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Potentiometer Selection

Discussion in 'General Electronics Discussion' started by Staz, Nov 9, 2011.

  1. Staz

    Staz

    3
    0
    Nov 9, 2011
    Hi All,

    I have a 14w 14v pump I want to power using a 240v ac to 12v 2A power supply. I'd like to limit the voltage using a pot so that I will be able to vary the voltage to the pump to control its speed. What spec potentiometer should I use and how is this calculated?
     
  2. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    Hi Staz.
    A potentiometer cant carry that much current, a rheostat can, but waste power, you could build a pulse width modulation control circuit, or by a pre made unit, maplins sell a unit for about £ 15.00, or build one, google low voltage DC motor control circuits.
    Dave. :)


    http://www.maplin.co.uk/panel-mounted-speed-regulator-module-30310

    This can handle 3 amps continuous.
     
  3. davenn

    davenn Moderator

    13,345
    1,774
    Sep 5, 2009
    HI Staz
    welcome to the forums :)

    ok for a start have a 240VAC to 12AC 2A transformer then you need to rectify that to DC with a bridge rectifier. Follow this with a variable voltage regulator circuit like a LM388 or a LM317
    google search the datasheets on how to use them :)

    Dave
     
  4. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    The LM317 will need a pass transistor, the OP was for 2 amps plus, the former will only carry 1.5 amps comfortably, ive tried more, they dont like it even H sink air cooled.

    The LM338K would cope easy, the LM338T cant handle much more than 25 watts, 125 C, more sturdy the LM338K and can handle the full 5 amps so long as the voltage differential is modest, i did not mention the transformer and rectification, as i thought you had a psu already and wanted to control its output, if no psu already then transformer rectification etc.

    But pwm would be better for motor pump use.
    Dave. :)
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,211
    2,695
    Jan 21, 2010
    It may be worth looking in this thread for an explanation of one reason why a variable resistor is not the device of choice.

    There are a number of reasons:

    1) it reduces torque at low speeds (when you want/need it)
    2) it's wasteful of power
    3) it simply won't work on some motors
    4) it doesn't directly control speed
    5) the power rating of a pot to do this would be much larger than you expect.

    In the thread I have pointed you to, I calculate the power rating of the pot. In that case 1800W for a 360W motor.
     
  6. Staz

    Staz

    3
    0
    Nov 9, 2011
    Thanks for the welcome and advice guys,
    The out put of the ps is 12vdc 2a so sorry for not making that clear.
    It looks for every reason mentioned that pwm is the way to go.
    I found a pwm unit rated at 12v 8a on ebay for under $5 so have bought it as I couldn't buy the components for a self build for that. I'll post a link later on.
     
  7. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    So long as the unit is rated for its use above your application that should work fine, you said up to 12 volts at 8 amps should be fine.
    Good luck with the project.
    Dave. :)
     
  8. Staz

    Staz

    3
    0
    Nov 9, 2011
    Thanks Dave and all who answered my post.:)

    This is only one component of the project. I'm building a 10 gallon capacity brewery in my garage and this is to control the pump speed to shift water and wort (unfermented beer) around the 3 x 10 gal tanks required to produce the beer. I'm working on the control panel for the brewing system which consists of two pid units with inputs from 2 temp sensors switching 2 x 3 kw elements via two ssr's. One tank maintains water at 78c using one pid while the other maintains a heat exchanger at 66c which the wort is recirculated through and back to the grain in a loop. The pump needs to pass the wort through the heat exchanger and spray it onto the grain at about 1.5 litres per min hence the need for the pump control. The last tank is to boil the wort and hops so doesn't require a pid controller.

    Cheers,
    Steve
     
  9. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    Hope your brew turns out ok, cheers
    Dave. :)
    PS, many years ago i had a dabble small scale with the stuff you can buy from the chemist boots in the uk in them days, your sounds large scale.
    Enjoy the end result.
    Dave. :)
     
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