Question about Power Supply circuit

Discussion in 'General Electronics Chat' started by YazanJuve, Dec 18, 2009.

  1. YazanJuve

    Thread Starter New Member

    Dec 13, 2009
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    hello :)

    I was just wondering if this power supply circuit will operate correctly...if no what is missing or how should i modify it??

    thanks :)
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It's a fatal accident waiting to happen. Any responsible person will use a transformer to isolate his circuit from the line. That circuit cannot be made safe to use.

    The circuit is so limited that even adding a transformer won't improve performance. Voltage regulation is poor, and the current capability is very limited.

    What are your power requirements? A power supply is easy to design with some parameters.
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    As Beenthere says, it is dangerous to work on due to it being 'live to mains'.

    I'd estimate the maximum output at 25mA per rail or so.

    It should have a 47R resistor in series with the 1uF cap to limit inrush current if the input voltage is anything other than zero at the instant it's connected.

    If you try to use it, test using an isolating transformer!
     
  4. YazanJuve

    Thread Starter New Member

    Dec 13, 2009
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    well i got this circuit from Microchip so i though it will be working fine...what should i do to improve its performance?!? and if you got any better designs please provide me with it :) the output of this power supply should drive a 16F876A PIC so its output must be 5 V
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here's a simple one you can use with an 8v to 12v "wall wart" type plug supply:

    [​IMG]

    D1 can be any 1N400x or 1N540x diode; it's just there to protect the circuit in case the wall wart polarity is backwards.

    C1 is a filter capacitor.

    U1 is a fixed 5v regulator.

    C2 on the output is required for stability. You should also have a 0.1uF cap across the PIC's Vdd/GND terminals.
     
  6. YazanJuve

    Thread Starter New Member

    Dec 13, 2009
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    Thx SgtWookie :) but what if i want the circuit i got before to perform in a better way? what should i add/remove from it? thx for your help :)
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    It is better to throw the other circuit out completely than to try to fix it.

    It does not have a transformer, therefore it cannot be made safe.
     
  8. YazanJuve

    Thread Starter New Member

    Dec 13, 2009
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    well its from Microchip :S can it be useless? in my circuit PT is going to an LM324AD and POWER_DETECT is going to the 16F876A pin 21...so how can i use your circuit but with these two wires still going to their places in the circuit?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    If it's from Microchip, it's still dangerous.

    Try this instead:

    [​IMG]

    The transformer should have a secondary of about 9VAC.

    In the States, our power plugs have a round pin that connects to earth ground, and two flat prongs.
    Our narrower prong is the "live" or "hot" wire.
    Our wider prong is the neutral wire.

    You must use a fuse. The switch is optional.
    You may have to adjust the values of R1/R2.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    This is like the last one, with a power detect output added.

    If Power Detect starts falling or you stop getting PT outputs, better prepare to shut things down.

    [​IMG]
     
  11. YazanJuve

    Thread Starter New Member

    Dec 13, 2009
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    0
    aha :D well thx a lot really i appreciate it...lets hope everything goes right :)
     
  12. Wendy

    Moderator

    Mar 24, 2008
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  13. YazanJuve

    Thread Starter New Member

    Dec 13, 2009
    28
    0
    thxxx Bill ;)
     
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