Wrongly assumed rating of cheap transformers

Discussion in 'General Electronics Chat' started by recklessrog, Mar 24, 2016.

  1. recklessrog

    Thread Starter Member

    May 23, 2013
    I was asked to help a friend repair a small cheap guitar amplifier today which had a burnt out mains transformer. The original transformer was simply 230v primary, 18 volt @ 1amp secondary and had expired because A, the cabinet was fully enclosed with no ventilation, and B, the amp had been run flat out for long periods.
    My friend had replaced the transformer with an off the shelf 9-0-9 volt transformer rated at 1 amp and not used the center tap taking the output across the two 9 volt leads where he expected 18 volts.
    He had noted that it was smaller than the original, but the supplier had told him that as it was the same current rating, it would be fine. On testing the amp, it failed to produce the original output power and the transformer got red hot and also failed. He then assumed that there was something else that was breaking down causing an overload so he brought it to me to investigate further.
    After doing the usual visual and smell checks, I hooked the bridge rectifier up to a decent 18v transformer and wound it up on a variac. I played my Les Paul through the amp for an hour flat out and it was perfectly ok.
    Then I explained to my friend, his and the suppliers mistake. The original transformer was able to supply 18volts @ 1amp to a bridge rectifier, but the 9-0-9 volt one was designed to supply 9 volts at 1amp. The type of rectification it was designed for is where only one half of the winding is conducting in any half cycle. By feeding the 18 volts to a bridge rectifier where current is drawn on each half cycle, the transformer VA rating (which wasn't specified) was exceeded causing it to overheat.
    It is a shame that there are so many suppliers of components that don't have the full spec of these (and many other) components available. Just a brief primary- secondary voltage, and an ambiguous current rating. It also appears that some only refer to the voltage after rectification and smoothing at a specific current. I have also noticed that they have terrible regulation. The manufacturers (probably Chinese) try to use as little core material that they can get away with.
    Last edited: Mar 25, 2016
    #12 likes this.
  2. RichardO

    Well-Known Member

    May 4, 2013
    Thanks for your insight. A question: Was the under-rated transformer specified to work at 50 Hz?
  3. recklessrog

    Thread Starter Member

    May 23, 2013
    Yes they are sold as "mains transformers" here in the U.K so it is reasonable to assume that they are meant for 50 Hz.
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    To me a good troubleshooting log reads like a detective story. Thanks for a good read.

    HUGE hint right there. "There ain't no such thing as a free lunch" (TANSTAAFL) applies in electronics all the time. Transformers are just hunks of metal and it takes so many pounds to carry so much energy.

    It is YOUR job to determine if a replacement component is suitable or not. Instead of grousing over specifications published in unobtanium simply do not buy anything you cannot verify.

    This may cut down your sources of supply, but why use sources of dubious parts?

    (Actually this sounds like a real brick and mortar shop you can visit and hold parts in your hand before you buy and have them shipped. Oh how I miss those places...)

    Did you ever buy a car that "seats six" only to find an extra seat or two squirreled away?

    EVERY manufacturer does that. It is just common efficiency for cost effectiveness.
  5. recklessrog

    Thread Starter Member

    May 23, 2013

    The problem that my friend had is that he rightly expected the assistant to give him accurate advice. It transpired that it is too much to expect.