how to drop 12vdc to 3.7vdc

Discussion in 'General Electronics Chat' started by born2win, Feb 17, 2009.

  1. born2win

    Thread Starter New Member

    Feb 17, 2009
    I made a bluetooth headset into a over the radio handsfree in my car(i was bored). But sorry to say i forgot how to drop voltage using resistors. can anyone help? -Thank you
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    Dropping a voltage with resistors only works when you need a reference level. To change the voltage when providing power, use a voltage regulator instead. :)
  3. Dan2


    Oct 16, 2008
    variable regulator would work if u want 3.7v
  4. born2win

    Thread Starter New Member

    Feb 17, 2009
    Can i use zener diodes?
  5. mik3

    Senior Member

    Feb 4, 2008
    If the voltage across the device can vary within some limits you can. I ma saying that because a simple zener diode regulator does not provide a really constant output voltage independed of the load current. In reality the voltage across the zener varies with load current.

    A bluetooth device won't need much current thus you could use a zener. However, the better solution will be an adjustable voltage regulator like the LM317.
  6. Salgat

    Active Member

    Dec 23, 2006
    Zeners are the cheapest and easiest to use, you simply put a resistor in series and a zener in parallel with the load.
  7. Mike33

    AAC Fanatic!

    Feb 4, 2005
    Get an LM317. 2 caps and 2 resistors will set you up with the proper voltage. You might need to heat sink the thing, though - dropping over 8V can cause some power dissipation! The data sheet for the 317 can be found online, and has the basic circuit for doing this in the application data section.
  8. SgtWookie


    Jul 17, 2007
    I suggest National Semiconductor's datasheet for the LM117/LM317; it gives a good bit of information besides lots of applications.

    I recommend using a 120 Ohm resistor from the OUT terminal to the ADJ terminal (R1); this guarantees a minimum 10mA output current, which will ensure that you will get good output regulation.

    R2 will need to be somewhere between 189 and 195 Ohms (ADJ to GND) in order to get an output of 3.7v.