Planar Spiral Coil Design Guide
A planar spiral coil is a flat winding made on copper foil. It is common in wireless power pads, RFID loops, sensors, and compact tuned circuits. The layout looks simple, but small geometry changes can move the inductance, resistance, and self resonant point. A calculator helps you test those changes before drawing the board.
Geometry Matters
Outer size, turn count, trace width, and spacing control the magnetic path. More turns normally raise inductance, because the field links the conductor many times. Wider copper lowers resistance, but it also reduces the open window when the outside size stays fixed. Narrow spacing increases fill ratio. That can raise inductance, yet it may also increase parasitic capacitance.
Electrical Limits
A coil is not only an inductor. It is also a resistor and a small capacitor. At low frequency, direct current resistance is often enough. At higher frequency, skin effect reduces the useful copper thickness. Proximity effect can add more loss when turns sit close together. Quality factor compares reactance with loss. A high value usually means a sharper tuned circuit and less heating.
Practical Use
Use the result as a layout estimate. Then check it with simulation or measurement. Board copper tolerance, plating, solder mask, vias, nearby metal, ferrite sheets, and temperature can change real performance. The formula works best for regular flat spirals with uniform pitch. Irregular shapes need stronger validation.
Design Tips
Start with the required inductance and the maximum board area. Choose a trace width that supports current without excessive temperature rise. Increase spacing when voltage, capacitance, or manufacturing rules demand it. Keep leads short, because leads add resistance and stray inductance. For power coils, inspect RMS current and copper loss. For resonant coils, compare operating frequency with the estimated self resonant frequency. Keep the operating point comfortably below resonance.
Review Workflow
Enter dimensions in one unit. Select the shape that best matches your layout. Set copper thickness, material, temperature, current, and frequency. Read inductance, resistance, reactance, Q, skin depth, power loss, and resonance together. Export the result when you need a design note or comparison sheet. Document assumptions beside exports. Future checks become easier. Engineers can review the same numbers months later with confidence.