Power supply

Discussion in 'General Electronics Chat' started by Prophet Bill$, Dec 2, 2008.

  1. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
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    [​IMG]


    Parts:
    P1____________500R Linear Potentiometer
    P2_____________10K Log. Potentiometer
    R1,R2___________2K2 1/2W Resistors
    R3____________330R 1/4W Resistor
    R4____________150R 1/4W Resistor
    R5______________1R 5W Resistor
    C1___________3300΅F 35V Electrolytic Capacitor (see Notes)
    C2______________1΅F 63V Polyester Capacitor
    D1,D2________1N5402 200V 3A Diodes
    D3_____________5mm. Red LED
    Q1____________BC182 50V 100mA NPN Transistor
    Q2____________BD139 80V 1.5A NPN Transistor
    Q3____________BC212 50V 100mA PNP Transistor
    Q4 __________2N3055 60V 15A NPN Transistor
    T1_____________220V Primary, 36V Center-tapped Secondary 50VA Mains [COLOR=blue! important][COLOR=blue! important]transformer[/color][/color] (see Notes)
    PL1____________Male Mains plug
    SW1____________SPST Mains switch​

    HEY TECH BUDDIES
    I AM A TECH STUDENT AT THE COLLEGE OF THE BAHAMAS AND I AM BUILDING THIS VARIABLE DC SUPPLY BUT MY PRESENTATION IS IN TWO DAYS AN I NEED TO KNO THE PURPOSE AND ROLE OF EACH OF THE COMPONENTS.
    PLEASE HELP A.S.A.P.
    THANKS IN ADVANCE
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You posted a copy of the project from Electronics-Lab that has errors. The original correct project is here: http://www.redcircuits.com/Page36.htm .

    It is a very simple circuit using only 4 transistors. You need to learn about how transistors work in order to understand the function of each part. Ask your teacher.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    In simple words:

    Q4 is the transistor which controls how much current has to flow as to keep the output voltage constant. Think of it as an electrically controlled variable resistor (actually is not like that).

    Q3 senses the output current and limits it to the limit you set with P1 (pot).

    Q1 senses the output voltage as to be constant at the value set by P2 (pot). If the output voltage varies due to changes on the load or input voltage from the transformer, then it gives the appropriate signal to Q2 and then Q2 controls Q1 as to keep the output voltage constant.
     
  4. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
    22
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    THE CAPACITOR ACTS AS A SMOOTHER TO SMOOTHE OUT THE RIPPLE EFFECT RIGHT, SEEING THAT IT IS VERY HIGH (1UF) OH! AND HOW DO I CALCULATE THE RIPPLE FACTOR/EFFECT? ANDWHY IS R5 SO LARGE?..ALSO I CHANGED C2 TO 1uF 100V AND T1 TO 120V PRIM. 12 CENTER TAPPED AND 25VA WOULD EFFECT WOULD THIS HAVE ON THE CIRCUIT AS A WHOLE?
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    ripple factor=Vrms(ripple)/Vdc(across the capacitor)
     
  6. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
    22
    0
    I CHANGED C2 TO 1uF 100V AND T1 TO 120V PRIM. 12 CENTER TAPPED AND 25VA WOULD EFFECT WOULD THIS HAVE ON THE CIRCUIT AS A WHOLE?
     
  7. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
    22
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    ANDWHY IS R5 SO LARGE?..
     
  8. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
    22
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    WHY IS P1 A LINEAR POT AND NOT LOG.?
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    C2 is 1uf in the project. Its voltage rating does not affect how it works.

    The original project has a 36V center-tapped transformer that makes an unregulated +24.5V. Your 12V center-tapped transformer will make an unregulated +7.5V.
    The original project has a max output of almost 24V. Yours will have a max output of almost 7V.

    It is rated at 5W. The max output current of the original project is 2A. the power dissipated by R is (2A squared times 1 ohm)= 4W.
     
  10. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The larger this resistor is the easier the current sensing becomes. And one ohm is not very large.
     
  11. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    By large you mean size or value for R5? :)
     
  12. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
    22
    0
    I didnt find a 500ohm linear potentiometer but i have managed to find a 1k linear potentiometer. What would be the effect of this to my circuit?
     
  13. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
    22
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    By the size i mean value.
     
  14. mik3

    Senior Member

    Feb 4, 2008
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    If you use the 1K pot, Q3 may not be able to divert all the current needed as to keep the current limited to the set value. I haven't analyzed the circuit to know exactly but to b sure use the 1K pot and put a 500R fixed resistor between pins 1-2 and another 500R fixed resistor between pins 1-3.

    where

    pin 1=wiper pin
    pin 2 and 3=the other two pins
     
    Last edited: Dec 2, 2008
  15. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
    22
    0
    why is it that the LED remains on for a bit even after i turn off the power switch?
     
  16. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
    22
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    which of these pins is the wiper pin MIK3? top middle or bottom? and what would be the effect of inserting these resistors?[​IMG]
     
  17. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Because the filter capacitor can power the led for a short time.
     
  18. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Usually is the middle pin.
    If you connect the resistors I told you, the 1K pot will appear in the circuit as a 500 R pot.
     
  19. Prophet Bill$

    Thread Starter Member

    Dec 2, 2008
    22
    0
    MIK3 I CHANGED C2 TO 1uF 100V, WHAT EFFECT WOULD THIS HAV ON THE CIRCUIT AND WHAT IS THE PURPOSE OF THIS CAPACITOR
     
  20. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    It won't have any effect on the circuit. The purpose of this capacitor is to provide power to the circuit if it causes a rapid change in current and the transistors can't respond such fast.
     
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