Explain this hum behaviour?

Discussion in 'General Electronics Chat' started by daviddeakin, Aug 19, 2012.

  1. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    Hi,
    can anyone explain this hum behavior to me?

    I have a 6V transformer powered from the mains, and attached to one leg is a 68 ohm resistor- that's it, no other connections at all. Now, if I connect my oscilloscope probe as shown, I read the ugly hum voltage shown in the photo. But if I reverse the scope leads I measure no hum at all! How is this possible?

    [​IMG]
    [​IMG]
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    It isn't hard, the resistor is a shunt resistor, it reflects the current flowing through it.

    Opps, reread your post.

    Don't forget, the scope is grounded though.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    This is probably due to capacitive coupling between the transformer windings.
    There is some current flowing through the transformer to the ground, so one way you are measuring the drop across the resistor and the other way there is no current through the resistor (the current leaves at the first connection) so you measure no voltage.
     
  4. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    Sounds plausible- thanks for the tip!
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    I tried to duplicate your experiment but did not get the same result.
    In both cases I got no measurable signal.
     
  6. chrissyp

    Active Member

    Aug 25, 2008
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    The scope is picking up the resonance (or feedback whichever you prefer to call it)of the transformer through its own grounding route . Put a 2 pin mains cable into your scope,ie no earth and the hum will not happen.
     
  7. #12

    Expert

    Nov 30, 2010
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    68 ohm shunt on the scope input? It takes a pretty good hum field to make a voltage on that. Better check for crusty connections in and around the scope lead.

    I like the answer from kubeek but I also worry about intermittent connections. Probably because my scope is 35 years old.

    Does it quiet down when you unplug the transformer?
     
  8. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    Yes, it is only present when power is applied. I can also turn the level up and down with a variac. I get the same result on both channels of the scope, and with different scope leads, and with the resistor attached to either of the transformer leads.
     
  9. #12

    Expert

    Nov 30, 2010
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    That pretty much proves kubeek is right.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    I did a rough calculation and it would take about 600pF primary-secondary capacitance to generate the measured output voltage across 68 ohms.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Does that assume 240V on both sides of the primary, relative to scope ground? One end should be close to ground potential. I believe that would double the capacitance estimate.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    I assumed 240V from primary to ground. But, now that you mention it, it's probably 120V to ground from each winding to ground with the two voltages 180° out of phase. That would indeed appear to increase the capacitance estimate since the capacitive currents from the two inputs would tend to cancel.
     
    Last edited: Aug 20, 2012
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This site says that house wiring is 230V, neutral, and ground. Since neutral ≈ 0V relative to ground, then one leg of the primary would be at 230v, the other at 0v. The average voltage across the interwinding capacitance would be ≈115v.
     
  14. maanga

    New Member

    Aug 20, 2012
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    It is NOT possible to generate a small wave form without a current in a 68 ohm resistor. this resistor will shunt all voltages on the high impedance probe. This must be the residual voltage on the scope itself with high amplification. What is the value (voltage) are you measuring?
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    The only plausible explanation I can think of is what you think is a 68Ω resistor is not 68Ω at all.
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Sure it can. Kubeek explained it in post #3. Attached is a simulation.
     
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