Handbook for Sound Engineers

382 Chapter 13

ating in close proximity. The potential for magnetic
coupling must be taken into account when installing
densely packed single- or multichannel relays.

An example of magnetic interaction is shown in Fig.
13-14 where two relays, K1 and K2, with identical coil
polarities are mounted adjacent to each other. When K2
is “off”, relay K1 operates at its designed voltage. When
K2 is activated, the magnetic fields oppose so the effec-
tive magnetic flux within K1 is reduced, requiring an
increase in coil voltage to operate the reed switch. For
closely packed relays without magnetic shields, a
10–20% increase in operate voltage is typical, which
can drive the relays above their specified limits. The
opposite effect occurs if K1 and K2 are polarized in
opposite directions making the operating voltage for K1
less.

There are several ways to reduce magnetic interac-
tion between relays:

• Specify relays that incorporate an internal or external
  magnetic shield.


• Apply an external magnetic shield to the area where
  the relays are mounted. A sheet of mu-metal or other
  high-magnetic-permeability ferrous alloy 2–5 mils
  thick is effective.


• Provide increased on-center spacing between relays.
  Each doubling of this distance reduces the interaction
  effect by a factor of approximately four.


• Avoid simultaneous operation of adjacent relays.


• Provide alternating coil polarities for relays used in a
  matrix.

13.2.5.3 Environmental Temperature Effects

The resistance of the copper wire used in reed relay

coils increases by 0.4% /1°C rise in temperature. Reed

relays are current-sensitive devices so their operate and

release levels are based on the current input to the coil.

If a voltage source is used to drive the relays, an

increase in coil resistance causes less current to flow

through the coil, so the voltage must be increased to

compensate and maintain current flow. Industry stan-

dards define that relays are typically specified at 25°C

ambient. If the relay is used in higher ambient condi-

tions or near external sources of heat, this must be care-

fully considered.

For example, a standard relay nominally rated at

5 Vdc has a 3.8 Vdc maximum operate value at 25°C as

allowed by the specifications. If the relay is used in a

75°C environment, the 50°C temperature rise increases

the operate voltage by 50 × 0.4%, or 20%. The relay

now will operate at 3.8 Vdc + (3.8 Vdc × 20%), or

4.56 Vdc. If there is more than a 0.5 Vdc drop in supply

voltage due to a device driver or sagging power supply,

the relay may not operate. Under these conditions there

will be increases in operate and release timing to

approximately the same 20%.

13.2.5.4 Dry Reed Relays

Because of the tremendous increases in low-level logic

switching, computer applications, and other business

machine and communication applications, dry reed

relays have become an important factor in the relay

field. They have the great advantage of being hermeti-

cally sealed, making them impervious to atmospheric

contamination. They are very fast in operation and

when operated within their rated contact loads, they

have a very long life. They can be manufactured auto-

matically and therefore are relatively inexpensive. A

typical dry reed switch capsule is shown in Fig. 13-15.

In this basic design, two opposing reeds are sealed

into a narrow glass capsule and overlap at their free

ends. At the contact area, they are plated with rhodium

over gold to produce a low contact resistance when they

Figure 13-14. Adverse magnetic interaction. Courtesy Coto
Technology.

N

S

+

N

S

+

Relay K1 Relay K2

Figure 13-15. Construction of a switch capsule of a typical

dry reed relay—Form A. Courtesy Magnecraft Electric Co.

Supporting

terminal

Normally

open

contacts

Glass

capsule

Supporting

terminal