Power Supply Inrush Current

Discussion in 'General Electronics Chat' started by KCHARROIS, Sep 8, 2014.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Hi All,

    So I designed an amp requiring a maximum current of 2.5 Amps. There for I have purchased a toroid transformer rated at 25VA at 7V AC at 3.57 Amps. So I thought the rating of the toroid was reasonable but I noticed that the inrush current charging the rectifier filter capacitors was 5 Amps even though the circuit is only outputting 2.5 Amps at most. The inrush current is only 0.001 second, should I be worried about this?

    Thanks,
    Kevin
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Transformers traditionally are pretty rugged devices, I would not worry in the least, it looks normal.
    Max.
     
  3. bertus

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  4. MrChips

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    Oct 2, 2009
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    I would say the transformer will be fine but I would watch the rating of the rectifier diodes.
    What is the size of the capacitor and the part # of the diodes? Is it a bridge rectifier or full-wave center-tap configuration or center-tap bridge bipolar supply?
     
  5. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Its a bridge rectifier, no center tap. I will have 2 capacitors in parallel rated for 20V 10 000uF which I believe is more then enough for what I need. I forget which diodes but there rated for 5 amps which that has me worried now I'll look at the rating since I assume the diodes have an inrush current rating. Should have mentioned that the 5 amp 1ms peaks are constant and not only at the beginning when the circuit gets turned on
     
  6. MaxHeadRoom

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    I don't see a transformer self destructing in .001 seconds, even if you cycled it rapidly the inrush would be much reduced due to the capacitor retained charge.
    Another indication is heat produced.
    Max.
     
    Last edited: Sep 8, 2014
  7. MrChips

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    Oct 2, 2009
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    2 x 10,000 uF = 20,000uF is a heavy AC load. Do you really need that much filtering?
    I would bump up the bridge rating or try it first with only one capacitor. Measure the DC and ripple voltage under full load and report back.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    The charging surge for the reservoir electrolytics can in extreme cases damage the rectifier.

    A lot of SMPSU circuits with the bridge rectifier connected directly to the mains, use a NTC thermistor to limit inrush, at room temperature the thermistor has high resistance, but as soon as you switch on; the current through it heats it up and lowers its resistance.

    I can't recall ever having seen a NTC in front of a transformer input - its far more usual to find a series current limiting resistor that's shunted out by a time delayed relay.
     
  9. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    My only concern is that I will damage the toroid transformer because of the high peaks of current I get when it charges the filter caps.
     
  10. MrChips

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    You are not going to damage the transformer. You are more likely to fry the bridge rectifier.
    Make sure you put an appropriately sized fuse on the input to the primary windings of the transformer.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Read the previous posts!.
    What do suppose is going to happen in .001 sec. ?
    It is heat caused by prolonged excessive current that usually destroys a transformer.
    Max.
     
  12. ian field

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    Oct 27, 2012
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    In *REALLY* extreme cases, the sudden rise in magnetic field causes the windings to contract.

    Most transformers are vacuum impregnated and cured in an oven to prevent any "vibration" in use, but as the OP mentioned; its a toroid with high peak currents - so its always going to be just that little bit nearer to pushing the envelope.

    What I'd suggest is; survey any similar designs that can be found online and study the measures other people have used.
     
  13. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    a modest sized choke and 1500 uf would filter about as well as your single large capacitance arrangment and would reduce the size of your surges.
    you should research such alternative filter methods.
     
  14. ian field

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    Choke-input filters went out with the valves - there's probably a very good reason for that.
     
  15. Kermit2

    AAC Fanatic!

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    cost was increased so the bean counters really frowned on iron core copper wound parts vs. newer(cheaper) large value electrolytics. Performance was not as big an issue as you might like to think... but suit yourself. i merely suggested the OP do some research.
     
  16. MaxHeadRoom

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    ...Also I think to some extent with the advent of smaller-portable-solid state devices, weight and size becomes an issue, however the R/L π filter is/was an efficient one.



    Max.
     
    Last edited: Sep 9, 2014
  17. ian field

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    With that bit of research - the OP would find out what those very good reasons were.
     
  18. Kermit2

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    care to share? You seem very sure that old is inferior. Is there a reason other than the cost/weight/size considerations we have already listed?
     
  19. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Thanks for expressing the concerns but I have found that in my case if I put small valued resistance such as 0.33 or 0.66 ohms works fine and limits the current.

    Thanks
     
  20. ian field

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    AFAICR; there was a fad that followed soon after the advent of solid state amplifiers, for getting the best possible transient response possible by the brute force & ignorance method of making the PSU as "stiff" as possible.

    The belief that any resistance *AT ALL* between the secondary and rectifier ruined performance, persisted and inevitably led to some amp builders letting the magic smoke out of said rectifiers and/or transformers.

    The method I described previously; a current limiting resistor in the primary circuit, that is shorted out by a relay after a short time delay became the most popular solution to inrush current, as once the relay contacts closed there was no unwanted resistance in the supply feed.

    I can't recall ever having seen a NTC thermistor used in the primary circuit of an amplifier - after all, they can't go down to 0 Ohms as they need some I2R to keep them warm.

    That little bit of resistance would go against the "holy scriptures"!
     
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