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6V to 3V without a regulator

Discussion in 'Electronic Basics' started by Jack B. Pollack, Dec 14, 2006.

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  1. I want to "wall-wart" power a 3V battery operated device (2 C cells). The
    device is decorative LED display controlled by a "blob" I think it is going
    to be difficult to find a 3V regulator and it is also probably overkill.

    What is the best way to do this and please provide a simple schematic,
    recommend supply voltage, etc for a newbee.

    Thanks
     
  2. Guest


    Go to Goodwill, and select a 3VDC wall wart. They're $1 apiece.

    Michael
     
  3. SUrely 2 C cells will last a fair amount of time. It's the smaller AA and
    AAA that don't,, or for that matter the gadgets using button cells (which
    have the further disadvantage of being costly to replace). If life
    is really an issue, run it off two external D cells.

    Or, put 4 or 5 silicon diodes in series, and with the cathodes facing
    towards ground. Then figure a dropping resistor to limit current. It
    will be sort of like a zener diode, and the about .7v voltage drop on
    the diodes will add up to 3V. Or just use the didoes in series between
    the adaptor and the display.

    Michael
     
  4. Thanks
     
  5. ehsjr

    ehsjr Guest

    For the future, when you *do* want a 3V
    regulator, here's a couple that are easy:

    ---- D1 D2 D3
    +6 ---+---|7805|---+--->|--->|--->|--- Vout ~2.9V
    | ---- |
    [.33uf] | [.1uf]
    | | |
    Gnd --+------+-----+----------------- Gnd


    For more precision:

    -----
    +6 ---+---|LM317|---+------+------ Vout 3V
    | ----- | |
    | Adj [240R] |
    | | | |
    | +------+ |
    | | |+
    [.1uF] \ [1uF]
    | / 500 |
    | \<-+ |
    | / | |
    | | | |
    Gnd --+------+--+----------+------ Gnd

    You adjust the 500 ohm pot for 3 volts at Vout.
    Alternatively, you could replace the pot with
    2 resistors in series - a 330 ohm and a 6 ohm.
    Or just use a 330 ohm resistor in place of the
    500 ohm pot to get ~2.96 volts.

    Either of the above circuits can be used with
    a higher supply voltage - up to 35 volts, but
    it is best to use a lower voltage like 12 or 6.
    The higher the supply voltage, the hotter the
    7805 or 317 will get.

    Ed
     
  6. Thanks
     
  7. I don't know about overkill, but it's not hard to find them- Digi-key
    lists about 700, some between 50 cents and $1.50.
     
  8. Why don't you just use a resistor dropper? This only works if the
    current drawn is sesibly constant. If you are driving the same number
    of LED's at all times then you could use this approach. Failing that
    then you could just use a high power zener diode, and a resistor. If
    all you're driving is LED's then this should be manageable.

    You can 3.3v zener diodes quite easily, or you could use a chain of 5
    rectifiers, such that 5 * 0.6 = 3.

    Could it be easier?

    Rob.
     
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