Brochure

ESG
8.578.57
8.578.57
8.57
I
L
=
28 volts
4 ohms
I
L
= 7 amperes
t
r
=
1
5
t
r
= 0,2 second
P =
7
2
x
0,0025
0,2
P = 0,613 watt
where:
IL = Load current in dc Amperes
L = Load inductance in Henry
tr = On/Off repetition rate in seconds
Example: A load with a resistance of 4 ohms and an
inductance of 0.0025 henry is driven from a 28 volt dc
supply while being switched on and off 5 times a second:
A protective diode or zener with a ¾ watt rating would
suffice.
The zener diode is the ideal choice for protecting low
voltage dc SSRs (less than 100 volts dc) used in parallel
with the output. In the forward current mode (reverse for
the SSR), the zener diode typically clamps as a single
diode would at approximately one volt, thereby provi-
ding added reverse-voltage protection. When two zeners
are used back-to-back (in series) with equal stand-off
voltages, they can be used to protect SSR outputs
bidirectionally when switching ac loads. At higher volta-
ges (greater than 100 volts) ac or dc, economics versus
performance may suggest another transient protective
device such as the MOV (metal oxide varistor) being the
most popular.
MOVs (Metal Oxide Varistors)
For more hostile environments, the MOV can be used as
follows:
across the incoming line to suppress external transients
before they can enter the system;
across the load to suppress load generated transients;
or, more frequently, across the SSR to protect it from all
transient sources. In the latter case, the MOV can be
conveniently mounted to the same SSR output termi-
nals as the load wiring. With the impedance of the load
in series with the MOV to limit current, a 30 joule unit is
usually adequate for brief spikes and also small enough
to be supported by its own leads.
If a MOV is connected directly across the power line, the
current limiting impedance will only be that of the power
generating source plus the wiring. In order to absorb the
possibility of high energy line transients form such a low
impedance source, the larger panel mount (300 - 600
joule) variety of MOV may be required. The greater
expense of such a device might be justified in that
suppression across the line is required in one place only.
Individual MOV specifications should be consulted for
precise information regarding energy absorption, clam-
ping properties and physical size, since the relationships
of these parameters will vary from one manufacturer to
the next.
Fuses
Semiconductor fuses are usually used in conjunction
wiht SSRs and are specialist fuses designed to protect
while operating at close to their full ratings.
They are sometimes referred to as current limiting fuses,
providing extremely fast opening, while restricting let
through current far below the available fault current that
could destroy the SSR. Although these fuses are not low
cost, they do provide a means of protecting SSRs against
high current overloads where survival of the SSR is of
prime importance.
The following are the main parameters used in the
selection of a semiconductor fuse:
• Fuse voltage rating
• Fuse current rating
• Available system fault current
• Fuse peak let through current
• Fuse total clearing (or let through)
• Surge withstand capability of the SSR
Diodes and Zeners
The diode shown across the load in A of Fig. 12 is the
most effective way of suppressing the possibly hundreds
of volts of bak EMF that can be generated by the coil at
turn-off. The disadvantages of this method are the SSR
is not protected from other transient sources, and the
dropout time of the load may be extended by several
milliseconds.
The general rule in the selection of protective diodes
and zeners is that their peak nonrepetitive (pulse)
current ratings (Fig. 12) should be equal to, or greater
than, the minumum load current. Conservative steady-
state power ratings for these devices may be ascertained
from the following equation:
P
Watt
=
I
L
2
L
t
r