Step down transformer amperage

Discussion in 'General Electronics Chat' started by imraneesa, May 27, 2015.

  1. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    Dear Sirs,

    I have a transformer i salvaged from a circuit board. it is not having any details over it. but i know it is used in the circuit to step down from 110ac to 12ac. i want to find out the amperage of it.

    what load i can use to find out the current it is giving. can i use some 40w bulb? i want to check the current rating of the transformer.
    i want to build AC to DC 12v 1.5A adapter.
    Thanking you in advance.
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    It will give as much current as you take from it up to a point were it will probably overheat and die!

    You can't "measure" how much current a transformer will supply, they are given a VA rating that you are expected to stay within.

    Compare it's physical size to other 110/12 transformers for a rough idea of it's rating.
     
  3. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    how to know the current using VA rating sir. if the VA rating is 40VA and the transformer is 110v~24v. how to know the current rating.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,387
    1,605
    Simple.

    40VA / 24V = 1.67 A
     
  5. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    ok thats nice. so can i use this transformer to give me 12v DC with 1.5A rating? how can i decrease the 24V AC to 12 V DC in best way using bridge rectifier.
     
  6. paulktreg

    Distinguished Member

    Jun 2, 2008
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    You can't know it's VA rating if there's no information on the transformer.

    If you are sure it's 12VAC out then all you can do is slowly increase the load current on the secondary whilst ensuring 12VAC is present and make sure the transformer doesn't overheat.

    Without the manufacturers specification it's a combination of testing and guess work.
     
  7. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    ok i am not going with that transformer without the details on it. but i also have a transformer 110ac to 24ac with 40VA. i want to use this to regulate 12v DC with 1.5A current.
    can i use lm317 for this after bridge rectifier. or is there any better way to decrease the voltage from 24v to 12v?
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    You can estimate the power capability (in VA) of the transformer by comparing its physical size with similar transformers in catalogs. Signal Transformer is good for this. If you have a good selection of power resistors, another way is to measure the transformer output voltage with no load, then apply load resistors until the output voltage drops by about 10%. Ohm's Law gives you a good first-pass estimate of the full load output current.

    ak
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    Using a 24Vac winding to make 12Vdc is problematic, and very inefficient. The full-wave rectified, filtered voltage at the input of the regulator will be much too high, so you will wasting a lot of power in the regulator.

    Ideally, to make regulated 12Vdc, the transformer secondary should be 30 to 34Vac with a center tap (17-0-17) for two diode rectification. If using a transformer without a center tap for a four diode bridge, then the secondary should be ~18Vac
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Well you can measure it, sort of - but its time consuming.

    Keep adding parallel resistors and see at what point it starts to get significantly warm.

    When you decide you don't want it to get any warmer than that, calculate what all those parallel resistors come to and work out the current - or just measure the current.
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    An LM317 will burn off 18 watts when droppping from 24V to 12V at 1.5 A.

    Make sure you use a big heat sink.
     
  12. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    You need to derate a transformer to about 50-60% of it's AC rating when it's used to generate DC with a diode/capacitor.
    Thus a 1.5A AC rated transformer should not be used for more that about 0.8Adc output otherwise it can overheat.
     
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