Calculating Joules from Watts for Microwave Capacitor

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Discussion Overview

The discussion revolves around the role and function of a .9 microfarad capacitor rated for 2100v in a microwave rated at 1800 watts. Participants explore the relationship between the capacitor's energy storage capacity and the microwave's power requirements, questioning the purpose of the capacitor and how it operates within the microwave's power supply system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants calculate the energy stored in the capacitor using the formula 1/2 CV², arriving at approximately 1.98 joules.
  • Others question the relevance of calculating capacitor energy, suggesting that the capacitor's role is not to power the microwave for extended periods.
  • There is uncertainty about how the microwave can draw more power than the capacitor can provide, with some participants noting that the power supply is not charging to voltages higher than its rating.
  • One participant explains that the microwave power supply provides high voltage AC current to a doubler circuit, which is then rectified for use in the magnetron.
  • Another participant suggests that the capacitor's purpose may be to smooth the output DC current/voltage, indicating that it is constantly charged and discharged.
  • Concerns are raised about the ripple effect if the capacitor is too small for the output requirements.
  • Some participants express a lack of knowledge regarding the formulas needed for further calculations.

Areas of Agreement / Disagreement

Participants express differing views on the function of the capacitor and its relationship to the microwave's power supply. There is no consensus on the calculations or the implications of the capacitor's energy storage capabilities.

Contextual Notes

Participants mention limitations in their understanding of the formulas involved in calculating the operational dynamics of the capacitor within the microwave system.

Samson4
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How can a microwave I took apart have a .9 microfarad capacitor rated for 2100v? The microwave is rated at 1800 watts. What am I missing?
1/2 CV*2=1.98 joules
 
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The capacitor won't be able to power it for a long time then (can you calculate for how long?), but that was never its purpose.
 
Samson4 said:
How can a microwave I took apart have a .9 microfarad capacitor rated for 2100v? The microwave is rated at 1800 watts. What am I missing?
1/2 CV*2=1.98 joules
I don't know why do you calculate capacitor energy. Do you know how a microwave power supply works?
 
mfb said:
The capacitor won't be able to power it for a long time then (can you calculate for how long?), but that was never its purpose.

I don't know the formula for such a calculation. How can it draw more power then the capacitor is able to provide? The power supply isn't charging to higher voltages than it's rated for.
 
zoki85 said:
I don't know why do you calculate capacitor energy. Do you know how a microwave power supply works?

I don't understand what you're asking me.

A microwave power supply works by providing a high voltage ac current to a doubler circuit. It then feeds the rectified current into a magnetron. I am definitely not the one to be answer question of the sort though.
 
Samson4 said:
I don't know the formula for such a calculation. How can it draw more power then the capacitor is able to provide? The power supply isn't charging to higher voltages than it's rated for.

Total energy stored in a capacitor is not the same thing as the power it is able to provide. I assume the purpose of this capacitor is to help smooth the output DC current/voltage? If so, then it's constantly being charged and discharged. The current and voltage during capacitor discharge should be nearly the same as they are at the peak of the AC signal (unless you have a poor quality filter circuit) so the output power should remain relatively constant with only a small ripple. Having too small of a capacitor for your output would greatly increase the ripple effect.

I'd provide formulas for determining all this, but I don't happen to know them. Looking around on google did net me the following link though. Maybe it can help?
http://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=4856&context=ujmm

I think the key point here is that the capacitor is only discharging for a short time before being charged again. If sized correctly, the drop in voltage and current between the start and end of discharge is small, so output DC power is relatively constant and equals the input power (ignoring losses).
 
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Samson4 said:
A microwave power supply works by providing a high voltage ac current to a doubler circuit. It then feeds the rectified current into a magnetron. I am definitely not the one to be answer question of the sort though.
Then, if you know how it works, I don't know what is bothering you...
 
zoki85 said:
Then, if you know how it works, I don't know what is bothering you...
I thought it was a power capacitor; that is what was bothering me. Drakkith already helped me, but thanks anyway.
 

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