5V to 3V

Discussion in 'General Electronics Chat' started by tznuk, Dec 14, 2007.

  1. tznuk

    Thread Starter New Member

    Nov 29, 2007
    7
    0
    I am making a small elevator for my daughter and the motor I am going to use for it is a 1.5-3V motor. I am trying to figure out how i can use an old cell phone charger that puts out 5V and converting that down to 3V for the motor, that way I will not be replacing batteries all of the time. I dont know anything when it comes to electronics, so please help me with this and use lamens terms. Thanks
     
  2. eeboy

    Active Member

    Sep 27, 2007
    90
    1
    Assuming this is a small motor (minimal current draw) and it runs only in one direction you could simply place 3 diodes in series. Should drop the voltage down slightly below 3V.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Use 1N4001's. The motor may be happy if you only use 2 diodes.
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    Two diodes = 1.1+1.1 = 2.2
    5-2.2 = 2.8v

    You can also use a single zener but it's much more expensive or a fixed 3v or three terminal adjustable regulator which even more expensive. ;)
     
  5. eeboy

    Active Member

    Sep 27, 2007
    90
    1
    Curious... Where does the 1.7V come from?

    Nominal forward voltage on a silicon diode is between .6V and .7 V. Nominal forward voltage on a germanium diode is closer to .2V.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yeah, I agree with EEboy - except I think the germanium is closer to 0.3V

    Maybe you two were thinking about the drop across a fullwave bridge rectifier from end to end?

    Tznuk, another way wind up with a more efficient circuit would be to use a 555 timer to construct a PWM circuit with a base frequency around 1KHz with 60% on time, 40% off time, or 3/5 duty cycle.

    Of course, that will require more work than just using a few diodes in series to drop the voltage down - but it'll also be easy to fine-tune the speed of the motor once that's done.
     
  7. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Depends on the current. The 1N4001 at 1A has a nominal Vf of 1.1V.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    [nitpick]According to the Fairchild datasheet, 1.1V is the max at 1A. Typical is about 0.93V.[/nitpick]:D
     
  9. h.d

    Active Member

    Oct 22, 2007
    150
    0
    you can use zener diod which thresholding voltage eaqual(3V)
    parallel with the motor.
     
  10. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Why not using an LM317 for safe regulation?

    The voltage drop across a diode is quite unstable. It may range from 0.6V to 0.9V. Zener diodes are not a good option either, as they tend to waste energy and the drop voltage also varies with current (except when we are dealing with very small currents).
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Two volts is pretty iffy for headroom for an LM317, especially since we don't know the load current. There are lots of low dropout regulators that will work, though.
     
  12. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    For the LM317, the minimum is 1.5V from the input to the output. For this objective I think it will serve. Looking at the datasheet it is possible to see that for a load current of 500mA and a temperature of 25ºC, the dropout voltage will be of 1.9V. It will be the worse case.

    See the datasheet here:
    http://www.fairchildsemi.com/ds/LM/LM317.pdf
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    You're probably right. It will probably work as a one-off. However, that graph you read is not worst case, it is typical. If you look at the Electrical Characteristics table, the part is only specified for (Vin-Vout)>3V.
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Personally, I think SgtWookie has the right idea. A simple 555 PWM circuit like this will allow speed control. I would change the MOSFET to one specified for 5V logic level drive. Fairchild has dozens of them that will work, and I'm sure other mfrs do also.
     
Loading...