Testing for proper transformer wiring

Discussion in 'General Electronics Chat' started by dwurmfeld, Sep 11, 2009.

  1. dwurmfeld

    Thread Starter New Member

    Apr 27, 2008
    5
    0
    I have a set of PCB's to test that have a simple stepdown transformer to convert 115/230 VAC down to 12VAC.

    The transformer has a dual primary; when on the PCB for 115V, the traces connect them in parallel. When used on the pcb for 230 VAC, the primaries are wired in series. (see attached PDF)

    I am trying to detect if the board is a 115 VAC board or a 230VAC board, from the terminals feeding the primary. I will use this to properly power up the board for further testing.

    I only have access to the two terminals; A & B. (see attached PDF)
    The traces on the board are hidden by the transformer.

    I cannot rely on the test operator (out of my control) to "look at the board" and select the proper input voltage. The way the circuit is designed (also out of my control) the board will be damaged if powered with the wrong input voltage.


    I have two ideas:
    1. Inject a small AC voltage, and read the secondary voltage to determine the board type.
    2. Make points A&B part of a LRC "tank" oscillator circuit. If I did it correctly, it should not oscillate if it is the 230 VAC traces (direct short between A&B)
    Idea 1 will work, I just have to come up with a direct method to measure the output voltage, then drive a relay to select the proper input voltage.


    Idea 2 is promising, does not involve energizing the primary, but I have no idea how to:
    • Design an oscillator with the appropriate R & C for the parallel primary inductance.
    • Detect the oscillation (LM567 tone decoder?) to then drive a relay controlling the input voltage.
    Any ideas would be appreciated.
     
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    hook it up to the lower of the two possible input voltages and read the secondary. How could that injure the circuit when a yet lower 'test' voltage not?
     
  3. dwurmfeld

    Thread Starter New Member

    Apr 27, 2008
    5
    0
    You are correct, it would not "hurt" the circuit to start by powering up with 115, checking the output, then switching it out for 230.

    Thanks for the help.
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The following method does not require feedback from the secondary.

    Put a small 230V indicator lamp in series with the winding and connects to 230V.

    Depending on the brightness of the lamp, switch to lower voltage as necessary.

    A 110V connected transformer winding will give a very bright lamp while a 230V winding would give lower brightness.
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Another possibility would be to measure the DC resistance of the primary side. It appears the low voltage setup should be about double the high voltage setup. You'll likely want to use a Kelvin-type measurement to make this measurement though (easy to do with a constant current DC power supply and a digital multimeter). The only way it wouldn't work is if the process variation of the transformers is large enough to swamp the signal you're trying to discriminate.
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I would say the resistance of the high voltage setup is about 4 times that of the low voltage setup.
     
  7. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    You're right; thanks for the correction. Makes it easier for the OP...
     
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