Transformer with dual secondaries question

Discussion in 'DIY' started by Bassblaster, Sep 13, 2017.

  1. Bassblaster

    Bassblaster Super Member

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    Im looking into getting an Antek toroidal transformer to upgrade my LM317 with a puny transformer that cant keep 12V at half an amp.
    I Know you can wire the windings in series to make split rails, but a 15V 0.8A x2 transformer, can i wire them so i get the same voltage but double the current? or would i bet better getting a lower voltage transformer with the same VA rating and putting its 2 secondaries in series?

    OFF TOPIC, not important but i have a pulled transformer from an RCA VBT200, how can i test a transformer to find its current rating?
     
  2. petehall347

    petehall347 the brandy coffee man

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    there will be a formula . something like when secondary is overloaded voltage sags if overloaded so dont let it sag ..transformer warms up too much if overloaded . . measure voltage across load or use an ammeter in series . sorry i cant do the maths bit .but this is should be known too .
     
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  3. whoaru99

    whoaru99 Epic Member

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    Dual independent secondaries can be wired in series for twice the voltage or in parallel for twice the current. Just have to get the polarities/phasing correct. The Antek information shows this.

    I think whether you do it series or parallel largely depends if you need +/- supply or just a single rail.
     
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  4. Bassblaster

    Bassblaster Super Member

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    Pete, i thought it was something like measure voltage at a load, but i wasnt sure. Guess its time i get a DC Load.

    Whoaru, So a 30VCT i would wire both windings in parallel since i dont need a split supply. Simple!
     
  5. whoaru99

    whoaru99 Epic Member

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    Well, the Anteks I've dealt with, for what they call a 30V transformer, have two separate windings that could be connected in series resulting in 60VCT or could be connected in parallel for 30V but double the amps.

    Either way, the net VA is the same. For example if it was a 15V, 15VA transformer, the series connection would be 30V @ 0.5A, the parallel connection would be 15V @ 1A.
     
  6. gadget73

    gadget73 junk junkie Subscriber

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    just make sure you phase it right otherwise its basically a dead short.
     
  7. BinaryMike

    BinaryMike Pelagic EE Subscriber

    This question comes up repeatedly. Unfortunately there's no simple answer. There are a few good methods of estimation, however, and that's often sufficient if your application isn't stressful:

    [1] Measure transformer weight and compare with catalog items having similar weight and output voltage.

    [2] Find regulation figures for transformers of similar design style and weight, then load the unknown device until its output voltage falls by the same percent regulation. This method is complicated by the fact that few transformer manufacturers make regulation figures available. AnTek is an exception, but they only make toroids. Hammond coughed up the data (to me) following an inquiry to their engineering dept.

    [3] Measure secondary DC resistance, then take a (conservative) guess at the current rating, load the unknown device accordingly and run for several hours, then measure DC resistance again. A resistance increase of 20% occurs for 50C temperature rise. Without information about the transformer's insulation system, you want to keep internal temperature rise down to no more than 40~50C. This method requires use of a four-wire ohmmeter for low-voltage windings, but ad hoc solutions are readily available.
     
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  8. shelly_d

    shelly_d Not An Audiophool

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    For the Antek - wire the primaries in parallel (red to red and black to black) to set it up for US voltages (110 to 120). Wire the secondaries in parallel (green to green and blue to blue) to get a single 15 volt output with current capacity of ~ .8 amps. When drawing .87 amps with a primary winding being fed by 115 volts, you will see 14.7 volts output.

    Also, with the information Antek provides it is possible to get a first order prediction of the output voltage based upon the current draw. This won't be exact but it will be close.

    For an input voltage of 115 V RMS AC the output voltage can be approximated by this formula (for this transformer only)

    V = (-.625) * I + 15

    (Note that this is based upon a linear response to current draw. It is an ideal approximation that does not really happen in real life. The actual voltage drop is non-linear. This was computed from the information Antek supplied for this transformer. Do NOT use it for any other)​

    Once you have that voltage you can adjust it for your local conditions by measuring the voltage you get from your power company. Sometimes it can be high. I've measured as much as 122 V in my home. Here is how:

    (Calculated Voltage) * (Your measured Voltage) / 115​

    This is again based on a linear application of input to output.

    These are not exact but they will get you close. I used an Antek transformer to rebuild the power supply of Stereo 70 using these same calculations and the output voltages I read when finished were within 10% of what I had been predicting using these methods. Not a ringing endorsement, I know, but it got me close enough to get the right transformer and get the amp up and running.

    Good Luck

    Shelly_D
     

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