capacitive divider

Discussion in 'General Electronics Chat' started by lokeycmos, Jul 13, 2011.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    i have a transformer that has a higher output than what i need. using a variac i determined that i need 89 volts into the primary to get what i need out. i need some way to drop from 120 to 89. dont want a resistor, it would probly burn up. i was reading that caps can be used as an ac voltage divider. could someone please critique my circuit and offer some advice and how to calculate the values i need? thank you!
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    A capacitor or resistor will cause the output voltage to change when the load current changes.

    If you use a proper transformer then the output voltage will barely change when the load current changes.

    You could add a second transformer (a 31V one) in series with the original transformer but connected so that it reduces the voltage by 31V.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    300
    It is possible to drop voltage with a capacitor, but this is fraught with difficulties. Amongst other things, with an impedance in series with the primary, the output voltage from the transformer will depend on the secondary load current.

    You will also need to know what your primary inductance is and select the capacitance value with care. If you make the capacitor too big it could even resonate with the primary inductance, with potentially disastrous results - increased voltage off load and a burnt transformer.

    Next, the capacitor would have to be of a type which is safe for continuous AC service - don't use a polarised electrolytic. Finally, a parallel discharge resistor would be required to avoid the thing being a shock hazard even when disconnected from the mains.

    All in all, you would probably be better advised not to do it this way.
     
  4. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    could you please elaborate?
     
  5. #12

    Expert

    Nov 30, 2010
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    I have used the AG method. It works!

    As an air conditioner repairman, I have access to lots of free, used, 24 volt transformers with 120v, 208v, and 230v inputs. Wiring the 125 volts that I get from the local utility to the 208v or 230v inputs gives me 14.4V and 12.5V buck voltages. As long as the secondary current does not exceede the original current rating, it's good to go.

    I'd never try wiring my available 250 VAC to a 208 or 120v input because that would exceede the ratings, the allowable magnetic field, and fry the transformer.

    edit: Good timing. I elaborated within one minute of your request!
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    A series resistor or series capacitor causes the output voltage to change when the load current changes. You probably don't want that.

    You can connect a second transformer in series with the original transformer. Connected in phase then the voltages add. Connected out-of-phase then the voltages subtract. The output voltage will change only a small amount when the load current changes.

    Why not buy a proper transformer?
     
  7. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    will connecting them out of phase have any adverse effects? im not very wealthy but i have a s**t ton of parts laying around. i try to get by with what i have.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Transformers in series simply add or subtract their voltages. No problem if you don't overload them.
     
  9. #12

    Expert

    Nov 30, 2010
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    AG is talking about the current limitations of the transformers. The buck transformer must be able to carry the current you are using.
     
  10. billbehen

    Active Member

    May 10, 2006
    39
    1
    If you do use a cap in the input side, make sure it's bipolar for safety's sake!
     
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