Field Coil Calculators
Electromagnet & Electrodynamic Speaker Handbook
SI units

Rewind for a New Supply

Restorers often want to run a vintage field-coil speaker from a modern low-voltage supply, or to replace an open-circuit field. Keep the bobbin, keep the ampere-turns and dissipation, change only the wire. This plate finds the new gauge and resistance.

Parameters

Original field resistanceR1R_1Ω
Original operating voltageV1V_1V
Original wire gaugeMeasure with a micrometer after scraping the enamelAWG
Mean length of turnℓMLT\ell_{MLT}π × mid-depth diameter of the windingcm
New supply voltageV2V_2V
Original · AWG 340.16 mmRewound · AWG 250.455 mm×2.84 Ø
Fig. 4Bare copper (bright) and enamel (dark ring) drawn at the same scale.
Rewind specification
New gaugeAWG 25
Exact gauge25.1
New resistanceR2R_226.7 Ω
New turnsN2N_21,793
Original turnsN1N_112,518
New currentI2I_2450 mA
Old → new NI793 → 807
Old → new P6.02 → 5.4 W
Wire needed251 m
☞ Ideal resistance at 12 V is 23.9 Ω. With the nearest standard gauge, NI changes by +1.8 %.

Gauge required to keep NI vs. supply voltage

101002025303540originalnewSupply voltage (V)Wire gauge (AWG)

How it works

Theory · assumptions · formulas

The scaling law

The original turns are recovered from the measured resistance: N1=R1A1/(ρ ℓMLT)N_1 = R_1 A_1 / (\rho\,\ell_{MLT}). For a full bobbin of fixed window, the number of turns is inversely proportional to the wire area, N∝1/AN \propto 1/A, so resistance goes as R∝N/A∝1/A2R \propto N/A \propto 1/A^2.

Keeping the ampere-turns constant while the voltage changes requires (see Plate II) NI=VA/(ρℓMLT)NI = VA/(\rho\ell_{MLT}) to stay fixed, hence

A2A1=V1V2,d2d1=V1V2,R2R1=(V2V1)2\frac{A_2}{A_1} = \frac{V_1}{V_2}, \qquad \frac{d_2}{d_1} = \sqrt{\frac{V_1}{V_2}}, \qquad \frac{R_2}{R_1} = \left(\frac{V_2}{V_1}\right)^{2}

and the power P=V2/RP = V^2/R is unchanged. A well-filled bobbin rewound for a different voltage runs at exactly the same temperature and produces exactly the same gap flux.

Real wire, real numbers

The calculator rounds to the nearest whole AWG and recomputes the turns using the insulated diameters, since thin wire carries proportionally more enamel. That is why the reported NI drifts a few percent from the original.

Low-voltage cautionA 12 V field drawing the original 6–10 W pulls almost an ampere. Use a regulated supply with adequate current rating and remember that thick wire makes a stiff, springy winding. Wind under tension and secure every layer.