Handbook for Sound Engineers

388 Chapter 13

Fig. 13-20 shows three methods of operating a reed
switch using a coil. Fig. 13-21 shows four ways to
operate a reed switch using permanent magnets.

13.2.6 Solid-State Relays^9

Solid-state relays (SSRs) utilize the on–off switching
properties of transistors and SCRs for opening and
closing dc circuits. They also use triacs for switching ac
circuits.

13.2.6.1 Advantages

SSRs have several advantages over their electrome-
chanical counterparts: no moving parts, arcing, burning,
or wearing of contacts; and the capacity for high-speed,
bounceless, noiseless operation. Many SSRs are avail-
able that feature optical coupling; thus, the signal circuit

includes a lamp or light-emitting diode that shines on a

phototransistor serving as the actuating device. In other

types of SSRs, a small reed relay or transformer may

serve as the actuating device. A third type is direct

coupled and therefore not actually an SSR because there

is no isolation between input and output. These are

better called an amplifier. All three types are shown in

Fig. 13-22.

Ac relays turn on and off at zero crossing; therefore,

they have reduced dv/dt. However, this does slow down

the action to the operating frequency.

13.2.6.2 Disadvantages and Protection^4

Solid-state relays also have some inherent problems as

they are easily destroyed by short circuits, high surge

current, high dv/dt, and high peak voltage across the

power circuit.

Figure 13-21. Energizing a dry-reed switch with a permanent magnet. Courtesy Coto Technology.

S N

90 o

N

N

N

N

S

S S S N S N

S N S

N

S

Off

On

Hold

Off Off

On

Y +Y

Y +Y

N S

On

Y +Y

On

Off Off

Y +Y

X

Hold Hold

Hold Hold

A. Movement with the magnetic field

parallel to the dry-reed switch.

B. Rotational movement with a bar-shaped

permanent magnet.

C. Movement with the magnetic field

perpendicular to the dry-reed switch.

D. Rotational movement with two or

more ring magnets.

180 o 180 o

X

N

S

Off