Load vs. Short Circuit

Discussion in 'General Electronics Chat' started by Farlander, Dec 23, 2010.

  1. Farlander

    Thread Starter Active Member

    Oct 14, 2008
    158
    0
    Why is it that a short circuit draws infinite current while a load only draws what it needs?


    Situation: When I connect the primary of a 120V/220V AC transformer to mains AC 120V, my 20amp circuit breaker blows immediately. If I put a load in series with the primary, i.e. a light bulb, will it draw only the amperage needed to power the bulb?

    Thank you
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    Are you sure about what you are telling us?
    If you are tripping a 20a breaker you are doing something wrong and dangerous.
    Playing with 120v mains power is potentially lethal.
    Are you sure the transformer is what you claim? Is it a 120/240 primary with some other voltage secondary?
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Let us know some more specifics about the transformer and the load it is supplying.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,766
    928
    There is a small surge drawn by transformers until the magnetic field builds and self induction limits the current. If its a standard transformer it should not trip the input circuit breakers, unless perhaps the secondary is shorted. This will make it draw a massive surge current at initial power up. You should check out the transformer with a good meter.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    A light bulb acts as a resistor, it limits the current that can flow through it.

    Transformers provide inductance, a form of resistance to AC, but if the inductance isn't high enough, it acts like a short circuit and draws as much current as possible.

    There is also "inrush" for very large transformers and motors, where they appear to be a near short circuit at startup, but these are typically very large and not common items for most people.
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,740
    759
    OP Is crazy.....u guys are telling someone who dunno how a transformer works to connect it to Mains....

    Inrush or no inrush...OP doesn't even understand ohm's Law....

    Nice one guys..go ahead and tell him to connect the transformer again.

    OP...stop what you are doing before u kill ur self
     
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,766
    928
    Where exactly in those short responses did anyone tell the OP to connect the transformer up again?

    He was told its dangerous, he was told it shouldn't do what he says it did. He was told to check it with a meter. Yet on the third re-read, I still don't see where we are encouraging him to re-energize the thing.
     
  8. Farlander

    Thread Starter Active Member

    Oct 14, 2008
    158
    0
    Thanks for the replies I've gained valuable insight. Most likely it is the type of transformer I am using, it is from a UPS and is not made with wire, but with wide copper straps looped around a solid core. I suspect there is not enough inductance to limit the AC current, or perhaps since it is so heavy and large (about 5"x5") the inrush current is too great.
    Thanks!
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,740
    759
    Just checking who is more concerned :D
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,669
    804
    Transformer from UPS is definitely not 120/240V, and also it is not made for operation at 60Hz. Do not try connecting it to mains again!
     
  11. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    The answer is back EMF. The second coil generates a counter voltage from the magnetic flux. I suspect you had the secondary shorted when you tried the primary. If the secondary can not generate a voltage (as with a dead short) then there is no back EMF. It is the same reason a motor only draws the current it needs. A spinning motor is also a generator, and generates back EMF that resists the EMF from the outlet. Reduce the speed of the motor, the back EMF reduces, and more current is drawn.

    For the other commentors, my UPS plugs into 120VAC, how can it generate 120VAC otherwise? 120VAC in, 120VAC out. There should be two transformers in a UPS, the one with the wide copper straps was likely the output transformer. To create 120VAC 5A will require 12VAC 50A (and this is the reason for the heavy duty conductors), it is a conversion process remember.
     
  12. Farlander

    Thread Starter Active Member

    Oct 14, 2008
    158
    0
  13. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,740
    759
    Then you must have connected them wrong.
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    That isn't a transformer from a UPS, it appears to be manufactured as a simple stepdown with dual secondaries.

    If you want transformers from a UPS most any small to medium size company throws the darn things away because they won't bother replacing the batteries when they get old and they usually want to step up in size anyway. I've got a lot of them hanging around but they're just too expensive to ship when you can find the same thing for free if you just get on the phone.
     
Loading...