How Can I Prevent My LC Circuit Capacitor from Overheating?

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Discussion Overview

The discussion centers around the overheating of capacitors in an LC circuit operating at resonance, specifically focusing on the power dissipation in ceramic capacitors and potential alternatives. Participants explore the implications of capacitor specifications, including equivalent series resistance (ESR) and ripple current ratings, as well as the effects of circuit configuration and measurement techniques.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes that the current through the capacitor is approximately 2 Amps AC, leading to significant power dissipation calculated using the formula P = I^2 * R, suggesting around 2.24 watts of heat generation.
  • Another participant mentions that using a different set of capacitors in series (1uF, 0.04 ohms ESR) also resulted in overheating, despite the calculated power dissipation being only about 0.2 watts.
  • Questions arise regarding what constitutes "a lot" of power for ceramic capacitors, with participants expressing difficulty finding relevant specifications in datasheets.
  • Some participants inquire about the type of resonance (series or parallel) and the voltage driving the circuit, indicating that these factors may influence the heating issue.
  • There is a suggestion to measure the actual current more accurately, as multimeters may not provide reliable readings at 20 kHz, and a recommendation to use a resistor in series for better measurement.
  • Participants discuss the absence of a specific power rating for capacitors, emphasizing the importance of voltage and ripple current ratings instead.
  • One participant suggests exploring low ESR capacitors and mentions that larger capacitors may perform better in terms of heat dissipation.
  • Another participant raises a question about the dimensions of the capacitors being used, implying that size may relate to the observed heating effects.
  • A link to a potential alternative capacitor from Radio Shack is provided, although the effectiveness of this suggestion is not confirmed.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the overheating issue, with no consensus reached on the best approach to mitigate the problem or the specific characteristics of capacitors that would be most effective. Multiple competing views on capacitor specifications and measurement techniques remain unresolved.

Contextual Notes

Limitations include potential inaccuracies in current measurement at high frequencies, the lack of clear power ratings for ceramic capacitors, and varying interpretations of what constitutes excessive power dissipation.

lankan_ice_405
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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.
 
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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
 
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?
 
- Is this series or parallel resonance?

- What voltage are you driving the circuit with?
 
lankan_ice_405 said:
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?

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.
 
lankan_ice_405 said:
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.

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?
 
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.
 
skeptic2 said:
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

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).
 
Whast the typical power rating of ceramic capacitors?
I'm thinking of making a bank of about 10 caps
 
  • #10
lankan_ice_405 said:
Whast the typical power rating of ceramic capacitors?
I'm thinking of making a bank of about 10 caps

Please re-read my post #6 above.
 
  • #11
lankan_ice_405 said:
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?

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?
 
  • #12
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.
 
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