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Wiring a rectifier

  1. Oct 5, 2008 #1

    DaveC426913

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    I'm wiring my aquariumlet up for light. It's one of those standard 3x20W puck light sets, but I only need one light. I want to wire up a cooling fan using one of the other light terminals.

    The light set has a transformer that outputs 12V 0.5A, and each puck is 20W max.

    My CPU cooling fan wants 12VDC 0.08A.

    Presumably, the light is AC, so I've got me a bridge rectifier. I'm not sure I got the right one, it was what the guy behind the counter recommended based on my needs.

    It says:

    GI
    W02G
    644M

    Even if it's not perfect, he says I likely won't burn it out.

    He also said though, that it will likely put out something closer to 14V.

    So: do I need anythnig else for this circuit? A resistor?
     
  2. jcsd
  3. Oct 5, 2008 #2
    If you are connecting a CPU fan then that's all you need. I assume the rectifier has four pins. Two pins are for AC input, and plus-minus pins for the rectified output. To stabilize the voltage coming out of the rectifier it is customary to put a capacitor across the output. But for a motor that's not necessary.
     
  4. Oct 5, 2008 #3

    DaveC426913

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    Yeah, I wasn't sure about that. One pin is labelled +. If I call that North, then pins E and W seem to be labelled with a vertical ~. And the south pin is not labelled. The North-West edge of the otherwise-round rectifier is flat (like a transisitor).

    Oh. I thought maybe the rectifier included the capacitor. Cool. I'll need to figure out what size cap.
     
  5. Oct 5, 2008 #4

    Redbelly98

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    You don't need the cap for a motor. And if the transformer output is 12-14 Vac, then a cap will get you 16-19 Vdc. You're better off without the cap for running a 12Vdc motor.
     
  6. Oct 6, 2008 #5

    DaveC426913

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    OK.

    What about input vs output wattage? Do I need a resistor in there?
     
  7. Oct 6, 2008 #6

    Redbelly98

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    Hi Dave,

    Since the failure mode for a motor would be overheating, as opposed to electrical arcing, I think that rectified 12Vrms should be safe for a 12Vdc rated motor.

    If it turns out to be 14 Vrms you may be pushing things and a series resistor would drop that voltage down. Suggest you measure actual motor current, then choose appropriate resistor for a 2V drop (or whatever drop you need). 1/2W power rating will suffice for up to about 5V drop, for 0.08 A current.

    Disclaimer: I haven't actually run a DC motor on rectified AC myself. Others, feel free to chime in here (agree or disagree with my reasoning in 1st paragraph?)

    Regards,

    Mark
     
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