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 = V max /I limit 

 Accept reduced Q factor as tradeoff 

 

 Calculating Wire Length 

 Wire Length for N Turns: 

 l wire ≈ N × π × d coil 

 Where d coil is the average coil diameter. 

 Resulting DCR: 

 R dc = ρ × l wire / A wire 

 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 →