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Transformer HELP

  1. Nov 5, 2009 #1
    Transformer HELP!!

    A small transformer with 1200 turns In the primary coil is used to power a appliance that has a output voltage of 6V. The label o the transformer says that the input is 5W at 120V. What will be the output power of the transformer?

    Given: V output= 0.6*10V (6V)
    V output= 3.0*10^2 mA
    Required: P Output
    Analysis: P=IV
    Solution: 3.0*10^2mA *0.6*10V
    = 1.8*10^3W will it be the output power of the transformer?


    B: Will the secondary coil have more, fewer or the same number of turns as the primary coil? Why?
    It will have fewer turns.
     
  2. jcsd
  3. Nov 5, 2009 #2

    Mark44

    Staff: Mentor

    Re: Transformer HELP!!

    The transformer will have fewer turns in the secondary windings, so you're right there, but I believe you're way off in everything else. You're given that Vin = 120 V and Vout = 6V, so this is obviously a 20:1 stepdown transformer.

    Since you are given that Vout = 6V, how can you say this right after your given V out?
    The units of voltage are volts, not amps or milliamps. How did you come up with 300 mA for the output voltage?

    Aside from any power losses due to internal resistance in the windings, the power in should be equal to the power out. I am assuming that the intent of this problem is to ignore internal resistance. What changes are the voltage and current. If the voltage is stepped down by a factor of 20, the current will be stepped up by a factor of 20.

    Since this is a stepdown transformer, the number of windings in the secondary should be 1/20 of the number of windings in the primary.
     
  4. Nov 5, 2009 #3
    Re: Transformer HELP!!

    I revised it :
    Given: V output= 0.6*10V (6V)
    I output= 300 mA or (300*10^-3)
    Required: P Output
    Analysis: P=IV
    Solution: 300*10^-3A *0.6*10V
    = 1.8W will be the output power of the transformer?
    is this correct?
     
  5. Nov 5, 2009 #4

    Mark44

    Staff: Mentor

    Re: Transformer HELP!!

    Where are you getting your Ioutput? There is no mention of current in your problem description.
    The power into the transformer should be essentially the same as the power out of it. What changes are the current and voltage.
     
  6. Nov 6, 2009 #5
    Re: Transformer HELP!!

    OMG! I missed the part of the Q, Here is the it:
    "a appliance that has an output curret of 300mA & an output voltage of 6V. The label on the transformer says that the input is 5W at 120V. What will be the output power of the transformer? "
     
  7. Nov 6, 2009 #6

    Mark44

    Staff: Mentor

    Re: Transformer HELP!!

    Here is what you have said over the course of this thread:
    Your description is not clear as to what is the output of the transformer and what is the input to the appliance.

    An appliance should not have an output current and an output voltage. The transformer takes line voltage (120 V) at some number of amps and steps it up or down to an output voltage at some different number of amps. The appliance takes as input the voltage and current (amps) produced by the transformer.

    Please provide the exact, word-for-word problem description.
     
  8. Nov 6, 2009 #7
    Re: Transformer HELP!!

    HI! Here is the exact word-to-word problem:
    Q3: A samll transformer with 1200 turns in the primary coil is used to power a portable tape player that has an output current of 300 mA and an output voltage of 6.0V. The label on the transformer says that the input is 5W at 120V, if there were 1200 turns on the primary coil,
    a) What is the output of the transformer?
    b) Will the secondary coil have, more, fewer, or the same # of turns? Why?
    (My answers are same that I have posted last)
    Thanks alot!!!!!!
     
  9. Nov 6, 2009 #8

    Mark44

    Staff: Mentor

    Re: Transformer HELP!!

    This probably isn't word-for-word (the actual problem probably doesn't have "samll" in it), but it's close enough.
    Notice that the problem does not ask for the output power of the transformer. As I said earlier, the input power and output power are going to be essentially the same. The only things changing are the input and output voltages and the input and output currents.

    Since the output voltage on the transformer is 6V and the input voltage is 120V, this is a 20:1 stepdown transformer. The current will be stepped up by a factor of 20. If the output current of the transformer is 300mA what will be the input current to the tape player?

    For the windings in the secondary coil, yes, there will be fewer turns, namely 60, which is 1/20th of 1200.
     
  10. Nov 6, 2009 #9
    Re: Transformer HELP!!

    SORRY:(
    I have actually again missed 1 word in part a of the Q, so Im writing it again:
    a) What is the output power of the transformer?

    b) Given: V primary = 120V
    N primary 1200 turns
    V secondary 6 v
    Required: n secondary
    Analysis: V1 N2= V2 N1
    N2= V2* N1/V1
    Solution: N2= 6V*1200 turns /120V
    = 60 turns.
     
  11. Nov 6, 2009 #10

    Mark44

    Staff: Mentor

    Re: Transformer HELP!!

    If the transformer is an "ideal" transformer, power in = power out, so the output power is also 5W. The information in this problem leads me to believe that what you're working with is an ideal transformer.
     
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