10. WINDING AND CONDUCTORS RESISTANCE
by Sam Belkin, MSEE
Lets see how to determine the primary winding resistance R1, the secondary circuit resistance R2 and the primary leakage inductance Ls1. their values. Resistance of the wire or the other conductor may be found from the following formula:
R = r x l / S,
where r is specific resistance in Ohm x cm2/cm,
for copper r = 1.7241 x 10-6 Ohm x cm2/cm,
for brass r = 3.9 x 10-6 Ohm x cm2/cm,
l is the length of the conductor in cm,
S is the cross sectional area of the conductor in cm2.
For example primary coil was wound by AWG 14 wire of 167.3 ft length. First step is to find the length. The value of 167.3 ft is equal 167.3 x 30.48 = 5099.3 cm. Next from the Wire Table we will find that the bare area for AWG 14 wire is 20.82 x 10-3 cm2. Now the resistance of this coil is:
R1 = r x l/S = 1.7241 x 10-6 x 5099.3/20.82 x 10-3 = 0.42 Ohm.
In another case we want to determine resistance of the brass beam between secondary terminal and electrode. The beam has a size 1.0" x 1.0" and the length between terminal hole center and electrode hole center is equal 8". For this case the variables will be:
l = 8 x 2.54 = 20.32 cm and
S = 1.0 x 1.0 x 2.542 = 6.45 cm2.
And the sought resistance
R = r x l/S = 3.9 x 10-6 x 20.32/6.45 = 12.29 x 10-6 Ohm.
Figure 4. The measurement of the winding resistance
In cases when the length of the wire is unknown the primary winding resistance R1 may be easily measured by a milliOhm meter or as shown on the schematic in Figure 4 as a ratio of measured voltage over current. Note that both the Ammeter and the Voltmeter are connected directly to the coil's terminal. All wires must be as short and as big AWG as possible, especially for the connection of Ammeter and coil. The best is if the Ammeter will be connected directly to the coil's terminal because any resistance between Ammeter and coil will distort the result. Resistance of electrodes or conductors in the secondary circuit may be measured according to schematic in Figure 1. The Voltmeter or DSO voltage probe must be connected across the measured part. The sought resistance will be equal to the ratio obtained voltage over current according to Ohm's law.
© Digiweld 1999