In Episode 27 of No Dumb Questions, we explain electrical capacitance in wire and cable components and demonstrate how it is measured in a testing environment.
Capacitance is the property of an electrical component or cable that quantifies its ability to store energy in an electrostatic field. Represented by the letter $C$ and measured in farads (most commonly picofarads, or pF, in wire specifications), capacitance occurs between two conductive elements separated by an insulating dielectric.
Three primary physical parameters determine a cable’s capacitance:
- Conductor Geometry: The surface area, shape, and size of the conductors.
- Conductor Spacing: The distance separating the positive and negative (or signal and shield) paths.
- Dielectric Constant ($\varepsilon_r$): The specific insulating material between the conductors.
A cable with higher capacitance holds more electrical charge and inherently opposes rapid fluctuations in voltage. In signal transmission lines, excessive capacitance can round off high-speed digital pulse transitions and cause signal attenuation.
To measure capacitance, New England Wire uses an LCR meter (measuring inductance, capacitance, and resistance). For a coaxial cable, test leads are connected between the center conductor and the outer braided shield. Triggering the meter measures the total capacitance across the entire length, which is then divided by the cable’s length to express capacitance as a standardized value, such as picofarads per foot (pF/ft).
Key Technical Takeaways
- Definition: Capacitance ($C$) measures the ability of two conductors separated by an insulator to store electrical energy in an electrostatic field.
- Unit of Measure: Expressed in farads (F), typically specified as picofarads per foot (pF/ft) or per meter (pF/m) in cable design.
- Design Drivers: Governed by conductor surface area, dielectric wall thickness/spacing, and the dielectric constant of the insulating compound.
- Electrical Behavior: Higher capacitance stores greater charge and resists rapid voltage changes, directly impacting high-speed signal integrity.
- Testing Procedure: Measured using an LCR meter connected across the signal path and ground/shield, normalized over total linear distance.
Coming up next…Episode 28: What is inductance?
Episode 27 Transcript – What is Capacitance?
Welcome to No Dumb Questions
Hello. Looks like it’s time for another episode of No Dumb Questions. Let’s find out what today’s question is.
All right. Today we are going to be talking about capacitance. What is capacitance and how is it measured?
Defining Capacitance and Farads
Capacitance is a measurement used to describe an object or component’s ability to store energy in the form of an electric field. Capacitance is typically represented with the letter C and described in units of farads.
Factors Influencing Cable Capacitance
Capacitance is measured between two conductors where the size and shape of the conductors, the insulation spacing between the conductors, and the dielectric constant of the insulation all factor in the amount of charge that an object can hold.
An object with high capacitance will hold more charge and oppose quick changes in voltage.
Measuring Capacitance with an LCR Meter
We measure capacitance here at New England Wire with an LCR meter. The LCR meter stands for inductance, capacitance, and resistance, with C standing for capacitance.
So an example that we’re going to do today is measuring a coax. When we measure a coax, we measure from the center conductor to the outer shield. So we’ll connect one component to the center conductor and the other terminal to the outer shield.
Once we are connected, we hit trigger on the LCR meter and it will return a value for the complete length of cable. So this coax is a ten-foot sample measuring 335 picofarads, so we will rate that as 33.5 picofarads per foot.
Well, thank you for joining us again on No Dumb Questions. If you have any more questions, please feel free to reach out.