Power Supply Filter Capacitor Overheating..... ??

Discussion in 'The Projects Forum' started by tmiuser, Dec 22, 2010.

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  1. tmiuser

    Thread Starter New Member

    Dec 22, 2010
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    OK, guys..this one has me stumped.
    I am definitely NOT that intelligent when it comes to Electronics, but everything SEEMS ok with this circuit....but the Filter Capacitor is getting really warm.....and I have NO idea why.

    A image of the circuit is located at:
    http://i52.tinypic.com/vcrqyd.jpg.....
    C5 is the one that gets really hot.....I have checked the supply across it, and can see nothing except nice 5VDC . No AC is visible at all. No HF noise..just a smooth 5VDC. but...that cap is getting WOW warm....the circuit operates great (for days..if desired), and I am only pulling about 86 mA total thru this supply....but, I know that the Cap is going to fail very soon...because it is so hot, it is certainly baking that sucker!!!
    (p.s...the Zener is also fairly warm, but...that seems more normal to me!!)

    Any Ideas??? I'd love to correct this!
    [​IMG]

    Looking forward to the GURU Responses!!

    -Steve:confused::confused::confused::confused::confused::confused:
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    What is the supply voltage? This seems like it is made for high AC voltage, but I will assume that it is connected safely, for example through isolation transformer. What is the load current?

    You said it there is no AC or noise, is that based on oscilloscope measurement or simple DVM?
    C4 is cool but C5 is hot, not the other way around? That would be much more like it, as C4 is the only part that limits voltage before it reaches R5.
    Does D1 get hot?
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Don't outrule a bad capacitor. Swap 4 & 5 to see what happens if you don't have an extra.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    C5 likely has excessive internal leakage. When electrolytic capacitors are left on the shelf for a long period of time, the dielectric basically gets dissolved by the electrolyte.

    Electrolytic capacitors can frequently be re-formed by charging them slowly to their rated voltage using a current-limited voltage source; for example a variable voltage DC power supply with a 5k or 10k Ohm resistor in series with the cap. The capacitor's dielectric is re-formed when the charging current falls below an acceptable value.

    What is the voltage rating of C5?
     
  5. tmiuser

    Thread Starter New Member

    Dec 22, 2010
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    SGTWookie:
    OK...C4, is actually rated at 100V
    C5 is (actually) rated at 25V, even though it says 50V on the schematic.
    C5 is a 're-used' capacitor from an old PC Power Supply...so, what you say could be true....I'll go ahead and replace it with some new ones I have.

    kubeek:
    The Input Voltage to the entire thing is 120VAC.....it is NOT connected thru an isolation transformer. It is connected directly to the AC Main Lines. (that is by design....the circuit is safely isolated from human contact.)
    The 'noise' measurement is based on O'Scope...The power looks remarkably clean........
    ...Yes, C4 is cool....and C5 is -holy crap- :eek::eek::eek:hot....
    D1 does get pretty dang warm...but I sort of expected that at the load current i pull.
    The Load current is: 23mA at 'rest'.......(nothing in circuit active....)
    and then....when I turn on the LCD backlight....81mA.

    81mA is the MAXIMUM amount that this entire thing draws.
    ...and, the capacitor getting hot...is WITHOUT backlight on LCD.
    (so...I am only pulling 23mA from the circuit.)

    You guys are great.....appreciate the input...hope to solve this Thread with your assistance!!

    -Steve
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    You MUST use a transformer or other inductive means to isolate the circuit from mains power. There is NO WAY to make a capacitively coupled mains supply safe, as capacitors very frequently fail shorted, which will expose a human user to mains power.

    You can purchase a small transformer very inexpensively. Funerals are VERY expensive.
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    You might need to increase the zener to 2W type.

    The C5 is definitely faulty if it's voltage is rated properly
     
  8. tmiuser

    Thread Starter New Member

    Dec 22, 2010
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    R!f@@:
    So, you believe a 2 watt Zener would be required for a 80mA Max Load?
    Seems a bit much.....
    I calculated it to be a .5 watt Zener, so I put a 1 watt in there....
    :confused:

    SgtWookie:
    This board is mounted -inside- of a 240VAC/120VAC distribution panel. The lethal areas of electrocution are much easier to touch within the panel than this board. (This board is completely contained within it's own little box)
    But, I do understand your point completely.
    Adding a transformer would basically defeat the purpose of having the this as a capacitive supply. (but, could be done, obviously....)
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    The Zener option is tht u said it heats up. A 2W will last longer.

    On second it could heat up due to the faulty capacitor, better replace it with a Low ESR type
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Replace that cap and you should be fine so long as the ones you've got in series with the main are rated to deal with main line electricity. Look for "suppression capacitors" as they're of quite a better build and also usually internally fused to some extent.
     
    Last edited by a moderator: Feb 11, 2013
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