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Question about transformer input/output rating

Discussion in 'Electronic Basics' started by Ken, May 7, 2004.

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  1. Ken

    Ken Guest

    I bought a desklamp that has 35 Watt lamp, and a transformer.
    The transformer input rating is 120V AC 60Hz 70W
    Output rating is 12V AC 3000mA

    Based on my limited knowledge, the output power should be 12*3000mA =
    36 Watt.

    My question are
    1. since the transformer's input rating is 70W, what is the actual
    power consumed by this transformer? or actually what is the power
    consumed when I use this lamp (when the lamp itself is rated 35 watt)?

    2. What is type II transformer?

  2. CFoley1064

    CFoley1064 Guest

    Subject: Question about transformer input/output rating
    1) Assuming you're running straight 120VAC into the transformer, it probably
    draws less than 40 watts total power from the line. As a "first cut", you can
    assume a power transformer is better than 90% efficient when it is loaded
    between 25% and 90% of rated load.

    2) Class 2 is a U.L. term which indicates that the part meets specific safety
    requirements as defined in U.L. 1585 or U.L. 1310.

    Good luck
  3. Power isn't consumed by a transfomer. The power rating indicates how
    much power you can get out of it without overheating and possibly
    destroying it. The connected devices consume the power, and determine
    how much power is passed through it.

    So, when you have a 35W lamp connected up, the transformer is passing
    35W, and wasting perhaps a couple of watts in heat.

    Bob Monsen
  4. Ken

    Ken Guest

    Thanks all for your reply.
    The transformer is kind of big, and the lamp is pretty cheap. Is it
    possible that this is a inefficient transformer that could need much
    more power than output rating?
    I wonder why the input rating is so high compared to the output rating
    (70 W vs 36 W).

  5. I'm not an expert on the meaning of "type II" but it might mean that
    the transformer is capable of shorted secondary operation while it
    limits the input power to no more than 70 watts. This implies a lot
    of leakage inductance between primary and secondary and would make it
    have very poor voltage regulation and the secondary voltage would be
    expected to be quite a bit higher at no load than it is with a 36 watt
    load. Try measuring the output voltage with the normal and open
    circuit secondary.
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