Servo Power

Discussion in 'Homework Help' started by NCH, Dec 5, 2013.

  1. NCH

    Thread Starter New Member

    Feb 28, 2013
    12
    0
    Hey gang!
    Im trying to find a smart way to cut of the power to a Micro Servo(HS-55)
    by using some good transistor or maybe mosfet technology.
    Any ideas? I have had a hard time using mosfet and BJT's as switches to work.

    I want 5V to the microservo to power it and i want to controll the supply to it with a controlsignal from a PIC.

    Im trying to save the power that leaks thru the servo when its being powered by 5V for a project im doing.

    So i haven't had any luck connecting it since it doesnt connect like a normal "load" with a pin in and a pin out. It just has 3 connectors(Signal,power,gnd).

    Any tips are greatly appreciated! :D
     
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,790
    828
    Most RC servos require power at all times to maintain their position.
     
  3. NCH

    Thread Starter New Member

    Feb 28, 2013
    12
    0
    Ah okey i didn't know that! But iam using it for only 2 positions, so.

    1. Power the servo
    2.Use it.
    3.Unpower, and if it moves back to its original position its ok.

    Its made for at battery application so i want to save as much power as possible.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I've done this. A PNP transistor or PFET can be used to cut the +5v power to the servo.

    Regarding the losing position, it depends if there is a load pulling on the servo (like gravity etc). If there is not much load, you can fix it in either of two positions using a circle (cam) with two detent positions or two small magnets.

    It's a very common situation in things like electronic locks.
     
  5. NCH

    Thread Starter New Member

    Feb 28, 2013
    12
    0
    Well i tried the PNP transistor with a normal load and it seems to work fine(4.8v)
    When i try with my servo it puts Vce to around 2 volts which i think is well over saturation so im having a bit of problems getting the right voltage to the servo.
    Its only around 2 volts at the servo when its plugged in. Im not really sure why the difference is so big. Anyone?

    [​IMG]

    So this is a picture of my circuit. for R1=4.7k
    R2=100k

    And as the load i've connected the servo.
    Note that my voltage is 5 V instead of 12 V.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,767
    1,102
    The transistor probably isn't supplying enough current for the load. Try reducing R1 to 1k, or even 470 Ohms.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    As well as that, I would use a better transistor, like a BC327 which will only drop about 0.1v when turned properly on.
     
  8. NCH

    Thread Starter New Member

    Feb 28, 2013
    12
    0
    Well i try reducing R1 with a potentiometer to as low as 50Ω without any luck. Maybe i should try a different transistor i guess.:confused:
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,767
    1,102
    What is the peak current your servo draws?
     
  10. NCH

    Thread Starter New Member

    Feb 28, 2013
    12
    0
    I'm using a Micro Servo SG90. I've gotten the right amount of voltage over the servo now, but when i try to use the servo it drops to 2 V over the servo again. Thinking about trying a p-Mosfet since im having such trouble with this configuration.
     
  11. NCH

    Thread Starter New Member

    Feb 28, 2013
    12
    0
    the transistor im using is a BC557C. I've heard the servo draws around 75-150 mAh.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,767
    1,102
    Then the BJT base resistor needs to be ~330 Ohms.
    Any MOSFET you get needs to be a 'logic level' type so that it can work with a 5V supply and be switched on fully by the PIC.
     
  13. NCH

    Thread Starter New Member

    Feb 28, 2013
    12
    0
    Yeah i've found some p-Mosfet of the logic level types, im not really sure how to draw the line "low" from the PIC though. Or do i have to keep it as a Pull-up from the PIC at all times and then just draw it low as i want to power the Servo?
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,767
    1,102
    For a p-FET, yes.
     
  15. NCH

    Thread Starter New Member

    Feb 28, 2013
    12
    0
    but if i use a pnp-bjt i also have to gnd to activate the servo? so i have to use a pull-up from the PIC either way right?
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,767
    1,102
    Yes.
    The PIC will need to pull the pnp base high to turn the pnp off.
     
  17. NCH

    Thread Starter New Member

    Feb 28, 2013
    12
    0
    Okey i'll give this a try and hopefully it will work! Thanks alot for the info, i really appreciate it and its great to learn something new!
    :)
     
  18. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    The stall current of those RC microservo's depends on the applied DC voltage (power supply).

    On a 5v supply it will be about 150-200mA max.

    That is a worst case situation, a normal load would be much less torque than a stall situation and they have enough gearing to be quite usable even on 50-100mA. For very light loads like turning a sensor they can be directly powered from a 25mA capable PIC output pin.
     
Loading...