Linear Power supply - Large current on the trafo side before the caps (schematic inside)

Discussion in 'Power Electronics' started by FurryLemon, Aug 30, 2016.

  1. FurryLemon

    Thread Starter New Member

    Aug 14, 2016
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    Hi there,

    Sorry for the vagueish title. I seem to be having a problem. The circuit is a variable dual polarity linear power supply.
    I have already built it physically and it is working. I have only been testing it with large loads so the current drawn is quite small. But everything works fine.

    However, I have since gone back to the simulation and have noticed that the current on the transformer side before the smoothing capacitor is very large when the load is quite small (draws a larger current), the current on the trafo's side is anywhere from 2A to 4A. My physical transformer is rated at 0.8A peak, ouch. The diagrams below show this large current. The blue wave is the current before rectification, the green is the current just before the smoothing capacitor.

    My final question is, what is causing this? the large capacitor? and how do I reduce this? I know that if you place a small resistor between the rectifier and capacitor that will reduce it, but at the same time that reduces the voltage quite a bit.

    hmmm tricky.

    Any help would be greatly appreciated.


    EDIT: Forgot to say, my transformer is rated at 22Vrms, at 0.6Arms. I am using lm317 regulators.
     
    Last edited: Aug 30, 2016
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    At startup the capacitor has no voltage and the voltage difference is at it largest and also the current.
    After some time, the capacitor gets the peak voltage of the input voltage and the current will drop.
    In your simulation you use 1N4148 Diodes wich are good for only 200 mA max.
    Better use diodes from the 1N540X series, wich can handle 3 Amps.

    Bertus
     
  3. FurryLemon

    Thread Starter New Member

    Aug 14, 2016
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    ah, of course, I = C*dv / dt , describes this behavior. Ahh, and thats why some gigantic transformers make that soft doosh! sound when powered on, large startup currents.

    In regards to the diodes in the simulation. I have used the 1n400A diodes rated at 1A.

    Okay, the thing is, is there a way to reduce the large current after startup? The current is still quite large and my trafo is only rated at 0.8A peak.
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    The 6800uF caps are an 'overkill' for 600mA.
    Try 1000uF and also add some serial resistance to your voltage sources.

    E
     
  5. #12

    Expert

    Nov 30, 2010
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    Are you asking about a simulation or reality?
    In reality, the current will drop as soon as the filter capacitors are charged. In simulation, I don't have a clue.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You could think of an NTC resistor.
    That has a high resistance at startup and lowers when heated (by the current).

    Bertus
     
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  7. FurryLemon

    Thread Starter New Member

    Aug 14, 2016
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    You're right, the current does drop, but it continues to be quite high, larger than 0.8A. I guess im talking about the simulation at the moment. With the prototype I've built I can't exactly see the variation in the current. I have no oscilloscope. I've been only testing on very small currents.
     
  8. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Are measuring the peak current ot RMS or Average current in your Sim.?
     
  9. FurryLemon

    Thread Starter New Member

    Aug 14, 2016
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    It is peak current I'm measuring. The image I uploaded in my first post shows a peak current of almost 3A after some time.
     
  10. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Your simulation looks perfectly normal for a transformer/rectifier/filter circuit. I see absolutely nothing to worry about, including the large current surge in the beginning; that's nothing more than the filter capacitors charging up during the first few power line cycles. The power transformer doesn't care about short-term, transient overloads, only sustained overloads that will result in overheating.

    You're fine. Quit worrying.
     
  11. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Hi FL,
    Basically your design is flawed, you have a 22vRMS transformer and your are trying to set the regulated Vout to +24v.?
    The resistive load is 50R, that is 480mA current load.

    The LM317 is showing only 1v drop, it requires at least 2v to 3v to ensure good regulation.

    The 6800uF caps will have to draw a high peak current for a very short time in order to keep the caps close to (22V * 1.414) - 2*0.7v diode drops.

    What is the purpose of the power supply.?

    E
     
  12. FurryLemon

    Thread Starter New Member

    Aug 14, 2016
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    huh? really? I understand the large current surge is due to the charging of the filter capacitors. But after the large current surge the current continues to peak at around +/-3A. And like I said my trafo can only supply 0.8A peak. This won't gradually harm any components?
     
  13. FurryLemon

    Thread Starter New Member

    Aug 14, 2016
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    It is a variable power supply for any circuit that I plan to build in the future.

    I'm trying to understand what you've written.

    So lets say the peak input current to the regulator is (22V * 1.414) - 2*0.7v = 29.7V

    The output voltage from the regulator needs to be, lets say 25V.
    But lets say the dropout voltage of the lm317 is around 1.5V. So 29.7- 1.5=28.2V

    so dv= 28.2 - 25 = 3.21 V.

    Now I= C * dv / dt

    2.2A =(6800uf * 3.21) / 0.01 (0.01 =1/100Hz)


    Is this what you mean ?
     
    Last edited: Aug 30, 2016
  14. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Rather than look at the current through the diodes, which never will accurately reflect the current through the transformer, better to look a the voltage and current into the LM317 after the bulk filter cap. The amount of ripple relative to the average voltage or current will tell you how hard the filter cap is working. The negative voltage peaks will tell you if the 317 is in danger of pulling out of regulation. The average value of the current will be much closer to the RMS value of the transformer secondary current.

    ak
     
  15. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi FL,
    If +/-18V is OK, look at this option.
     
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  16. OBW0549

    Well-Known Member

    Mar 2, 2015
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    No, it won't.

    In a circuit with a transformer, rectifier and filter capacitor, keep in mind that the rectifier diode typically only conducts during a very brief period right around the peak of the transformer output voltage. During the conduction period the current is normally very high, but that current persists for only a short time, and the average current (which is what will determine how hot the transformer gets) is no more than the average current being drawn out of the capacitor by the remainder of your circuit. (I don't know what the "0.8A Peak" spec on your transformer refers to, but I'm sure it doesn't refer to those brief 3A spikes on each half-cycle.)

    Again: your circuit, as far as I can tell, is behaving exactly as I would expect.

    EDIT: this web site illustrates some of what I've been saying.
     
    Last edited: Aug 30, 2016
  17. dannyf

    Well-Known Member

    Sep 13, 2015
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    What you observed is normal and has been known for decades by the audio folks.

    Essentially the bulk capacitor is a reservoir of energy: it gets discharged by the load and refilled at the peak by the transformer through the diodes.

    So if the loading is low, the capacitor is not as depleted and it's voltage remains high. That means that the transformer has a shorter period of time to charge up the capacitor, thus the peak current is high.

    When the loading is high, the capacitor gets more depleted and the voltage on the capacitor is low. This the transformer has more time to charge up the capacitor so the peak current is lower, everything being equal.

    What happens is that with high capacitance, and fast diodes, the power supply can become a significant source of HF and sometimes rf noise.

    Thus the use of soft recover diodes in high end audio amps.

    There is an aes article on this.

    PS: it shows you that 99.99 percent of the waveform shown in textbooks about rectification is all wrong. :) but that's another story for another day.
     
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  18. crutschow

    Expert

    Mar 14, 2008
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    I assume the peak 0.8A transformer limit you are referring to is the peak sine current of the 0.6Arms rating.
    There is a transformer limit but it's not the peak current per se, it's the RMS current which results from those high peak currents.

    The high peak currents, due to the filter capacitor, cause the RMS transformer current to be about twice the DC output current, thus you should derate the transformer RMS current rating about 50% as compared to the DC output current.
    This means you should draw no more than about 0.3ADC from your 0.6Arms transformer.
    (You can see this RMS current in the simulation by doing a CTRL left-click on the transformer current waveform title in the plot window.)

    If you include the actual transformer resistance in the simulation, that will reduce the RMS transformer current a small amount, since it reduces the peak current.
     
    Last edited: Aug 30, 2016
  19. FurryLemon

    Thread Starter New Member

    Aug 14, 2016
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    Yes you're assumption is correct

    Right Ok, In the simulation though, when I'm drawing 300mA, the RMS current on the transformers side is about 1.4A RMS. Which is more than 0.6A RMS.
     
  20. FurryLemon

    Thread Starter New Member

    Aug 14, 2016
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    Interesting. I don't suppose you know where the AES article is ?
     
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