potentiometer efficiency

Discussion in 'General Electronics Chat' started by admin, May 12, 2010.

  1. admin

    Thread Starter Member

    May 5, 2010
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    Roughly how much energy is wasted in a 24vdc pot controlling 2 0.1A fans rated at 24vdc, Power supplys output is 24vdc 1.5A also powering a 24vdc 700ma buck driver
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    The lower the rpm the more energy is wasted provided that the pot can withstand the power dissipation which is highly unlikely for standard pots
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You should be using PWM to control the fan speed.
     
  4. admin

    Thread Starter Member

    May 5, 2010
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    i see you always recommend a pwm, but how much more efficent is a pwm? how much is it going to cost? fan operates at 14-26 vdc, input power 2.40 watt
     
    Last edited: May 13, 2010
  5. Norfindel

    Active Member

    Mar 6, 2008
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    If you want to go that way, you need at least an emitter follower to buffer the potentiometer. The disadvantage, is that any voltage not dropping on the fan is going to drop on the transistor.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I don't always recommend it. However in your particular application, PWM just makes good sense.

    If you simply use a pot/rheostat (or some form of linear regulator) to limit the current flow through the fan, the pot/rheostat/linear regulator will dissipate power as heat.

    With PWM, the connection to power is either ON or OFF, so there is comparatively little power loss in the circuit.
    [eta]
    Cost is minimal; a wide-range PWM circuit can be built from just a few components; a 555 timer, a timing cap, a pot, a resistor, a couple of diodes and a power MOSFET. Since your supply voltage exceeds the limit of a 555 timer, you would also need a voltage regulator and associated caps for the input and output.
     
    Last edited: May 13, 2010
  7. admin

    Thread Starter Member

    May 5, 2010
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    got a diagram and parts list? fans are 2 24vdc brushless .15 amp
     
  8. R!f@@

    AAC Fanatic!

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    Search the forum. AAC has plenty
     
  9. admin

    Thread Starter Member

    May 5, 2010
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    got any searching tips?
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    for god sake use the search button or click all the link on top of this page
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    See the attached.

    Sure, using a pot and a voltage follower is initially much cheaper to build and quite a bit easier. However, you'll pay for the inefficiency in energy costs and poor reliability.
     
  12. admin

    Thread Starter Member

    May 5, 2010
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    thanks, never made a circuit before this will be my first

    is the lm317 going need to be heatsinked?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    An LM317 almost always needs to be heat-sinked.

    However, not in this case. You might get a 15°C rise internally, but the power dissipation is only about 240mW after the start-up.
     
  14. admin

    Thread Starter Member

    May 5, 2010
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    do you know where i can order all these parts in one place?
     
  15. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you want to order parts, it is good to know where you are.

    You could take a look at digikey or mouser.

    You can edit the profile and set the location.
    It can be found under User CP (at the left almost top).
    Then "Edit Your Details", in the Additional Information paragraph.

    Bertus
     
    Last edited: May 16, 2010
  16. admin

    Thread Starter Member

    May 5, 2010
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    im in nebraska, usa
     
  17. admin

    Thread Starter Member

    May 5, 2010
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    hey sarge, do you have a partlists for that circuit?
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    --- Bill of Materials ---

    Ref. Mfg. Part No. Description
    C1 -- -- capacitor, 1nF
    C2 -- -- capacitor, 100nF
    C3 -- -- capacitor, 100nF
    C4 -- -- capacitor, 100nF
    C5 -- -- capacitor, 10µF
    D1 -- 1N914 diode
    D2 -- MURS320 diode
    FanMot -- -- inductor, 1mH
    M1 -- IRF510 MOSFET
    R1 -- -- resistor, 1.2K
    R2 Linear Technology Rtot=100K wiper=.5 integrated circuit
    R3 -- -- resistor, 47
    R4 -- -- resistor, 1K
    R5 -- -- resistor, 120
    R6 -- -- resistor, 10K
    R7 -- -- resistor, 47, 2W
    U1 Linear Technology LM555 integrated circuit
    U2 Linear Technology LM317 integrated circuit
     
  19. admin

    Thread Starter Member

    May 5, 2010
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  20. SgtWookie

    Expert

    Jul 17, 2007
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    Have you considered basket weaving for a hobby?

    Perhaps you should.
     
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