Handbook for Sound Engineers

282 Chapter 11

inductance decreases rapidly as the number of layers is
increased.

11.1.2.4 Winding Capacitances and Faraday Shields

To allow the maximum number of turns in a given
space, the insulation on the wire used to wind trans-
formers is very thin. Called magnet wire, it is most com-
monly insulated by a thin film of polyurethane enamel.
A transformer winding is made, in general, by spinning
the bobbin shown in Fig. 11-10 on a machine similar to
a lathe and guiding the wire to form a layer one wire
thick across the length of the bobbin. The wire is guided
to traverse back and forth across the bobbin to form a
coil of many layers as shown in Fig. 11-15, where the
bobbin cross-section is the solid line on three sides of
the winding. This simple side-to-side, back-and-forth
winding results in considerable layer-to-layer capaci-
tance within a winding or winding section. More com-
plex techniques such as universal winding are
sometimes used to substantially reduce winding capaci-

tances. These capacitances within the windings are rep-

resented by CP and CS in the circuit model of Fig. 11-13.

Additional capacitances will exist between the primary

and secondary windings and are represented by capaci-

tors CW in the model. Sometimes layers of insulating

tape are added to increase the spacing, therefore reduc-

ing capacitance, between primary and secondary wind-

ings. In the bi-filar windings of Fig. 11-14, since the

wires of primary and secondary windings are side by

side throughout, the inter-winding capacitances CW can

be quite high.

In some applications, interwinding capacitances are

very undesirable. They are completely eliminated by the

use of a Faraday shield between the windings. Some-

times called an electrostatic shield, it generally takes the

form of a thin sheet of copper foil placed between the

windings. Obviously, transformers that utilize multiple

layers to reduce leakage inductance will require Faraday

shields between all adjacent layers. In Fig. 11-15 the

dark lines between the winding layers are the Faraday

shields. Normally, all the shields surrounding a winding

are tied together and treated as a single electrical

connection. When connected to circuit ground, as

shown in Fig. 11-16, a Faraday shield intercepts the

capacitive current that would otherwise flow between

transformer windings.

Faraday shields are nearly always used in trans-

formers designed to eliminate ground noise. In these

applications, the transformer is intended to respond only

to the voltage difference or signal across its primary and

have no response to the noise that exists equally (or

common-mode) at the terminals of its primary. A

Faraday shield is used to prevent capacitive coupling,

via CW in Fig. 11-13, of this noise to the secondary. For

any winding connected to a balanced line, the matching

of capacitances to ground is critical to the rejection of

Figure 11-14. Layered windings.

Figure 11-15. Bi-filar windings.

= Primary = Secondary

= Primary = Secondary

Figure 11-16. High frequency equivalent circuit of a

transformer with Faraday shield and driven by a balanced

source.

CC 1

C 1 C 3

RG 1 RP Ideal LLS RS

CS Sec

RL

CL

Faraday

CC 2 Shield

C 2 C 4

RG 2

1 N

Xfmr

Pri CPRP