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Burnt condenser

  1. Mar 28, 2009 #1
    Hi

    I am a physics teacher but I studied theoretical physics so I am not very good in experimental physics. Recently, I burnt a condenser but I have no idea why. I tried to charge it with about 10 volts. His capacity was 1000 microF and there was written 25 volts on it so what did I wrong? Must there be a resistance interconnected between the cables?

    Thanks for any help

    Ulrich
     
  2. jcsd
  3. Mar 28, 2009 #2
    Was it an electrolytic capacitor?If so this must be connected with the correct polarity(+to+).If not it may have been just a faulty capacitor.
     
  4. Mar 28, 2009 #3
    If not due to incorrect polarity, is there any chance before it fried that it actually read 2.5 volts? In which case, 10 volts exceeded its rating by 4 times?
     
  5. Mar 28, 2009 #4
    I dont now if it was an electrolytic capacitor but it had a cylindrical form so it was sure wrapped with a dielectric. The polarity was correct and the capacitor was not faulty. In fact I burnt 2 capacitors because I also thought that the first one was faulty. The same thing happened with the second. I also controlled the voltage with a voltmeter which also measured around 10 volts. The amperage was zero at first then increased slowly. This was when it burned. Its really amazing.

    How long does it go normally to charge such a capacitor? And is it dangerous to touch it when it is charged?
     
  6. Mar 28, 2009 #5
    I will check this next time I will be at school but I am almost sure that it was 25 volts. What is usual for such a condensers of about 1cm diameter (0.4 inches) and long about 2cm (0.8 inches)?
     
  7. Mar 28, 2009 #6
    :surprisedFrom the shape of it it sounds like it was an electrolytic.The charging time depends on the resistance times the capacitance and if you connected it straight to the supply with no resistor it should charge in a fraction of a second the current dropping to zero and not rising.Did you use a power supply?If so there may have been an a.c. output component which zapped the dielectric shorting it out.Try using a battery.
     
  8. Mar 28, 2009 #7
    Prior to connecting the next cap, connect your DC voltmeter to your power supply, adjust to 10 volts DC. Then put your voltmeter in AC mode and measure for any AC component that may be on the output of your power supply. It it only reads a small ripple voltage (in the milli-volts), all is well enough with your supply.

    Next, try turning your supply voltage up to 2.5 volts ONLY and see if the next cap fries. Also, make certain negative is connected to negative, positive to positive.

    Sometimes the decimal point is difficult to see (not to imply that it isn't actually a 25 volt rated cap). Also, there's a chance you received a batch that were marked incorrectly.

    One last note, I've seen some caps that have a weird way of denoting which end is negative and which is positive that is, there's an arrow pointing to the end that's negative, but they have the minus sign stamped on the positive end of that arrow! So right off the bat, you think the negative end is where you've spotted the minus sign when it's actually as per the other end of that arrow. I thought this worthy of pointing out per the problem you're experiencing.
     
    Last edited: Mar 28, 2009
  9. Mar 29, 2009 #8
    Yes it was a power supply but I connected it for sure on DC.

    Thanks Gnosis, Ill try this out. Hope it will work. I wouldnt like to fry another cap in front of my students...
     
  10. Mar 29, 2009 #9

    uart

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    Science Advisor

    My thoughts on this.

    1. A voltage rating of 2.5 volts is a very uncommon for an electrolytic capacitor. I think 5.6 volts is the lowest rating I've ever seen in a regular type, though I have seen ratings as low as 3.9 volts for supercaps (these were several Farads capacity meant for stuff like CMOS memory backup). Anyway I suspect that the OP is correct in thinking it was 25V rated.

    2. A bad batch of capacitors is a possibility (though a fairly remote one).

    3. This is what I think is most likely. Common electrolytics are constructed using Aluminium however Tantalum types are also available. Tantalum electrolytic capacitors generally have more stable capacitance values and lower series impedance (especially at high frequencies) when compared with Al types. Tantalum capacitors are not usually meant to be used for bulk storage in the way that Al types are often connected directly across a DC supply (they are more suited for use in applications like timing and filter circuits). In fact they often don't have enough internal impedance to protect themselves from damage if they are suddenly slapped across a "voltage source" and indeed they often fail to a short circuit in this event.

    So it may be that the capacitors you have there are these Tantalum types. In that case you need a series resistance of at least a few ohms in series before connecting to an already running power supply.
     
    Last edited: Mar 29, 2009
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