Self Capacitance of Coils & Shielding: Calculate & Understand

In summary, the question is about self capacitance when shielding a coil. Some sources say grounding the conductive shielding affects capacitance while others say it doesn't. The questions also ask about calculating capacitance with and without an air gap, and if there is an online calculator for it. It is recommended to use copper tape as shielding with a dielectric layer between the shield and windings, and to avoid shorting the tape to itself.
  • #1
burnit
53
0
Hi,

I am reading conflicting things about self Capacitance when Shielding a Coil, can someone help..
IE: I have a coil of wire --say 25 turns, 250mm dia formed from looping in my hands & bound together with a non conductive thread.

I have read on some sites that there is no affect on Capacitance when the Conductive Shielding is Grounded & then on other sites they say it affects Capacitance grounded or not?

Now my questions are, how do i work out the coil to shield capacitance if i wrap a conductive shielding material around this to eliminate stray fields from interupting the received signals.

Firstly with no AIR gap between the coil & shielding material?

Then also by introducing an Air gap between the coil & the shielding material how does this effect the results?

Or is there an online Calculator to do this?

I understand i will have to leave a small gap in the ends of the shield material to avoid unwanted eddy currents circulating in it.

Thank You
 
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  • #2
You normally don't shield with a coil of wire, but copper tape. The capacitance you seem to be referring to developes between the sheild and windings. Place dialectric (tape) between the shield and outer winding. The capacitance is inversely porportional to the thickness of the dielectric of course.

By the way, don't short the tape to itself as it wraps around (This constitutes a single turn shorted secondary.) but overlap the ends with insulation between them.
 
Last edited:
  • #3


Hello,

Thank you for your questions about self capacitance of coils and shielding. I can understand how the conflicting information can be confusing. Let me try to clarify and provide some guidance on how to calculate and understand this concept.

Firstly, self capacitance is the capacitance between two parts of a single conductor. In the case of a coil, it is the capacitance between the individual turns of the coil. This capacitance is a result of the electric field that exists between the turns of the coil. When a conductive shield is added around the coil, it can affect the self capacitance in a few ways.

If the shield is grounded, it can act as a capacitor in parallel with the coil. This means that the total capacitance of the coil and shield will be higher than the self capacitance of the coil alone. This is because the electric field will now exist not only between the turns of the coil, but also between the coil and the shield.

On the other hand, if the shield is not grounded, it will not significantly affect the self capacitance of the coil. This is because the electric field will not be able to flow through the shield and will remain mostly confined to the coil.

To calculate the capacitance of the coil and shield, you can use the formula C = Q/V, where C is the capacitance, Q is the charge, and V is the voltage. You can also use an online calculator, but make sure to input the correct dimensions and materials for accurate results.

The presence of an air gap between the coil and shield can also affect the capacitance. If the air gap is small, it may not have a significant impact. However, if the gap is large, it can act as a dielectric and increase the capacitance between the coil and shield.

In terms of avoiding unwanted eddy currents, it is important to leave small gaps in the shield to prevent the flow of electric currents. These eddy currents can cause interference with the received signals.

I hope this helps to clarify the concept of self capacitance and shielding for you. If you have any further questions, please do not hesitate to ask. Thank you.
 

1. What is self capacitance?

Self capacitance refers to the ability of a coil to store electrical energy in the form of an electric field. It is typically represented by the symbol C and is measured in units of Farads (F).

2. How is self capacitance calculated?

The self capacitance of a coil can be calculated using the formula C = (π x d x n)^2 / (8 x l), where d is the diameter of the coil, n is the number of turns, and l is the length of the coil. This formula assumes that the coil is tightly wound and has a uniform diameter.

3. What is the purpose of calculating self capacitance?

Calculating the self capacitance of a coil is important for understanding its electrical properties, such as its resonant frequency and its ability to store energy. It can also help in designing and optimizing circuits that use coils, such as inductors and transformers.

4. What is shielding and how does it affect self capacitance?

Shielding refers to the use of a conductive material to surround and isolate a coil from external electric fields. Shielding can help reduce the self capacitance of a coil by preventing the electric field from extending beyond the coil's boundaries, thereby decreasing its capacitance.

5. How can I improve the self capacitance of a coil?

To improve the self capacitance of a coil, you can increase the diameter of the coil or decrease the length, since self capacitance is directly proportional to the diameter and inversely proportional to the length. Additionally, using a material with a higher dielectric constant or adding shielding can also help improve the self capacitance of a coil.

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