240V split phase and transformers

Discussion in 'General Electronics Chat' started by cgriffin, Feb 23, 2012.

  1. cgriffin

    Thread Starter New Member

    Feb 23, 2012
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    If I have a 10:1 step down transformer (standard 120v to 12v doorbell xformer) can I attach it the 120+ and 120- on my split phase 240v and get 24v AC at the output.

    I ask because I need to actuate a pair of relays that require 24v ac (to turn on 240V mains) and I would rather not have to find a 5:1 transformer and connect it between a single 120V line and the ground.
     
  2. #12

    Expert

    Nov 30, 2010
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    NO. The higher voltage will saturate the core and the magic smoke will get out.

    ps, If you have any dead central air conditioners laying around, they almost always contain a 240/24V transformer.
     
  3. cgriffin

    Thread Starter New Member

    Feb 23, 2012
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    Yeah, sadly I don't need much power at all, less than an amp to click over these 2 relays.

    So would connecting a 120v transformer between one hot and the ground be safe?
     
  4. cgriffin

    Thread Starter New Member

    Feb 23, 2012
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    I am building an electric boiler and I will be extending the ground wire all the way to the boiler. Would connecting something between a hot line and ground (like the transformer) cause the ground to electrify with the differential between the 2 120v lines
     
  5. #12

    Expert

    Nov 30, 2010
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    Yes. Applying 120 VAC and a ground wire to a 120 V primary will work without smoke but you will only have 12 volts from the transformer you described.

    Your second question seems odd. A proper ground wire will always be zero volts, give or take a few millivolts, under this kind of condition.
     
  6. cgriffin

    Thread Starter New Member

    Feb 23, 2012
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    Yes, I would have to buy a new transformer, but they are easily had at 120v.

    My second question is just out of my own ignorance. I was told once that it was always best to run an independent 120v line rather than run off one leg and ground of the 3 wire 240v circuit. I don't remember the consequence however so it might just have been a code thing.

    I appreciate your assistance. I'm usually a logic level DC guy forgive me.

    edited for clarification
     
  7. #12

    Expert

    Nov 30, 2010
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    I'm thinking the context is about how to hook up wires in the load area. Very often a 240 VAC line is run as a 12-2-g cable (3 wires) and the "g" wire is intended as "bond"...not a current carrying wire. The proper way is to provide a current carrying neutral wire AND the bond wire if you have any 120 volt loads.

    HOWEVER...if you use (2) 120 volt primaries and wire them in series with each other to form a 240 volt load, you get around the bond/neutral problem without having to run a current carrying ground wire for the 120 VAC loads because there aren't any 120V loads.

    On the other hand, I have a 240 volt clothes dryer with a 3 wire cord and it has a 120 volt light bulb in it. This machine obviously uses the bond wire to carry the neutral current for the light bulb. That's a no-no as far as I know. I wonder how Sears/Kenmore got away with that kind of design.
     
    Last edited: Feb 23, 2012
  8. crutschow

    Expert

    Mar 14, 2008
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    No you should not use the groundl in a 3 wire 240V circuit for a connection to get 120V.

    As #12 noted, a way to use a 120V transfromer from 240V would be to use two (identical) 120V -12V transformers in series. You wire both the primaries in series to the 240V and the two secondaries in series to get 24V.
     
  9. #12

    Expert

    Nov 30, 2010
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    You can see that I was still figuring out what you meant between post #5 and post #7. What you need to figure out is the electricians definitions of neutral and bond. Put simply, the bond wire is a safety ground for the metal case of any machine. It never carries any current except while it is saving your life by blowing the fuse (or circuit breaker). The neutral wire carries current to ground for the 120 volt loads. That's why 12-3-g cable was invented...to have both a bond wire and a current ground for the 120V loads.
     
  10. cgriffin

    Thread Starter New Member

    Feb 23, 2012
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    Ok I have acquired a 4 wire 240 12,500W cable for my control box (I will install a corresponding wall outlet for it later. Am I correct in stating Red and Black are hots, Green is bond and white is the ground I can us to get 120V off one of the hots?
     
  11. cgriffin

    Thread Starter New Member

    Feb 23, 2012
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    other way around erm...


    white bond
    green ground
    red hot
    black hot
     
  12. #12

    Expert

    Nov 30, 2010
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    Green or naked is always case ground, or "bond".
    White is current carrying neutral.
    Black and red are hots.

    What you said in post #10.
     
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