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  1. PLW
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    PLW Trainee Access

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    Bournemouth
    Hi guys I am currently training to be an electrician and have only been at college about 6 months. So my understanding of things isn't particular great.

    Anyway I have recently come across transformers and whilst I understand the general idea is to either increase or decrease the voltage for a circuit, would I be right in saying that step up transformer increases the voltage and therefore increases the current, and that a step down transformer decreases the voltage and therefore decreases the current.

    I got this idea from Ohm's law - The level of current flowing rises if the voltage is increased and falls if the resistance is increased.

    All help is appreciated sorry if this isn't very technical :)
     
  2. Rob
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    Rob Trusted Advisor

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    North west
    Quite the opposite.
     
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  3. PLW
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    PLW Trainee Access

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    Bournemouth
    Can you please elaborate?
     
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  4. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
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    Wilko Electrics
    Hi - If voltage goes up (step up) then current will go down. If it didn't, then there would be more power out than was put in.
     
  5. Barry White
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    Barry White Guest

    Ignoring transformer losses

    input volts x amps = volts x amps output
    240volts x2 amp =480VA= 12v x 40amp

    VA =volt amps

    Stepup means to go up in voltage and down in amperage
    Stepdown means to go down in voltage and up in amperes
     
  6. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

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    Northamptonshire
    Assuming you are talking isolating transformers a step down has more windings on the primary input side than the secondary output side. The current from the secondary is proportionate to the primary, so if the step down is 10:1 for the voltage the current output is a 10:1 step up. The reverse for a step up transformer.
     
  7. PLW
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    PLW Trainee Access

    Location:
    Bournemouth
    Thank you for your answers guys. So now I know that as the voltage increases the current decreases and vise versa. So why when Ohm's law states that when voltage increases current increases, is it the opposite for transformers?
     
  8. Wilko
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    Wilko Electrician's Arms

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    Berkshire
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    Wilko Electrics
    Without doing the physics (lazy me) - if it wasn't this way the transformer would be magically putting out more power than was put in.
     
  9. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    if you increase the voltage across a resistor, say, then the current will risebecause I=V/R according to ohms kaw. however, with a transformer, your load may be 10 A @ 24V wich , as power in =power oot, means @ 240V the primary current will be 10A x (24/240) = 1A.
     
  10. PLW
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    PLW Trainee Access

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    Bournemouth
    So in very basic terms as the voltage is stepped up the resistance is also stepped up causing the current to decrease vise versa
     
  11. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    no. you're looking at this from the wrong angle. look more using the simple formula P=IV. ignoring any losses for simplicity.
     
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  12. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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  13. charlie76
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    charlie76 Regular EF Member

    Location:
    Notts
    Business Name:
    CES Midlands
    As telectrix said you are looking at it wrong. Don't use ohms law in the way you are. Consider that ohms law applies to either the input circuit (the primary winding) our the output circuit (the secondary winding). V = IR applies in the primary and V = IR applies in the secondary winding, independently, with R being the resistance of the winding/coil. What you are confusing it with is thinking it is one circuit, its not, think of it as 2 circuits, the primary winding is one, the secondary winding is one. You are right that if the voltage increases in the primary then the current will increase in the primary.

    Using P = IV, for a 100% efficient transformer power on the primary winding will be equal to power at the secondary winding.

    So lets say you have a transformer with 230v, 2A input and 12v output.

    Pin = Iin x Vin
    Pin = 2 x 230 = 480W

    So as we said 100% efficient so you know there will be no power loss so you will have 480W at the secondary winding and we know we have a 12v secondary so....

    Pout = Iout x Vout
    480 = Iout x 12
    Iout = 480 / 12 = 40A

    So as you see the voltage went down and the current went up.
     
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  14. HandySparks
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    HandySparks Trusted Advisor

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    Neish Electrical Services
    Er, yes, in a resistor (or any sort of purely resistive conductor). No point trying to apply a law of physics to a situation in which it has no relevance.
     
  15. Sikho
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    Sikho New EF Member

    Location:
    South Africca
    Hello,
    Upon visiting the site and performing some testing and inspection it has been found that all four of the CT’s installed in the starter cubicles are over heating. The burden is 2.5VA , ratio is 20:1 A @ Class 1 CT.

    What might be the problem? Please help
     
  16. Pete999
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    Pete999 Trusted Advisor

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    Northampton
    What currents are/is the cubicles taking? there will be some heat produced, what makes you think the CTs are OVER heati?ng
     
  17. Sikho
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    Sikho New EF Member

    Location:
    South Africca
    Each cubicle takes up to 20A. All the CTs are damaged;

    IMG-20161208-WA0010.jpg
     
  18. DPG
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    DPG Electrician's Arms

    Location:
    S Yorkshire
    It would be worth starting a new thread about your CT issue. Daz
     
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