Controlling AC voltage systems.

Discussion in 'General Electronics Chat' started by Rbeckett, Jul 4, 2012.

  1. Rbeckett

    Thread Starter Member

    Sep 3, 2010
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    I am looking for a simpl and safe way to drop the incoming household 115 VAC 60 Hz to 100 volts AC 60 hertz. I need to drop this voltage to energize the 100 VAC coils on a device I am building. I have already built a 12 volt and 5 volt regulator set up with transformers, but have no clue how to drop AC volts. The coils do not pull large amps but are labeled for 100VAC not 115 VAC household current. Will a pot or resistor network do this safely? Or will I need an isolation transformer to do this also. I have 6 solenoids that will be operated by 6 SSR's from a digital 5 volt logic circuit. Any help or guidance to do thiis safely and within the forum rules? TIA for the help.
    Wheelchair Bob
     
  2. #12

    Expert

    Nov 30, 2010
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    Individual resistors for each solenoid would work. The last time I did this, I used a transformer with the secondary in series with the line voltage to drop the volts. A 115 to 12 volt transformer would get you about right.
     
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  3. Wendy

    Moderator

    Mar 24, 2008
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    As long as you aren't using this AC to power DC there is no problem. Basically you are using a 12V transformer to drop the voltage using the transfomers own generated voltage out of phase.

    [​IMG]

    Get the polarity wrong on the output the voltage will be boosted by 12VAC (127VAC). The transformer much be rated for the current of the solenoid or more.
     
    Last edited: Jul 4, 2012
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  4. Rbeckett

    Thread Starter Member

    Sep 3, 2010
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    Thank you fellas. I knew there had to be a safe and simple way to do it, but it is better to ask than mess with line voltage in the blind. Safety always!!! Bill, I had planned to take the 5 and 12 VDC off first, the drop the 115 to 100 and call it done. The SSR's dont mind but the air control valve solenooids are definately 100 VAC coils. So I may just add another transformer and got that way. It is probably safer with a trans former than without.
    Bob
     
    Last edited: Jul 4, 2012
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Just wondering here, Doesn't this make the 12V transformer into a form of 'autotransformer'? Or is it more like a magnetic amplifier? Never seen this circuit before.

    Wouldn't this circuit violate the TOS?:)
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    My first thought would be to get a small Variac with a power rating that can handle your power demand, but they are not cheap.
    Might be a lot cheaper to find a step-down transformer.
     
  7. #12

    Expert

    Nov 30, 2010
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    @shortbus

    There is a difference between an auto-transformer and this one. This circuit uses the secondary as a buck winding.

    With an auto transformer, all the windings are in phase and you tap off somewhere besides the end connections. In a buck configuration, the line voltage is run through a secondary winding that is out of phase.

    Because a moderator answered the question, I have to believe it is not in violation of the TOS.
     
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  8. Wendy

    Moderator

    Mar 24, 2008
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    The ToS is concerned with making DC power supplies direct from mains with no isolation, such as a transformer provides.

    Line circuit will be with us always. If you have a component that is power from AC then it is not a power supply circuit. SCRs are for switching in this case.

    That does not mean that the DC circuit controlling the SCRs doesn't need isolation, that is a different kettle of fish.
     
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