Voltage Divider Circuit Help!

Discussion in 'The Projects Forum' started by fanman1964, Sep 6, 2008.

  1. fanman1964

    Thread Starter New Member

    Sep 6, 2008
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    Need to design a simple circuit that will reduce incoming 110VAC to about 85VAC. The two voltages will be switched independently to the input of a 110VAC fan. 85VAC will run the fan at "lo speed". Will a simple resistor divider work? Any other suggestions?
    Thanks

    Fanman
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    How many watts is the fan?

    Why dont you buy a fan speed controller, like the light dimmer?
     
  3. fanman1964

    Thread Starter New Member

    Sep 6, 2008
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    fans are 35 watts each, will drive two at a time. I use light dimmer now, but would like switch selectable hi / lo speed.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    If your dimmer is working fine with the fan, why not just add a SPDT selector switch so that one way the motor gets it's power from the dimmer output, the other way it gets power from the hot input line?
     
  5. fanman1964

    Thread Starter New Member

    Sep 6, 2008
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    The desire is to turn the fan on with a wireless remote control, and also remotely select hi / lo speed. This is a design that will end up in a marketed product, not a one time setup. Looks like I'll have to use a step down transformer and a relay to select either 110 or 85 volts. I found a Tamura xfrmer that provides a secondary output of 88 volts, which should suffice. Just thought a simpler circuit would work, hence the post.
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    You can still us ethe dimmer and a relay to switch between low and high speed. If you dont want the dimmer you can use an inductor or a resistor in series with the fan to reduce the voltage across it and hence reduce the speed. A resistor may be more practical but it will dissipate lot of energy as heat. However, an inductor (good designed one) will dissipate almost no power but still will do the job.
     
  7. fanman1964

    Thread Starter New Member

    Sep 6, 2008
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    Can you recommend an inductor that would work well?
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    The inductor might be a large one, so the transformer solution might be more cost effective. Do you know the current draw of the fan, or its rated wattage? This is good to know, as it will let us calculate the size of the inductor, and/or let you purchase the right sized transformer.
     
  9. fanman1964

    Thread Starter New Member

    Sep 6, 2008
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    I will need to drive two fans on the same circuit, each rated at 35 watts full RPM
     
  10. mik3

    Senior Member

    Feb 4, 2008
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    You can use an autotransformer if the isolation does not matter, its cheaper and smaller.
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    I come up with about 170 mH as the size inductor to drop 32 volts, reducing your fan voltage to 88. It may be that my catalogs are not all that comprehensive, but the largest value I can find is 20 mH.

    The transformer may be the easy way out.
     
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