Handbook for Sound Engineers

Heatsinks and Relays 381

current in the actuating coil. As a result, there is always

enough pull on the armature to hold it in the operating

position.

An ac differential relay employs two windings

exactly alike, except they are wound in opposite direc-

tions. Such relays operate only when one winding is

energized. When both windings are energized in oppo-

site directions, they produce an aiding magnetic field,

since the windings are in opposite directions. When the

current through the actuating coils is going in the same

direction, the coils produce opposite magnetic fields. If

the current through the two coils is equal, the magnetic

fields neutralize each other and the relay is nonoperative.

A differential polar relay employs a split magnetic

circuit consisting of two windings on a permanent

magnet core. A differential polar relay is a combination

of a differential and a polarized relay.

13.2.5 Reed Relays6,7,8,11

Reed relays were developed by the Bell Telephone

Laboratories in 1960 for use in the Bell System central

offices. The glass envelope is surrounded by an electro-

magnetic coil connected to a control circuit. Although

originally developed for the telephone company, such

devices have found many uses in the electronics

industry.

The term reed relay covers dry reed relays and

mercury-wetted contact relays, all of which use

hermetically sealed reed switches. In both types, the

reeds (thin, flat blades) serve multiple functions, as

conductor, contacts, springs, and magnetic armatures.

Reed relays are usually soldered directly onto a circuit

board or plugged into a socket that is mounted onto a

circuit board.

13.2.5.1 Contact Resistance and Dynamics

Reed relays have much better switching speed than

electromechanical relays. The fastest Coto Technology

switching reed relay is the 9800 series, with a typical

actuate time of 100ȝs. Release time is approximately

50 ȝs. Actuate time is defined as the period from coil

energization until the contact is closed and has stopped

bouncing. After the contacts have stopped bouncing,

they continue to vibrate while in contact with one

another for a period of about 1 ms. This vibration

creates a wiping action and variable contact pressure.

Static contact resistance (SCR) is the resistance

across the contact terminals of the relay after it has been

closed for a sufficient period of time to allow for

complete settling. For most reed relays, a few millisec-

onds is more than adequate, but the relay industry uses

50 ms to define the measurement.

Another contact resistance measurement that has

provided great insight into the overall quality of the

relay is contact resistance stability (CRS). CRS

measures the repeatability of successive static contact

resistance measurements.

13.2.5.2 Magnetic Interaction

Reed relays are subject to external magnetic effects

including the earth’s magnetic field (equivalent to

approximately 0.5 AT and generally negligible), elec-

tric motors, transformers external magnets, etc., which

may change performance characteristics. Such magnetic

sources include one common source of an external

magnetic field acting on a relay or another relay oper-

Figure 13-13. Various types and pin connections for
latching relays. Courtesy Magnecraft Electric Co.

B

+ Reset



Set +

A. Dc single coil,

1 Form C contact

B. Dc single coil,

2 Form C contacts.

+ Reset

Set +

C. Dc dual coil,

2 Form C contacts.

D. Ac coil,

1 Form C contact.

Reset

Com Set

E. Ac coil, 2 Form C contacts.

Reset

Set

Com

+ Reset



Set +

7

7 7

7 7

2

2

5 6

5

1

1

1

4

4

6

6

3

3

9

3

9

(^45)
6
A B
B B
A B
A
A A
9
8
B
9