Transformer Power Supply Issues

Discussion in 'General Electronics Chat' started by atrumblood, Apr 27, 2014.

  1. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    TITLE: Transformer Power Supply Issues:


    I recently built the transformer power supply circuit from the Experiments section of the online book.

    The issue I have is that when I have the switch in the off position I am reading ~250mV DC across the leads.

    I suspected the switch as the source of the issue. However I can not find any leaks in the switch. It reads infinite resistance on my meter, but somehow I am showing a small voltage. If I unplug the circuit from the wall the voltage I show goes to zero as it should with the switch in the off position.

    The switch is a rocker rated for up to 250V AC.

    I made sure to test all leads in the power cord to check for leaks and found none.

    Am I missing anything?

    I have attached a diagram of my circuit.
     
    Last edited by a moderator: Apr 27, 2014
  2. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    Sorry for the title the forum wouldn't let me post a title longer than that.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,652
    2,348
    Hello,

    Digital multimeters are very sensitive.
    The leak current may cause the voltage at the output.
    Try a resistor of say 1KOhm at the output of the bridge and measure again.

    Bertus
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,395
    1,607
    Interesting result which makes it appear as if this is a real voltage. bertus gives one good test to eliminate a phantom voltage, but this seems different. Still, his test is valid one.

    Is your voltmeter line powered or a battery hand held? I'll scratch my head even harder if it is a battery.

    To explain why you see something on a battery handheld I have to wave my hands a lot and get non-specific, but basically there are circuits and connections between things you did not connect, the "parasitic" connections. That includes things like the capacitance across the open power switch contacts, or between transformer windings.

    Typically you can ignore these secondary effects, but sometimes the appear as small ghost voltages such as you see. ("Ghost" because you cannot get any power from them.)

    They can drive engineers to near insanity in devices such as switching power supplies, where the switch noise seemingly gets everywhere, and finding how and where it does that is not so easy.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Does any of the unswitched part of the mains wire's go near the output of the transformer?

    Are you sure you are switching the hot side of the mains?
     
  6. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    I tried your suggestion Bertus and that did not eliminate the voltage I was reading. I am not reading any current from the ghost voltage either.

    ErnieM, Yes my multimeter is battery powered and handheld.

    crutschow, The transformer is a good 3 inches from the mains. Too close?
    I do have the switch connected to the Hot side.

    Thanks for the replies guys.
     
    Last edited: Apr 27, 2014
  7. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    No capacitors (that you didn't draw) in the circuit? Can you load the 1/4V with a 1 k resistor and get to zero volts?

    From a practical approach, it means nothing. From a curiosity approach, this is the stuff that learning is made of.
     
  8. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    When I add a filter cap between the dc leads the voltage jumps to 18V for some reason. Doesnt matter what Capacity I choose. That was not an expected result.

    Also As i stated I added the resistor as bertus suggested. It did not help.
     
  9. #12

    Expert

    Nov 30, 2010
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    Sorry. I missed that.

    If I assume the power switch is really a tiny parasitic capacitance, that would explain how 18 volts gets to the output capacitor. (You must have serious voltage on the primary to get serious voltage on the secondary.) At this point in my imagination, the available current must be dependent on Xc (the impedance of the parasitic capacitance). Your meter probably has an input impedance of 1 meg or 10 megohms, and that is making a voltage divider, with the Xc as the supply.

    I'm going to draw a model and see how many picofarads it would take to run this scenario.
     
    Last edited: Apr 27, 2014
  10. #12

    Expert

    Nov 30, 2010
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    And the answer is.....a bit more than 29.6 pf.
     
  11. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    Ah I see.

    However I forgot to mention that with out the filter cap between the DC leads i show ~12.2VDC. but when I add a Capacitor to the DC leads to attempt to filter the power it jumps to 18V.




    The main is 120V AC @ 60Hz.

    I will have to keep playing around with it.
    Also does it make a difference that I am using Polarized caps on the dc output to filter?
    Should I be using non-polarized?
     
  12. #12

    Expert

    Nov 30, 2010
    16,355
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    No, you didn't forget, You said in the very first post that the number was about 250 mv DC.

    What I forgot to mention was that I only have so much energy for rabbit holes.
    If the voltage is going to change that much, you're going to have to find it yourself.
     
  13. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    It is possible that your transformer is in close proximity to other sources of electromagnetic radiation an true to its nature transforms it, or you might just have an "optimistic" DVM.
     
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