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25v to lower voltage

Discussion in 'Electronic Basics' started by Guy Paddock, Jan 28, 2004.

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  1. Guy Paddock

    Guy Paddock Guest

    Is there a quick and easy way to drop the voltage in a circuit down from 25
    volts, to, say, 9v? I'm assuming a transformer, but I was wondering if there
    was a resistor that might handle it, or something of the sort...

    Also, if a transformer is required, where would I get one to match those

  2. JD Steffen

    JD Steffen Guest


    Might check into the LM7809 voltage regulator. I don't remeber what
    the voltage limit is for it, so you'll have to check out the data sheet. Try
    Motorola or Linear. This way you don't need a transformer. The
    regulator will output a clean 9 volts.

  3. Are you interested in 9V A/C? Transformers don't work with DC. For A/C,
    it'll be hard to find a transformer that will take 25VAC -> 9VAC.

    If its DC, using a 7809, like M. Steffen suggested, would work:

    25V .-----. 9V
    ---------|7809 |------+--------
    | | |
    | | ---
    | | --- 0.1uF
    '-----' |
    | |
    | |
    | |
    -------------+---------+-------- GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    However, the 7809 will consume 64% of the power through the circuit. Its
    also limited to some small current, say 500mA.

    There are more exotic chips used to drop DC voltage called 'switch mode
    power supplies' that are more efficient, and thus will generate less heat
    for larger power requirements. They are also more expensive.

    Bob Monsen
  4. Olaf

    Olaf Guest

    is the 25V AC or DC, do you need 9V AC or DC, what current do you need at
    9V? Without this information it will be quite impossible to give some
    advice :)

    bye, Olaf
  5. Guy Paddock

    Guy Paddock Guest

    I need 9v to power an electromagnetic coil. The current 25v makes the coil
    very powerful, but makes it heat up too much. I need it to run with less

  6. If you are not worried about loading the 25 volt supply the least
    possible amount, then just connect a resistor in series with the coil
    that will reduce its voltage drop to about 9 volts. If you measure
    the coil resistance, I can help you spec the resistor.
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