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

  1. #1
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    Arrow 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

    controller)




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

    size..


    here is what i got and what it says on the surface. Mostly i got from www.jameco.com 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
    29663
    5. 3.3K resistor Resistor, 1/4W 5%, 3.3K Ohm
    30672
    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???


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

    thank you
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  2. #2
    Retired Admin Aaron Cake's Avatar
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  3. #3
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    Post

    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
    j
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  4. #4
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    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?
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  5. #5
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    Post

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

  6. #6
    Variable Bitrate choyak's Avatar
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    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!!!!
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