Wire Selection
Wire Gauge & Material Selection
The wire used to wind an inductor directly affects its DC resistance, current capacity, and Q factor. Proper wire selection is essential for maximizing VIC circuit performance.
Wire Gauge Systems
Wire size is commonly specified using the American Wire Gauge (AWG) system:
| AWG | Diameter (mm) | Area (mm²) | Ω/m (Copper) | Max Current (A) |
|---|---|---|---|---|
| 18 | 1.024 | 0.823 | 0.0210 | 2.3 |
| 20 | 0.812 | 0.518 | 0.0333 | 1.5 |
| 22 | 0.644 | 0.326 | 0.0530 | 0.92 |
| 24 | 0.511 | 0.205 | 0.0842 | 0.58 |
| 26 | 0.405 | 0.129 | 0.1339 | 0.36 |
| 28 | 0.321 | 0.081 | 0.2128 | 0.23 |
| 30 | 0.255 | 0.051 | 0.3385 | 0.14 |
| 32 | 0.202 | 0.032 | 0.5383 | 0.09 |
Note: AWG follows logarithmic progression. Each 3 AWG steps doubles resistance, halves area.
Wire Materials
| Material | Resistivity (×10⁻⁸ Ω·m) | Relative to Copper | Use Case |
|---|---|---|---|
| Copper | 1.68 | 1.0× (reference) | Best for high Q |
| Aluminum | 2.65 | 1.6× | Lightweight applications |
| SS304 | 72 | ~43× | Corrosion resistance |
| SS316 | 74 | ~44× | Better corrosion resistance |
| SS430 (Ferritic) | ~100 | ~60× | Magnetic, high resistance |
| Nichrome (80/20) | 108 | ~64× | Heating elements, damping |
| Kanthal A1 | 145 | ~86× | High-temp resistance wire |
Effect of Material on Q Factor
Q Factor Relationship:
Q = 2πfL / R
Since R is proportional to resistivity, using high-resistivity wire dramatically reduces Q:
| Copper wire Q = 100 | → SS316 wire Q ≈ 2.3 |
| Copper wire Q = 50 | → Nichrome wire Q ≈ 0.8 |
When to Use Resistance Wire
Despite lower Q, resistance wire has valid uses:
- Current limiting: Built-in current limit without separate resistor
- Damping: Prevents excessive ringing
- Safety: Limits power in fault conditions
- Meyer's designs: Some original VIC designs used stainless steel wire
Warning: Using resistance wire in a resonant circuit dramatically reduces voltage magnification. A Q of 2 means you only get 2× voltage gain instead of 50× or 100× with copper.
Skin Effect
At high frequencies, current flows primarily near the wire surface:
Skin Depth (δ):
δ = √(ρ / (π × f × μ₀ × μᵣ))
For Copper:
δ(mm) ≈ 66 / √f(Hz)
| 1 kHz | δ ≈ 2.1 mm |
| 10 kHz | δ ≈ 0.66 mm |
| 100 kHz | δ ≈ 0.21 mm |
Skin Effect Mitigation
- Litz wire: Multiple thin insulated strands twisted together
- Flat/ribbon wire: More surface area for same cross-section
- Use finer gauge: If wire radius ≈ δ, skin effect is minimal
Magnet Wire Types
| Insulation Type | Temp Rating | Voltage Rating | Notes |
|---|---|---|---|
| Polyurethane (solderable) | 130°C | ~100V/layer | Can solder through coating |
| Polyester-imide | 180°C | ~200V/layer | Good general purpose |
| Polyamide-imide | 220°C | ~300V/layer | High temp applications |
| Heavy build (HN) | Various | ~500V/layer | Thicker insulation |
| Triple insulated | Various | ~3000V | Safety-rated isolation |
Wire Selection Guidelines for VIC
For Maximum Q (recommended):
- Use copper magnet wire
- Choose gauge based on skin depth at operating frequency
- Use largest gauge that fits the core/bobbin
- Consider Litz wire for frequencies >50 kHz
For Current-Limited Applications:
- Use stainless steel or nichrome
- Calculate required resistance: R = Vmax/Ilimit
- Accept reduced Q factor as tradeoff
Calculating Wire Length
Wire Length for N Turns:
lwire ≈ N × π × dcoil
Where dcoil is the average coil diameter.
Resulting DCR:
Rdc = ρ × lwire / Awire
VIC Matrix Calculator: The Choke Design tool automatically calculates DCR based on your wire gauge, material, and number of turns. It shows the resulting Q factor and voltage magnification for your design.
Next: Bifilar Winding Technique →