Handbook for Sound Engineers

Heatsinks and Relays 389

Short-circuit and high-surge current protection is
performed with fast blow fuses or series resistors. A
standard fuse normally will not blow before the SCR or
triac is destroyed since the fuses are designed to with-
stand surge currents. Fast blow fuses will act on high
in-rush currents and usually protect solid-state devices.
Using a current-limiting resistor will protect the
SSR; however, it creates a voltage drop that is current
dependent and, at high current, dissipates high power.
A common technique for protecting solid-state
switching elements against high dv/dt transients is by
shunting the switching element with an RC network
(snubber), as shown in Fig. 13-23. The following equa-
tions provide effective results:

(13-19)

(13-20)

(13-21)

(13-22)

where,

L is the inductance in henrys,

V is the line voltage,

dv/dt is the maximum permissible rate of change of

voltage in volts per microsecond,

I is the load current,

PF is the load power factor,

C is the capacitance in microfarads,

R 1 , R 2 are the resistance in ohms,

f is the line frequency.

RC networks are often internal to SSRs.

13.2.6.3 High-Peak-Transient-Voltage Protection

Where high-peak-voltage transients occur, effective

protection can be obtained by using metal-oxide varis-

tors (MOVs). The MOV is a bidirectional voltage-

sensitive device that becomes low impedance when its

design voltage threshold is exceeded.

Fig. 13-24 shows how the proper MOV can be

chosen. The peak nonrepetitive voltage (VDSM) of the

selected relay is transposed to the MOV plot of peak

voltage versus peak amperes. The corresponding current

for that peak voltage is read off the chart. Using this

value of current (I) in

(13-23)

where,

I is the current,

Vp is the peak instantaneous voltage transient,

R is the load plus source resistance.

It is important that the VDSM peak nonrepetitive voltage

of the SSR is not exceeded.

The energy rating of the MOV must not be exceeded

by the value of

Figure 13-22. Various types of solid-state relays.

Control

circuit

Light

emitting

diode

Trigger

circuit

Triac Power

circuit

ac line

A. Optically coupled.

dc

control

circuit

dc to ac

converter

Transformer

Trigger

circuit

Triac Power

circuit

ac line

B. Transformer coupled.

Control

circuit

Trigger

circuit

Power

circuit

C. Direct coupled.

Photo

transistor

ac line

Triac

R 1

L

V

--- dv

dt

= u----- -

R 2

1 – PF^2

2 Sf

---------------------------- dv

dt

= u----- -

C^4 L

R 22

=------

C^4

R 22

-------- V

I

u---^1 PF

-^2
2 SF

= u-----------------------

if

Figure 13-23. Snubber circuit for solid-state relay

protection.

Supply voltage

l

Solid-state

switch C

R 1

Snubber

R 2 L

Load

Snubber protection

V

VDSM Vp–= IR