damaging a potentiometer

Discussion in 'Homework Help' started by terrakota, Mar 22, 2005.

  1. terrakota

    Thread Starter Well-Known Member

    Feb 8, 2005
    67
    1
    Hi,
    I'm really new to electronics

    what I'm doing is the project to control the speed of a small DC motor that a got from a pile of junk, I connect the motor and the pot to a 6v battery but when i try to regulate the speed i burn the pot and the motor runs at full speed, and smoke starts in the pot,

    if i connect the motor to a 1.5 v battery it works fine but if a use the pot as voltage divider from a 6v to have aprox 1.5 volts the motor dont work, why is this?

    is this because the motor is not a variable speed motor?


    I'm using a 50k pot , 6v battery and small 1 inch motor, cant tell u anything else about the motor because dont have any printed clue

    thanks for any help and please excuse my poor english
     
  2. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    Its becase the pot is rated too low for the power your trying to drive through it. Most 50k pots are rated at 100mW to 500mW. Get a larger pot that has a lower resistance range. Forget up to 50k, do 10-1000 ohm, make sure it can handle up to 1 Watt. Should fix the issue. Though this is not a good method for controlling a motors speed, it will work.

    Being new, I don't want to inundate you with transistor and opamp circuit ideas just yet.
     
  3. terrakota

    Thread Starter Well-Known Member

    Feb 8, 2005
    67
    1
    :) forgot about the power rating,
    but is there a way to know a pot power rating? I cant find a way to notice that

    or all the 10-1000 ohm pot's are rated that power?

    thanks for your great help
     
  4. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Small DC motors can take quite a high current until they get up to speed. Using a series potentiometer is not the best way to control their speed because the speed will vary considerably with load.

    In any event , the usual carbon pot is quite unsuitable - they are not intended for that sort of application. If you want to continue along that route then go for a wire-wound type which will probably have a rating in excess of 2w.

    Better still, go the Pulse Width Modulation route. Do a Google for pwm-speed-control. You will get about 1000 hits with links to how to go about it
     
  5. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    The pots power rating should be posted on its specs. The specs are usually attached to the box the pot came in or on the leaflett with it. If all else fails, you go to digikey.com throw in the part number and find the particular pot and get its specs.

    If its cooking, your well over its rating.
     
  6. terrakota

    Thread Starter Well-Known Member

    Feb 8, 2005
    67
    1
    thanks for your great help ultra
     
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