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LC Question - Hot Components

  1. Jul 10, 2009 #1
    So I have an LC circuit at resonance except the capacitor gets very hot.

    There is about ~2Amps AC going across it.
    The setup is:

    2x 0.1uF capacitors in parallel
    ~304uH inductor


    the ESR of the cap is 0.56ohms and the resistance of the inductor is ~0.7ohms.

    I was looking for the power rating of the capacitor but I couldn't find it.
    its the Kemet Ceramic Capacitor; 240C330C104KCR5TA. Rated for 500V, but I'm pushing in well below that.
    Its resonating at 20kHz.
     
  2. jcsd
  3. Jul 10, 2009 #2
    Looks to me like you're dissipating 2.24 watts in a surface mount capacitor. No wonder it's getting hot. P = I^2*R = 2^2*0.54
     
  4. Jul 10, 2009 #3
    But when I used another set of 5 capacitors in series (1uF, 0.04ohms ESR), they also got very hot as well. It works out to about 0.2 Watts.

    What is considered "a lot" for a regular ceramic capacitor anyways? I looked on the datasheet for this thing but it wasn't much help.

    Are there any high power capacitors I can use as an alternative?
     
  5. Jul 10, 2009 #4

    uart

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    - Is this series or parallel resonance?

    - What voltage are you driving the circuit with?
     
  6. Jul 10, 2009 #5
    I don't know what you're application is so I don't know if you can use other types of capacitors. Do a search on "low ESR capacitor", or "low ESR ceramic capacitor."

    To reduce loss from ESR you would place them is parallel if you don't exceed the voltage rating.
     
  7. Jul 10, 2009 #6

    berkeman

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    Staff: Mentor

    Capacitors will generally not have a power rating. They will have a voltage rating, and a ripple current rating. Do you see a ripple current rating in the datasheet?
     
  8. Jul 12, 2009 #7

    vk6kro

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    It also depends on how you are measuring the 2 amps. Most multimeters would tell you lies when the frequency is 20 KHz. The current could be a lot higher than you are seeing.

    Try putting a small resistor (10 ohms) in series with the coil / capacitor series combination. Measure the AC voltage across the resistor with an oscilloscope.
    You should be able to get better capacitors by accepting a larger size than surface mount.

    If the capacitor is getting hot, just reduce the drive until it doesn't get hot.
     
  9. Jul 12, 2009 #8

    f95toli

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    Gold Member

    It might be even more if the LC resonator has a Q>1 (the energy stored in a resonator depends on its coupling factor as well as the Q value).
     
  10. Jul 14, 2009 #9
    Whast the typical power rating of ceramic capacitors?
    I'm thinking of making a bank of about 10 caps
     
  11. Jul 14, 2009 #10

    berkeman

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    Staff: Mentor

    Please re-read my post #6 above.
     
  12. Jul 14, 2009 #11
    Can you give us the dimensions of the capacitors referred to above? You're dissipating 0.16 W in each one and they would have to be fairly small to get warm with that low a power. Again, how did you determine the 2 amps?
     
  13. Jul 14, 2009 #12

    vk6kro

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    Have a look at these capacitors at Radio Shack:

    http://www.radioshack.com/product/index.jsp?productId=2102589

    That is probably what you need to be using.

    Just curious, how are you getting so much power at 20 KHz?

    Surface mount components' heat dissipation depends on the size, but the smaller sizes of resistors have dissipations of 50mW to 100mW. So, although the dissipation of a capacitor is not given, it is probably similar to a SM resistor of the same size.
     
    Last edited: Jul 14, 2009
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