Field coil calculators for the bench & the winding lathe.
Fifteen engineering plates for designing, rewinding and powering electromagnet field coils, from the pot magnet of a 1930s electrodynamic loudspeaker to laboratory Helmholtz pairs. Every plate has sliders, a live drawing, graphs you can scrub with a finger, and the theory spelled out underneath.
Tools
Select a calculatorBobbin Winding & Resistance
Turns, layers, wire length, DC resistance and copper weight that fit a given bobbin.
Ampere-Turns & Gauge Selection
Pick the wire gauge that delivers a target NI from a fixed supply voltage.
Magnet Wire Gauge Explorer
AWG diameters, resistance per metre, turns per centimetre and current rating.
Rewind for a New Supply
Re-gauge a field coil for a different voltage while keeping the same ampere-turns.
Pot-Magnet Gap Flux Density
Magnetic circuit of an electrodynamic speaker, with leakage and iron saturation.
Voice-Coil Force Factor Bl
Bl at rest, Bl(x) versus excursion, linear stroke Xmax and motor force.
Pole Tip & Bl(x) Throw
Fringe-field convolution of Bl(x), undercut T-pole symmetry and the 70 % throw limit.
Thick Solenoid Axial Field
Exact on-axis flux density of a multilayer air-core field coil.
Helmholtz Coil Pair
Uniform-field coil pair: centre field and ±1 % homogeneous region.
Field Coil Inductance
Air-core (Wheeler) or gapped iron-core inductance, stored energy and core flux.
Energising Transient & Flyback
Current rise, switch-off kick and decay with diode, resistor or Zener suppression.
Field Coil as Filter Choke
The classic CLC power supply: ripple attenuation, resonance and DC drop.
Field Supply Dropping Resistor
Series resistor to run a field coil at rated power from any DC supply.
Temperature Rise & Hot Resistance
Steady-state and warm-up temperature of a coil, plus the resistance-method thermometer.
Open-Baffle & Horn Tuning
Dipole cancellation, asymmetric baffle paths and the field current that sets Qts.
On field coils
Why an electromagnet?
Before Alnico and ferrite made strong permanent magnets cheap, loudspeakers, dynamos and motors drew their magnetic field from a winding of fine enamelled copper: the field coil. A few watts of DC through several thousand turns could magnetise a soft-iron pot to well over one tesla in the gap.
The double duty
Radio designers of the 1930s were thrifty. The speaker field was placed in series with the high-tension supply, where its inductance smoothed the rectifier ripple while the DC current that flowed anyway energised the magnet. Plates XI and XII treat that trick quantitatively.