Mains Transformer Current Rating - Help

Discussion in 'General Electronics Chat' started by Mad Professor, Apr 18, 2009.

  1. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    Good Day All.

    I would like to know if it is possible to work out the max safe working current for a Mains Transformers, without having the datasheet.

    I have a spare Mains Transformer from a project last year but I have lost the datasheet for it.

    240v AC Input (Pri Coil - 42.5ohm) Aprox.
    11.0v AC Output (Sec Coil A - 00.5ohm) Aprox.
    11.0v AC Output (Sec Coil B - 00.5ohm) Aprox.
    22.4v AC Output (Sec Coil A&B - 00.8ohm) Aprox.

    My Digital Volt Meter is quite old,

    With this data is there anyway to work out what the max safe working current is for this transformer?

    Thanks for your time.

    Best Regards.
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    Those resistances could be shorter runs of thinner wire or longer runs of thicker wire. If you have only one parameter available (the voltage) it is not possible to calculate any other value.

    Do you at least know the cross section area of the core? Rough guess at power rating? Best estimation of wire size? Anything?
  3. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    Thanks for your reply.

    The transformer metal core / cage is 63.60mm x 76.30mm x 25.15mm.
    The sec coil wire thickness is aprox 0.73mm.
    It's weight is 900g-1kg.
  4. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    A transformer that size would be good for a roughly 50VA resistive load in free air, so the two secondaries in series (you measured 22.4 VAC out) should be able to provide about 2 amps to a resistive load.

    If you drive a capacitor input bridge rectifier circuit, then the transformer VA capability will be reduced by a factor of 1.6 approximately.