Problem with measurements

Discussion in 'The Projects Forum' started by André Ferrato, Jul 31, 2015.

  1. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    Hello, i am working or a circuit for a desk lamp and having some troubles with the transformer. First of all, when i bought my transformer, i had no idea that when it is submitted to a high current some voltage losses would appear... But the true problem is this:

    The specs of the transformer are 500mA max current, 127VAC -> 15VAC. First, when i measure with no load, 16VAC are present on secondary and the rectified and filtered through a 2200uF eletrolytic cap is 23.05V. When i attach a load of 90mA, the secondary presents me with a 15.3VAC and the rectified and filtered through a 2200.... shows a 17.8V. That is really weird and it's happening with other transformers also, can someone explain this to me ? If the secondary has 15.3VAC the rectified should be at least 20.6V right ? Already accounting the bridge drop.

    I tested a 200mA max current with the same step-down, when i submitted it to 100mA the drop was insane, 12.2VAC on the secondary and 13V on the rectified side!

    It may be something newbish i am doing or asking.
     
  2. ISB123

    Well-Known Member

    May 21, 2014
    1,239
    527
    You need voltage regulator.
     
  3. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    The voltage regulator is an IC, i need an explanation about my measurements.. the RMS value is not matching with peak values, as i said in the first post
     
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    There's something wrong somewhere?

    Like you say:

    16VAC > Bridge Rectified And Smoothed > 22VDC Approx
    15VAC > Bridge Rectified And Smoothed > 21VDC Approx
    12VAC > Bridge Rectified And Smoothed > 17VDC Approx

    What meter are you using?
     
  5. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    I am using a Hikari-1000, it's a cheap multimeter, but i never let me down in many projects. Yes... but there's also another problem:

    When i measured the 15.3VAC and then got a 17.8VDC on the rectified side i started thinking there was something wrong with the multimeter, so i thought: i'll attach a 7815 here at this point, the datasheet say it needs at least 18v to regulate it to 15v, so if there is 17.8VDC on the rail, i'll measure at the output of the 7815 something below 15v and it happened as i thought, i measured 14.7v at the output and 17.8 at the input of the 7815. This leaves me two things, the measurement of the AC voltage is wrong and my multimeter is about to be thrown on the wall OR something really weird is happening.
     
  6. ISB123

    Well-Known Member

    May 21, 2014
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    Generally after rectifying you should gain DC voltage.

    15VAC X 1.414=22VDC
    Use a voltage regulator to keep the voltage steady at 15V.
     
  7. ebeowulf17

    Active Member

    Aug 12, 2014
    678
    79
    Would a bad cap cause this problem perhaps? Does your meter have a capacitance test mode? Or do you have other, comparable caps you could try in its place?
     
    RamaD likes this.
  8. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    Dial the meter to AC and measure what you expect to be DC. When you see how large the AC component is, you will know where to look next. Is a capacitor bad or is your DC measurement trying to average the noisy DC?
     
  9. RamaD

    Active Member

    Dec 4, 2009
    254
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    Capacitor polarity too.
     
  10. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    I have a lot of caps here, maybe that is the problem, i'll test today and post the results here.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Sounds like the transformer is from an "intrinsically-safe" wall-wart. These are purposely designed to have internal current-limiting to prevent a fire in the event the transformer is overloaded.

    Interesting article about how these are being obsoleted by legistlative fiat.
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,523
    1,247
    There are several things going on here. The AC output voltage of a transformer decreases as the load on the transformer increases for several different reasons. Two of them are a) energy loss in the copper windings because they have resistance, and the voltage decreeases as the current increases per ohm's law; b) the transformer core has an impedance to energy transfer between the primay and secondary windings, and the more the load increases the more energy is lost in the core.

    Separate from that, there is voltage ripple on the output cap. This is an AC voltage superimposed on the DC voltage after the rectifier. As the load current increases, the DC voltage goes down and the AC ripple voltage goes up. Most meters do not read this correctly.

    ak
     
  13. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    Thanks for the article MikeML, i am reading it now. AnalogKid yes i am aware of the energy losses in the transformer, i compare the transformer to fat dude, when you make him run it loses performance. The capacitor issue i'll address it today and see how it goes, also i intend to buy a new multimeter.
     
  14. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    A few things:

    Transformers do have a parameter called regulation and it's sloppy.
    There is such a thing called an energy limiting transformer. Those kind of transformers can be shorted in the secondary with no damage.
    The AC and DC currents are not the same and are dependent on the type of filter and rectification (half or full wave).
    Winding resistance contributes.
    Diode drops contribute.
     
  15. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    Yes, i am aware of that, but not aware if my transformer is one of this kind. I did not test because i was fixing my bike, tomorrow i shall test with another rectification.

    To test the winding resistance i could connect a battery to the primary and a resistor, and account the drop in the resistor to find for the resistance right? Or a wheastone bridge maybe? Will this give a precise result, or i am misunderstanding the concept of the resistance? I know it changes with temperature and theres also a formula to correct it. If what i said is correct, the cooper loss would change with frequency right?
     
  16. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,143
    202
    Take a look here: http://www.hammondmfg.com/pdf/5c007.pdf It's not 100% correct either, but it works really well for tube circuits. The diode losses and winding losses are not accounted for.

    Usually the winding losses are very minimal and you are concerned with the DC resistance or what would be considered "copper loss". Transformer efficiency is very close to 100%.

    With the three terminal linear regulators like the LM317T, you require a 3V minimum difference between the input and output.
     
  17. André Ferrato

    Thread Starter Member

    Apr 5, 2015
    206
    1
    I kinda did not understand these formulas.. the numbers used at the formulas...
     
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