Reactance VS linear resistance

Discussion in 'General Electronics Chat' started by AcousticBruce, Mar 25, 2014.

1. AcousticBruce Thread Starter Active Member

Nov 17, 2008
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In a AC circuit with a CONSTANT frequency, can you use a capacitor as resistance to save power loss?

Is there a time you would not want to use a capacitor for that?

When would you WANT to use a capacitor for for reactance resistance?

My questions are based on a non-changing frequency in a circuit.

2. #12 Expert

Nov 30, 2010
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It is common practice in high frequency circuits to use 2 capacitors as a voltage divider. It seems that resistors would work as well, but as you said, capacitors have no proportional heat generated.

3. shteii01 AAC Fanatic!

Feb 19, 2010
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Well, the "resistance" of the capacitor in AC circuit is well known. It even has a name: Impedance. Usually designated by: Zc. Units of Impedance: Ohm, same as any Resistor.
So.
Zc=1/(sC)=1/(jwC)=-j/(wC)=-j/(2*pi*f*C) Ohm

4. Wendy Moderator

Mar 24, 2008
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Circuits use capacitors or inductors as ballast all the time, since they don't actually use any power. Fluorescent and LED circuits can both be prime examples.

Since there is no power conversion into heat as with resistors efficiencies tend to go way up and stay there

RunFromYourWife likes this.
5. AcousticBruce Thread Starter Active Member

Nov 17, 2008
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That sounds really interesting. How high frequency? Like radio?

I was looking at the impedance formula $Z = sqrt{R^2 + X_c}$ and calculating the Xc with $\frac{1}{2\pi \cdot f \cdot R}$

I noticed that you can use the $\frac{1}{2\pi \cdot f \cdot R}$ when there is no resister, but if there is a resistor, you must also use $Z = sqrt{R^2 + X_c}$

6. shteii01 AAC Fanatic!

Feb 19, 2010
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http://en.wikipedia.org/wiki/Electrical_impedance#Capacitor
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/accap.html#c1

RC circuits are a little different.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/rcimp.html#c1

7. #12 Expert

Nov 30, 2010
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I learned about those in the old vacuum tube TV's.
Maybe not relevant to todays TV's, but still a valid method to be considered if your design is leading you toward needing a high frequency voltage divider.

8. alfacliff Well-Known Member

Dec 13, 2013
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I used some surplus caps to slow some fans for video game cabinets, I dont remember the cap values, just paralelled some till the desired speed was achieved. this made them a lot more quiet. one thing to watch out for is getting the circuit near resonance, the voltage and current can go way up at that point.

9. daviddeakin Active Member

Aug 6, 2009
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Yes. The technique is sometimes called a "wattless dropper". Some circuits that are powered directly from the mains use a capacitor to drop the voltage in this way.

10. #12 Expert

Nov 30, 2010
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Unfortunately, that subject is taboo on AAC. There will be no further discussion of this point or the moderators will shut it down, quickly.