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please help...with circuit

Discussion in 'Electronics & Electrics' started by effekt26, Nov 10, 2006.

  1. nux

    nux Member

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    250R is just over 500mW.
    Voltage output with the 80R fan load goes from 5V down to 4.134V then up to 12V.

    So not really ideal. I think using a 1W 100R pot would be a better option.
     
  2. Goth

    Goth Grumpy Member

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    Unfortunately, i don't think you can readily get a 1W pot.

    You go straight from 0.5W or less standard carbon pots up to 3W or so for wirewounds, which are expensive.
     
  3. sysKin

    sysKin Member

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    Ugh. Did you people forget that a voltage divider is a voltage diver ONLY if you're not drawing any current from it? As long as you connect a fan to the middle, this is not a voltage divider anymore.

    Problems with the circuit:
    1. It connects 12V and 5V with a resistor (10k one in original picture). As long as resistor is big, this is not a big problem (0.7mA here) but it's a waste of energy anyway.
    2. It feeds the fan VIA a resistor. If you set the pot to middle, you end up with 5k resistor to 12V and 5k resistor to 5V. These resistances are WAY to big to leave any significant voltage drop across the fan. You end up with fan voltage being close to zero. The only way to overcome this would be to have the pot small, but now look at point 1.
    3. There's no point connecting to 5V at all. If you only connect the pot to 12V, there will always be some pot value such that voltage drop across pot is 7V and voltage drop across fan is 5V, which is what you want. Just exactly what this value is is not easy to figure out (depends on the fan) but it's there.

    The original design would work ONLY if fan doesn't draw any current.

    [edit] oh my, is this my first post??? I've been here so long....

    [edit2] BTW if anyone is interested, my computer runs on several nice temperature-based controllers. I can share all the info about them. At least computer is completely silent as long as it's idle.
     
    Last edited: Nov 11, 2006
  4. Goth

    Goth Grumpy Member

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    It has already been established, theoretically and empirically, that if certain constraints are satisfied, then it is a perfectly workable method for regulating the fan voltage between 5V and 12V as desired.
     

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