Handbook for Sound Engineers

390 Chapter 13

(13-24)

13.2.6.4 Low Load Current Protection

If the load current is low, it may be necessary to take
special precautions to ensure proper operation.
Solid-state relays have a finite off-state leakage current.
SSRs also need a minimum operating current to latch
the output device.

If the off-state voltage across the load is very high, it
could cause problems with circuit dropout and compo-
nent overheating. In these applications a low-wattage
incandescent lamp in parallel with the load offers a
simple remedy. The nonlinear characteristics of the
lamp allow it to be of lower resistance in the off state
while conserving power in the on state. It must be
remembered to size the SSR for the combined load.

13.2.6.5 Optically Coupled Solid-State Relays

The optically coupled solid-state relay arrangement
(SSR) shown in Fig. 13-22A is capable of providing the
highest control/power-circuit isolation—many thou-
sands of volts in compact, convenient form. The triac
trigger circuit is energized by a phototransistor, a semi-
conductor device (encapsulated in transparent plastic)
whose collector-emitter current is controlled by the
amount of light falling on its base region.

A phototransistor is mounted in a light-tight chamber
with a light-emitting diode, the separation between
them being enough to give high isolation (thousands of
volts) between the control and power circuit.

The light-emitting diode requires only 1.5 V to ener-

gize and has very rapid response time. The power

circuit consists of a high-speed phototransistor and an

SCR for dc power source, as well as a triac for ac appli-

cation.

The relay not only responds with high speed but is

also capable of very fast repetitious operation and

provides very brief delays in turnoff. In some applica-

tions, the photocoupler housing provides a slotted

opening between the continuously lit light-emitting

diode and the phototransistor. On–off control is

provided by a moving arm, vane, or other mechanical

device that rides in the slot and interrupts the light beam

in accordance with some external mechanical motion.

Typical optically coupled SSRs have the following

characteristics:

13.2.6.6 Transformer-Coupled Solid-State Relays

In Fig. 13-22B, the dc control signal is changed to ac in

a converter circuit, the output of which is magnetically

coupled to the triac trigger circuit by means of a trans-

former. Since there is no direct electrical connection

between the primary and secondary of the transformer,

control/power-circuit isolation is provided up to the

voltage withstanding limit of the primary/secondary

insulation.

13.2.6.7 Direct-Coupled Solid-State Relays

The circuit shown in Fig. 13-22C cannot truly be called

a solid-state relay because it does not have isolation

between input and output. It is the simplest configura-

tion; no coupling device is interposed between the

control and actuating circuits, so no isolation of the

control circuit is provided. This circuit would be better

called an amplifier.

One other variation of these solid-state circuits is

occasionally encountered—the Darlington circuit. A

typical arrangement is shown in Fig. 13-25. Actually a

pair of cascaded power transistors, this circuit is used in

Figure 13-24. Metal-oxide varistor peak transient protector.

EV= DSMuIut

Manufacturer’s max

volt-ampere characteristics

Vp

VDSM

Peak

volts

Vnom maxV I–Apeak

nom rms

T

Metal-oxide varistor

surge suppression

R MOV

I

SSR IR > VP^ 

VDSM

Energy > VDSM × l × T

IR

VP

VDSM

Turn-on control voltage 3–30 Vdc

Isolation 1500 Vac

dv/dt 100 V/μs

Pickup control voltage 3 Vdc

Dropout control voltage 1 Vdc

One-cycle surge (rms) 7–10 times nominal

1 second overload 2.3 times nominal

Maximum contact voltage drop 1.5–4 V