Measuring AC wave from transformer

Discussion in 'General Electronics Chat' started by Gibson486, Mar 11, 2014.

  1. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    I am measuring a 300Khz AC wave (300V pk pk) from a transformer output. The problem is that, over time, it drifts to a lower value (depends on the cable length). I am trying to see what is causing the drift. If I remove the cable, the drift does not occur. I am guessing that the cable has a capacitance and the scope probes acts as a leak for the cable's capacitance. Does that sound feasible or am I just silly?
     
  2. #12

    Expert

    Nov 30, 2010
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    There is an interaction between all 3 elements. The supply of the signal might be changing with the power (heat) required to drive the capacitive load of the cable. I have actually calculated the maximum resistance allowed when required to drive a cable capacitance over x amount of distance at a particular frequency. This is usually a low number at 300KHz.
     
  3. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    Thanks.....I am going through the process doing calculations (arggg...opening up the Emag book sucks). On the cable spec sheet, it has 2 capacitance. 1 is C from wire to wire, the other is C to ground. Do I simply add those two? Also, they give the capacitance at 1kHz. I am guessing I have to resolve for C with my new frequency?
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    capacitance does not change with frequency. Xc does.
     
  5. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    ofcourse....hahahaha...need coffee....
     
  6. #12

    Expert

    Nov 30, 2010
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    Either add the capacitance or use a separate amplifier to drive the shield with the same changes that are on the data wire. Zero voltage difference causes zero current to flow and, mathematically, the shield disappears.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,235
    If it drifts with time then I would suspect that whatever circuit is driving the transformer is heating up due to driving the cable capacitance and that causes its output voltage to drop.
     
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