Transformer question

Discussion in 'General Electronics Chat' started by Spence, Jul 5, 2011.

  1. Spence

    Thread Starter Member

    Apr 23, 2010
    49
    3
    I'm concerned about the amount of heat generated by a small mains transformer because I want to place a small audio amplifier inside a wooden speaker enclosure.

    It's a 240v mains tranformer with a two sets of taps/outputs like so...
    2 x T400 m.a.

    24v 0v 24v 0v

    So I shorted the middle two to give me 1 x 48v (across pin 1 to 4). OK, runs without problem until I connect the bridge rectifier, then the transformer starts to get hot. Within just 2 or 3 seconds the laminates are really quite hot and I worry that this is not going to be a reliable device. I switch off, the transformer cools and a resistance check shows the two windings are OK and measure exactly equal.

    I've tried this with a few different bridge rectifiers, all 3A or 4A.

    I've built quite a few small amplifiers and none of the transformers get hot.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    48VAC has a peak voltage of 67.9V which is reduced to 65.9VDC by the bridged rectifier and is filtered.

    A class-AB audio amplifier fed 65.9V will produce an output of about 56W into 8 ohms. The power supply must provide about 90W which is an AC current of 375mA AC so the transformer will be near its max rating and will get hot. It will over-heat if it is sealed inside an enclosure.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    You claim that the laminates (i.e. the iron sheets in the core?) are heating within seconds. If that is an accurate description of the situation, then there is something drastically wrong, not a mild overload.

    Are you sure that you have the right type of transformer for your local mains voltage? If adjustable for different voltages, is it set up correctly?

    Transformers should not get uncomfortably hot in the space of just a few seconds.

    Edit: I see you have 240V mains. If the transformer is really set up for 120V, the core will be saturating and the transformer will burn out very fast. It will also deliver too much output voltage, possibly frying the rest of your project.
     
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  4. Audioguru

    New Member

    Dec 20, 2007
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    A transformer that over-heats in seconds might have a shorted turn in its coils.
     
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  5. Spence

    Thread Starter Member

    Apr 23, 2010
    49
    3
    That's the situation, so I'm going to bin the transformer, but why is it OK until I attach the rectifier. I was also wondering If bridging the transformer was causing a problem.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    If the transformer were at fault the overheating problem would presumably arise when it isn't connected to anything.

    Maybe you need to check the rectifier is OK before discarding the transformer.

    It would probably also assist others to make informed comments if you provided a schematic of the connections you make.
     
  7. PeeSeeBee

    Member

    Jun 17, 2011
    43
    7
    Does the bridge rectifier also get hot?

    Could yu post a photo of the transformer, as well as the schematic of your transformer/rectifier arrangement.
     
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