Plate Capacitors

Part of Energy Storage & Batteries

Flat plate capacitors built from foil, glass, wax paper, or mica can achieve useful capacitances for filter circuits, timing applications, and spark ignition systems.

Why This Matters

The parallel plate capacitor is the elemental form of all capacitors — two conductive surfaces separated by an insulator. Understanding its construction and performance gives you the ability to build capacitors from available materials for specific applications without relying on manufactured components.

In practical rebuilding work, capacitors appear in several critical roles: smoothing rectified AC to usable DC, timing oscillator circuits, blocking DC while passing AC signals, matching impedance in radio transmitters, and providing brief high-current pulses for ignition. Different applications require different capacitance values, voltage ratings, and loss characteristics. Building your own means selecting materials to meet these specifications.

The plate capacitor is also the best teaching model for understanding capacitance because the relevant parameters (plate area, plate spacing, dielectric material) are all directly visible and adjustable.

Capacitance Formula

For a parallel plate capacitor: C = ε₀ × εᵣ × A / d

Where:

• C = capacitance in farads
• ε₀ = permittivity of free space = 8.854 × 10⁻¹² F/m
• εᵣ = relative permittivity (dielectric constant) of the insulating material
• A = overlapping plate area in m²
• d = plate separation (thickness of dielectric) in meters

Design implications:

• Larger plate area = more capacitance (proportional)
• Thinner dielectric = more capacitance (inversely proportional)
• Higher dielectric constant material = more capacitance

Dielectric constants and breakdown voltages:

Material	εᵣ	Breakdown V/mm
Air	1.0	3,000
Dry paper	2.0–2.5	10,000–15,000
Wax paper	2.5–3.0	20,000–40,000
Glass (common)	6–8	8,000–12,000
Mica	6–9	70,000–200,000
Beeswax	2.7	25,000

For high-voltage capacitors, mica is dramatically superior to any other easily obtainable material — it combines high dielectric constant with exceptional breakdown strength. Mica occurs as thin, transparent sheets that naturally cleave to 0.1–0.5 mm thickness.

Flat Foil-Paper Capacitor Construction

Materials: Aluminum foil (or copper foil), wax paper (paraffin-impregnated newspaper or tissue paper), and connection leads.

Construction:

1. Cut two equal strips of aluminum foil: width W, length L (e.g., 5 cm × 50 cm)
2. Cut three strips of wax paper slightly wider and longer than the foil (add 1 cm on each edge to prevent shorting)
3. Stack in order: wax paper / foil strip 1 / wax paper / foil strip 2 / wax paper
4. Offset the two foil strips sideways by 1–2 cm so each extends beyond one side of the stack
5. Roll tightly into a cylinder, securing with tape or string
6. Attach connection leads to the two extending foil edges
7. Seal the assembly in melted paraffin or beeswax to exclude moisture and provide mechanical protection

Capacitance calculation for this example: A = 0.05 × 0.50 = 0.025 m²; d = 0.1 mm wax paper × 2 layers = 0.0002 m; εᵣ = 2.5 C = 8.854×10⁻¹² × 2.5 × 0.025 / 0.0002 = 2.76 nF

This is a small capacitance — suitable for radio tuning circuits. For larger capacitance, increase foil length, use multiple rolls in parallel, or use thinner dielectric.

Mica Capacitor Construction

For high-voltage or precision applications, mica capacitors offer superior performance.

Materials: Mica sheets (freshly cleaved to uniform 0.1–0.3 mm thickness), silver foil or aluminum foil plates slightly smaller than the mica, connection leads.

Construction:

1. Cleave mica sheets to uniform thickness using a thin blade along natural cleavage planes
2. Cut to size: mica 5 cm × 5 cm, metal plates 4 cm × 4 cm (leaving 0.5 cm border of mica on all sides)
3. Stack: mica / plate A / mica / plate B / mica / plate A / mica…
4. Alternating plates connect alternately to terminal A and terminal B
5. Clamp the stack tightly with wooden frames and bolts
6. Seal edges with paraffin

Capacitance per layer pair: A = 0.04 × 0.04 = 0.0016 m²; d = 0.0002 m (0.2 mm mica); εᵣ = 7 C_per_layer = 8.854×10⁻¹² × 7 × 0.0016 / 0.0002 = 497 pF

For 10 layer pairs: 4,970 pF ≈ 5 nF. Voltage rating: 0.2 mm mica at 100,000 V/mm = 20,000 V per layer — far exceeding typical needs.

Variable Capacitors (Tuning)

For radio receivers, a variable capacitor (trimmer or tuning capacitor) adjusts circuit resonance.

Air variable capacitor: Two sets of interleaved aluminum plates on a common shaft. One set is fixed; the other rotates. As the rotating plates mesh more or less with the fixed plates, effective overlapping area changes, varying capacitance smoothly.

Construction:

1. Cut 5–10 identical plates from thin aluminum sheet (0.5 mm thick)
2. Mount fixed plates on insulated spacers at equal intervals on one frame
3. Mount rotating plates on a shaft, aligned to mesh between fixed plates without touching
4. Bearing at each end of the shaft (wood, plastic, or brass)
5. Operating range: minimum (plates unmeshed, only fringe capacitance) to maximum (plates fully interleaved)

Practical capacitance range: A 10-plate variable capacitor with 5 cm × 5 cm plates, 2 mm air gap: 10–40 pF — appropriate for HF radio tuning (3–30 MHz).

Troubleshooting

High leakage: Moisture in the dielectric or on the surface of the capacitor causes current to flow between plates without flowing through the intended circuit. Dry thoroughly and reseal with paraffin.

Short circuit: Foil edges touching through thin spots in the dielectric. Visible as charred spots. Rebuilt with thicker or double-layer dielectric.

Low capacitance: Air gaps between foil and dielectric reduce effective contact area. Improve by pressing more firmly or using a spring-clamped construction rather than rigid frames.