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decreasing voltage to regulators.

Discussion in 'Electronic Basics' started by mark krawczuk, Oct 10, 2004.

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  1. -- HI, i am in the process of building up a power supply using lm 7805
    regs
    etc.....
    but the voltage before its had a capacitor across it , staright of the
    rectifier is about 37 volts dc, which is too much for the reg, plus with the
    cap across it would increase it abit.

    is there a simple way to reduce the voltage to the regualtor ?
    thanks, mark k



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  2. Indeed there is. Use a transformer with a lower output voltage.
    All you really need is a 12V center-tapped secondary rated for two or so
    amps.

    73 de KC7GR

    --
    Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
    (Known to some as Bruce Lane, ARS KC7GR,
    kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
    "If Salvador Dali had owned a computer, would it have been equipped
    with surreal ports?"
     
  3. Byron A Jeff

    Byron A Jeff Guest

    -
    -
    --- HI, i am in the process of building up a power supply using lm 7805
    -regs
    -etc.....
    -but the voltage before its had a capacitor across it , staright of the
    -rectifier is about 37 volts dc, which is too much for the reg, plus with the
    -cap across it would increase it abit.
    -
    -is there a simple way to reduce the voltage to the regualtor ?

    Almost all have some consquence related to heat dissapation
    associated with them. That 22 volts at whatever current draw
    you're pulling has to go somewhere.

    This is one of those times where you really need a switching
    regulator. Since almost all of the power goes to the load the
    heat generated is greatly reduced.

    A LM2574 (or 2594 or similar) operates a extremely high
    efficiency up to 37V of input.

    But a couple of high wattage resistors can make for an
    effective voltage divider if necessary.

    BAJ
     
  4. Sure there are ways, but why start with a difficulty? What is the
    source of the 37 volts DC? It might be possible to rewire it to lower
    its output.
     
  5. cpemma

    cpemma Guest

    Admittedly the easiest & cheapest solutions waste power, but if your maximum
    current requirements are fairly modest, knock the voltage down with a
    resistor + zener diode pre-regulator first. You can series 2 or 3 zeners to
    increase the voltage, spread the heat. Leave at least 3V headroom.
     
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