Handbook for Sound Engineers

380 Chapter 13

mechanical arrangement of the contacts may be such
that when the relay is at rest, certain circuits are either
open or closed. If the contacts are open when the relay
is at rest (not energized) they are called normally open
contacts.

Relays are wound in many different manners, Fig.
13-12. Among them are the single wound, double
wound, trifilar wound, bifilar wound, and two coil,
which are nonelectromagnetic.

13.2.4.1 Dc Relays

Direct current (dc) relays are designed to operate at
various voltages and currents by varying the dc resis-
tance of the actuating coils, and may vary from a few to
several thousand ohms. Dc relays may operate as
marginal, quick-operate, slow-operate, or polarized.

A marginal relay operates when the current through
its winding reaches a specified value, and it releases
when the current falls to a given value.

In the quick-operate type, the armature is attracted

immediately to the pole piece of the electromagnet

when the control circuit is closed.

Slow-operate relays have a time-delay characteristic;

that is, the armature is not immediately attracted to the

pole piece of the electromagnet when the control circuit

is closed. To accomplish this a copper collar is placed

around the armature end of the pole piece. They differ

from the slow-release variety in that the latter type has

the copper collar around the end of the pole piece oppo-

site from the armature.

A polarized relay is designed to react to a given

direction of current and magnitude. Polarized relays use

a permanent magnet core. Current in a given direction

increases the magnetic field, and in the opposite direc-

tion it decreases the field. Thus, the relay will operate

only for a given direction of current through the coil.

A latching relay is stable in both positions. One type

of latching relay contains two separate actuating coils.

Actuating one coil latches the relay in one position

where it remains until it is unlatched by energizing the

other coil.

A second and more modern type is a bistable

magnetic latching relay. This type is available in single-

or dual-coil latching configurations. Both are bistable

and will remain in either state indefinitely. The coils are

designed for intermittent duty: 10 s maximum on-time.

The relay sets or resets on a pulse of 100 ms or greater.

Fig. 13-13 shows the various contact and coil forms.

13.2.4.2 Ac Relays

Alternating-current (ac) relays are similar in construc-

tion to the dc relays. Since ac has a zero value every

half cycle, the magnetic field of an ac-operated relay

will have corresponding zero values in the magnetic

field every half cycle.

At and near the instants of zero current, the armature

will leave the core, unless some provision is made to

hold it in position. One method consists of using an

armature of such mass that its inertia will hold it in posi-

tion. Another method makes use of two windings on

separate cores. These windings are connected so that

their respective currents are out of phase with each

other. Both coils effect a pull on the armature when

current flows in both windings.

A third type employs a split pole piece of which one

part is surrounded by a copper ring acting as a shorted

turn. Alternating current in the actuating coil winding

induces a current in the copper coil. This current is out

of phase with the current in the actuating coil and does

not reach the zero value at the same instant as the

Figure 13-12. Types of relay coil windings.

A. Single.

B. Double.

C. Trifilar.

D. Bifilar.

E. Two coil.

1 3

2 4

13

24

3 4

1 2

1 2

Magnet

135 246

Noninductive