Shop OBEX P1 Docs P2 Docs Learn Events
servo power question — Parallax Forums

servo power question

mparkmpark Posts: 1,305
edited 2010-08-23 18:53 in General Discussion
I have a servo plugged into my Propeller Demo Board. It works most of the time, but every once in a while the servo runs into a little more resistance than usual (it's turning a door lock) and the system reboots (I assume because the servo is drawing too much current).

Right now the servo is getting its 5V from the demo board. I put a 470uF cap between 5V and ground but it didn't help. Should I try a bigger cap? Should I use two wall warts, one for the demo board, one for the servo? I'd prefer a one-wall-wart solution, of course.

Comments

  • John R.John R. Posts: 1,376
    edited 2010-08-20 12:40
    I don't know that the bigger cap will help, unless the current draw is a very short duration. What size wall wart are you using? Anytime I have a servo involved, I tend to grab the 7.5V 1Amp version. If you're using one of the lower power (current rating) versions, you may just need to swap out a "beefier" unit.
  • Peter KG6LSEPeter KG6LSE Posts: 1,383
    edited 2010-08-21 03:49
    attachment.php?attachmentid=72647&stc=1&d=1282387308

    Looking at the Power system on the board and knowing the Prop runs on the 3.3V rail . It is very likely that the servo is browning out the 5V reg thus starving the 3.3V reg resulting in glitching .

    Looking at the Nat Semi data-sheet the LM2937 is rated at 500mA.
    And I know some large servos draw some heavy current at times .

    If you can provide a connection from the DC RAW input on the Dev Board and feed it to a LM7805 or other 1+ Amp reg ON a heatsink you should be better off .

    OR

    Use a second regulated Brick for the servo .

    Use a O-Scope on the 5V rail to see if there is a dropout and do the same on the 3.3V rail . ( a fast refresh DMM or VOM will also work ).

    Pic from the Datasheet
  • RavenkallenRavenkallen Posts: 1,057
    edited 2010-08-21 20:36
    The demo board specs says it has two amp regs, so the servo must be pulling a lot of current. You just need to limit the power that is going into the servo, using a simple 1 amp or more diode would provide a quick fix. Of course you would need a higher power supply voltage, because of the diode voltage drop.
  • Peter KG6LSEPeter KG6LSE Posts: 1,383
    edited 2010-08-21 22:58
    I saw that rating too . I think its a typo .
    The Regs they are using are printed on the Parallax board Datasheet as LM2937IMP 5.0

    http://www.national.com/mpf/LM/LM2937.html#Overview


    it is Odd considering how its a 4X over rate .

    The Regs are SOT-223 . just from looking at them I would think around 500mA .
    They are linear after all . the heat has to go somewhere .
  • mctriviamctrivia Posts: 3,772
    edited 2010-08-23 18:53
    place a diode in series with the input of the demo boards 3.3v regulator and a large cap after the diode. this will fix your problem.
Sign In or Register to comment.