Calculator
Example Data Table
| Case | Length mm | Width mm | Dielectric µm | Relative permittivity | Expected use |
|---|---|---|---|---|---|
| Small test roll | 500 | 40 | 25 | 2.2 | Learning prototype |
| Bench capacitor | 1000 | 50 | 20 | 2.3 | Low voltage test |
| Larger foil strip | 2500 | 75 | 15 | 2.7 | Higher capacitance build |
Formula Used
The calculator uses the parallel plate capacitance model.
C = ε0 × εr × A ÷ d
Here, C is capacitance in farads. ε0 is vacuum permittivity. εr is relative permittivity. A is active overlap area. d is dielectric thickness in meters.
For multiple sections, the adjustment is:
Ctotal = Cbase × parallel sections ÷ series sections
Stored charge uses Q = C × V. Stored energy uses E = 0.5 × C × V². Reactance uses Xc = 1 ÷ (2πfC).
How to Use This Calculator
Enter the total foil length and foil width first. Add the unused edge margin. This margin reduces the active plate width.
Enter dielectric thickness in micrometers. Use the separator thickness, not the foil thickness. Add the dielectric constant for paper, film, wax paper, plastic sheet, or another insulating layer.
Set the overlap percentage. Use a lower value if alignment is poor. Add plate pairs, series sections, and parallel sections if your roll design uses them.
Press Calculate. The result appears above the form and below the header. Use the export buttons to save CSV or PDF results.
Article: Aluminium Foil Roll Capacitance Design
What This Calculator Does
An aluminium foil roll can act like a simple capacitor. Two conductive foil surfaces store opposite charge. A thin insulating sheet separates them. This calculator estimates that capacitance from practical shop measurements. It uses foil length, foil width, dielectric thickness, dielectric constant, overlap, and section layout.
Why Area Matters
Capacitance rises when active plate area rises. A longer foil strip gives more area. A wider strip also adds area. Edge margin reduces useful width. Poor alignment reduces overlap. That is why the calculator includes both margin and overlap factor.
Why Dielectric Thickness Matters
A thinner dielectric gives higher capacitance. A thicker dielectric gives lower capacitance. This relation is direct and important. However, thin insulation may fail at higher voltage. Always choose insulation that matches the voltage and safety margin.
Relative Permittivity
Relative permittivity shows how well the insulating material supports electric field storage. Air is near one. Many plastics are around two to four. Paper, waxed paper, and films vary. Use measured data when accuracy matters.
Roll Geometry Limits
The calculator uses an ideal plate model. A real roll has edge effects, wrinkles, gaps, pressure changes, and lead resistance. These effects can shift the final measured value. Use the result as a design estimate. Then verify with an LCR meter.
Sections and Connections
Parallel sections increase total capacitance. Series sections reduce capacitance but can raise voltage handling. The calculator lets you model both choices. This helps compare compact designs before cutting foil.
Energy and Reactance
The result also includes stored charge, stored energy, reactance, and RC time constant. These values help with timing circuits, filters, coupling tests, and educational demonstrations. They also show why even simple capacitors deserve careful handling.
Safe Practical Use
Do not connect homemade foil capacitors to mains power. Avoid high voltage unless you understand insulation ratings. Add discharge resistors for stored charge. Keep layers clean and dry. Build slowly. Measure often. Treat every charged capacitor with respect.
FAQs
1. What is aluminium foil roll capacitance?
It is the capacitance formed by rolled foil plates separated by insulation. The foil acts as conductors. The dielectric stores electric field energy between them.
2. Should I enter foil thickness?
No. Enter dielectric thickness. Capacitance mainly depends on plate area, dielectric constant, and spacing between plates. Foil thickness affects resistance and handling more than capacitance.
3. What is active area?
Active area is the overlapping conductor area separated by dielectric. The calculator reduces width by edge margin and adjusts area by overlap percentage.
4. What dielectric constant should I use?
Use the value for your insulating material. Plastic films often range near two to four. Paper varies with moisture and coating. Manufacturer data is best.
5. Why is my measured capacitance different?
Real rolls have wrinkles, trapped air, uneven pressure, imperfect overlap, and lead effects. These practical details can change the measured value from the ideal estimate.
6. Can this calculator estimate voltage rating?
It does not calculate safe voltage rating. Voltage rating depends on dielectric strength, thickness, defects, margins, humidity, and construction quality.
7. What do series sections do?
Series sections reduce total capacitance. They may help distribute voltage in special designs. Use proper balancing and insulation for serious voltage work.
8. Is a homemade foil capacitor safe?
It can be safe for low voltage learning. It is not recommended for mains, high voltage, or critical circuits without proper testing and safety design.