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Discussion in 'Electronic Basics' started by piller32, Jul 8, 2005.

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  1. piller32

    piller32 Guest

    Intro to transformers

    A transformer has 20 primary windings and 100 secondary windings.
    If the secondary voltage is 25v, Find the Primary voltage

    Vp/Vs =Np/N

    20p/100 =

    Now i'am stuck Please explain
  2. Chris

    Chris Guest

    Basic math.

    Vp / Vs = Np / Ns

    Now substitute the quantities you have:

    Vp / 25 = 20 / 100

    Get Vp on one side by multiplying both sides by the denominator...

    (Vp / 25) * 25 = (20 / 100) * 25

    Vp = (20 / 100) * 25 = 5 VAC

    So, the primary is 5 volts AC. Transformers can be step up and step
    down. This one is a step up transformer.

    It looks like you need to brush up on your basic algebra before
    continuing the Basic Electronics course. You can't do much of any
    electronics without at least a basic (1st year high school) math

    There are quite a few good books which can help you. Possibly, if
    you're up front with your teacher that you're a little light on math
    skills, he can recommend one he's familiar with. He might even
    recommend a good tutor or other resource to help you out.

    This group usually doesn't provide answers to homework questions.

    Good luck
  3. Tom Biasi

    Tom Biasi Guest

    You have expressed an equation. The equation has four variables. You are
    given three of them
    Come on!
  4. Genome

    Genome Guest

    Sometimes sums will hurt your head when you represent them as equations and
    try to eat them all at once.

    The secondary has 25V across it with 100 turns. So the secondary voltage per
    turn is 25/100 or 0.25.

    The primary voltage per turn is the same so the primary voltage is 20*0.25
    or 5V.

    Give yourself a new variable called the voltage per turn, Vt. Divide the
    secondary voltage, Vs, by the number of secondary turns, Ns, to get Vt, the
    voltage per turn.

    Vt = Vs/Ns
    = 25/100
    = 0.25V

    Having found the voltage per turn multiply it by the number of primary turns
    to find the primary voltage.

    Vp = Vt*Np
    = 0.25*20
    = 5V

    Since you had an equation for Vt you can substitute it in the above and get

    Vp = (Vs/Ns)*Np
    = Vs*Np/Ns
    = 25*20/100
    = 5V

    You can take your original equation...

    Vp/Vs =Np/Ns

    and convert it to what I had by multiplying both sides by Vs

    Vs*Vp/Vs = Vs*Np/Ns

    Vs/Vs, on the left hand side, is 1 so things cancel and you get

    Vp = Vs*Np/Ns

    Like before, then you plug in the values for Vs, Np and Ns and get

    Vp = 25*20/100
    = 5V

    Like before.

    What you're dealing with here is algebra, part of mathematics. You tried to
    jump too far and got lost. The almost nice thing is that it's a practical
    application so you can go back and say.... 'It's a transformer so the number
    of volts per turn is the same' and move forward.

  5. Fred Abse

    Fred Abse Guest

    I've just spent ten minutes puzzling how a transformer can have 20 primary

    Most have only one.

    I think you mean *turns*, rather than *windings*. Your example has two
    windings in the generally accepted sense of the term, a primary winding of
    20 turns and a secondary winding of 100 turns.

    Hence Np/Ns = 20/100 = 1/5

    So it's a 1:5 step-up transformer.

    Hence the primary voltage is 5V

    Since this looks like a homework problem, now go figure what the current
    in the primary will be if the current in the secondary is 5 Amps.
  6. ehsjr

    ehsjr Guest

    You can do this if you know fractions:
    The voltage ratio (Vp/Vs) is equal to the turns ratio (Np/Ns).
    We know the turns ratio (Np/Ns) 20/100 which equals 1/5.
    We know Vs is 25. The formula Vp/Vs = Np/Ns becomes:
    Vp/25 = 1/5 and the question becomes:
    What ratio, with 25 in the denominator, is equal to 1/5?

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