw your voltage will start to drop (I'm guessing this what is happening) Sorry. Try using a combination of MAX765 (voltage inverter from Maxium) and a 7805 and a 7908. The 7805 will give you your +5V you're looking for and use it to drive the input of the MAX765 and then take the -12V generated from it and run it through the 7908 to get a regulated -8V signal.

      Just a thought.

      j's Web Site
      [######----] 60% complete (1st try)


      • #4
        Ok.. thank you .

        i was testing the circuit with
        500mA outbout AC-DC PS..

        I checked the input of pin #2 of 7908
        it shows about -9V..
        it seems what i want to, right?

        and 7908 should output -8V using -9V input?

        i assume that 500mA is not very high

        and seems 7908 is dead.

        what do you guys think?
        <A HREF="" TARGET=_blank></A>


        • #5
          have u noticed that 79** has different pinorder than 78** ?
          It is heating up wery fast it's broke.
          Project almost ready


          • #6
            yeah 78 series pin1=input and pin2=common; 79 series pin1=common and pin2=input. If you 'tick-tock' (reverse) the wires the regulator emits a loud 'pop' and a section of the case flies away at a considerable velocity!!!!
            Carputer currently 'ghettoed' into car!!!

            EPIA MII-12000
            Ampie Case with rigged extra USB
            Hitachi 60GB
            Holux GPS mouse with iGuidance 2.0
            Lilliput 7" touchscreen
            Netgear WG121 USB WiFi
            Carnetix P1260