Confusing Small Transformer Data

Discussion in 'General Electronics Chat' started by gronkle, Oct 29, 2013.

  1. gronkle

    Thread Starter New Member

    Jun 2, 2012
    18
    2
    I have a tiny PCB-mounted, e-core, step-up transformer (probably 1:200 or 1:250) that is used in a small sparking circuit (images below). It uses a simple, transistor-induced square wave to turn a now-oscillating, battery-powered 3VDC circuit into a higher voltage AC-like wave on the secondary "coil" which feeds a 0.22uF capacitor by way of a standard diode. [​IMG]There is no continuity between the leads of the secondary coil - but the circuit works as intended...seemingly indicating a) the intentional use of open-winding coils...or b) that the excessive length of thin-gauge coil wire somehow does not produce a continuity reading on the meter. Thoughts?
     
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  2. gronkle

    Thread Starter New Member

    Jun 2, 2012
    18
    2
    There are 6 pin-outs on the transformer. There is continuity between 1-4 (red), and 2-3 (white) which are both looking like dual primary coils on the side of the oscillating 3VDC circuit - yet there is seemingly NO continuity between 5 and 6 (black), even though these are feeding the capacitor/diode circuit directly. There is ~0.6Ω between 1-4 and 5-6. There is also (strangely) ~236Ω of resistance on 5-6 (seems to point towards some sort of connection, but....). That would mean a 1:400!?

    Questions:
    1) Is there a chance that the longer length of very-thin-gauge wire would not produce continuity tone even if continuity exists? (equivalent of measuring continuity across a resistor)
    2) Could the imagined open-circuited windings on the transformer depicted in the above image drive the charging of the capacitor? (i.e Would an open-circuited coil still produce current movement in a single direction in pulses to feed either side of a capacitor?)
     
  3. Danm1

    Member

    Jul 19, 2010
    55
    7
    The thin gauge wire should not appear to be a discontinuity. If there was too much current in that thin wire, it would just fry, then it would be an open circuit.
     
  4. TorontoBob

    New Member

    Oct 5, 2013
    15
    4
    Of the low resistance windings, one will be your primary, another will be a feedback winding for the oscillator.

    Your high voltage secondary should not be open. As Danm1 suggested your secondary winding could have been fried, but when powered in the circuit could be arcing internally to allow it to work for now.
     
  5. gronkle

    Thread Starter New Member

    Jun 2, 2012
    18
    2
    Thank you for the reply - I have a couple of these boards that work fine, and they all show no continuity between 4-5 just like the one in image. As a test I already know the answer to, I tried to measure continuity across a 200ohm resistor using my meter....no continuity tone at that resistance. Perhaps the issue here is the sensitivity of the meter?
     
  6. gronkle

    Thread Starter New Member

    Jun 2, 2012
    18
    2
    So I pulled apart the high-frequency transformer, here is the data:

    Primary Coil is 35 turns, ~31 gauge wire, 0.8ohms, and 1395mm
    Fly Back Coil is 35 turns, ~31 gauge wire, 0.8ohms, and 1347mm
    Secondary Coil is 1663 turns, ~39 gauge wire, 235.5ohms, and ~53170mm (estimated via a calibrated resistance measurement of a known length of the wire)

    There was no observed breaks in the secondary winding - which seems to indicate that the resistance over that length of wire was too high to record continuity. Continuity over smaller lengths worked fine.

    35:1663 (primary:secondary) appears to point to a 1:47.5 ratio, but I know this transformer steps up to more like 1:250...not sure what is happening here...any thoughts?
     
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