Understanding CT selection

Discussion in 'The Projects Forum' started by mjscott, May 21, 2014.

  1. mjscott

    Thread Starter New Member

    May 21, 2014
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    0
    Hello,
    I need some way to monitor when a 220VAC circuit is in use. This circuit powers an outdoor radiant heater, which according to the specs, draws 12 AMPS. The header has two states, on and off, therefore the load should be consistent when the heater is on.

    My theory is to install a torroidal CT in the circuit which powers the heater (over one of the conductors), and drive an LED from this CT. I have never worked with CTs before, and I do not know how to select the correct one. Also, I don't know if I will need any other components in this circuit (diodes, resistors, etc...).

    The purpose of this little project is to have a reminder light (LED) inside my home to alert me if the heater was accidentally left on.

    Thanks in advance of any help provided.
    Mike
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    There are cts available with led built in.

    There have been threads on that subject in the last year. Search isn't working for me right now.:(

    I've connected cts direct to led with success.
    If you can find those threads, I won't have to redo the experiment.:)
     
  3. mjscott

    Thread Starter New Member

    May 21, 2014
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    0
    Thanks for the quick reply. I also tried the 'search' functions many times before posting, with only server errors returned.

    A CT direct to LED is an ideal solution... if I knew what specs to use when purchasing the CT and LED. I'm sure there is some 'electronics math' for this, but I don't know where to start.

    A CT with built-in LED is interesting, as long as the LED is on a lead I could extend. The circuit to the heater is in my crawlspace, and I want to install the LED in my kitchen.

    Cheers,
    Mike
     
  4. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
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    Search still not working.

    Remembered this one of mine.
    http://forum.allaboutcircuits.com/showthread.php?t=91368 post 23

    Find a wall wart supply with separately wound coils.
    Leave the mains powered side to connect to led.

    Remove low voltage windings and wind a few turns of #16. #14 if there's room.

    12 amps thru your new winding will illuminate led.
     
  5. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    It sure would help if I knew what country you are in or frequency your power is on. Then I might find the part for you.

    Anyway, a current transformer puts out current. Seems obvious enough. What isn't obvious is that if it doesn't have any where for the current to go, it makes voltage...Lots of voltage. So the first rule of current transformers is: The current must always have an escape route.

    I found some that have a 500 to one ratio. 12 amps would cause 24 milliamps on the output. With the following circuit, R1 is not included, R2 is a short, and the LED gets 24 ma on each half cycle. That averages to 12 ma and the LED is happy. The extra diode shorts out the unwanted polarity and the current transformer is happy.
     
    Last edited: May 22, 2014
  6. mjscott

    Thread Starter New Member

    May 21, 2014
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    0
    Hi
    I am in Canada. I believe our AC is 60Hz.
     
  7. DickCappels

    Moderator

    Aug 21, 2008
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    632
    Just be sure the transformer's secondary is loaded on both half cycles and to protect the LED from excessive reverse voltage. One way to do both is to connect two LEDs such that the conduct on opposite phases of the power line.
     
    #12 likes this.
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    just make sure the ct is loaded at all times, it is afterall, a 1 to 500 stepup transformer, and will deliver a nasty shock across an unloaded secondary.
     
  9. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    On a good day, you might even be able to find an LED with 2 leds inside it, one for each polarity. I use the red/green LEDs to make good/bad idiot lights.
     
  10. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    I pulled the sense coil from an old GFCI and tied an LED/diode to it. Threaded one lead of feed power through the hole and WALLA! A current indicator.
     
    inwo likes this.
  11. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    I'll have to remember that one.:)
     
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