Isolated solid state relay

A solid state relay includes a protective input stage at its control terminals to isolate the control circuit from line voltage or line current in the event of relay failure. The protective circuit includes a series combination of a voltage responsive element and a current responsive element. The current responsive element connects to the input or control terminal of the solid state relay and the voltage responsive element connects to ground. The interconnection of the current responsive element and voltage responsive element serves as the device input terminal for connection to a control circuit. In the event of relay failure, the voltage responsive element reacts to the increased voltage by providing conductivity, by way of the current responsive element, between the solid state relay input terminal and ground. The increased current flow opens the current responsive element and thereby protects the control circuit against the possibility of damage caused by current flow originating from the relay line voltage terminals.

FIELD OF THE INVENTION 
The present invention relates to relay devices and particularly to solid 
state relay devices with isolation against component damaging currents 
BACKGROUND OF THE INVENTION 
Traditional mechanical relay devices receive a control signal at a coil 
element and the coil element responds by producing a magnetic field for 
bringing together contact elements of the relay. The contact elements 
connect to nodes of a controlled circuit. A control circuit driving the 
coil element thereby determines conductivity, i.e., performs a switching 
function, between nodes of the controlled circuit. Such mechanical relays 
have suffered from relatively slow switching times and the need for a 
relatively large coil drive current to accomplish switching. Solid state 
relays address these concerns by providing high speed switching with 
relatively low drive current requirements. Accordingly, solid state relays 
are now a preferred form of control in many control applications. 
An important function of any relay is isolation between the control circuit 
and the controlled circuit. The relay provides a switching function 
between nodes of the controlled circuit, but no electrical connection 
exists between the control circuit and the controlled circuit. Thus, the 
control circuit may be a low power digital logic circuit with the relay 
device acting as a control interface to a much higher power circuit. For 
example, the control circuit could be a computer-based lighting control 
panel and the controlled circuit a high powered theater lighting system. A 
set of solid state relays interconnecting the control panel and the 
lighting system provide programmed control of the theater lighting system. 
An important aspect of the relationship between the control panel, 
operating at relatively low voltage and our rent levels, and the lighting 
system, operating at much higher voltages, is the electrical isolation 
provided by the solid state relays. If the high voltages or currents in 
the lighting system were introduced into the control panel, extensive 
damage to the control panel is likely. 
When a solid state relay receives excess currents, i.e., beyond specified 
ratings such as in a dead short situation, internal failures of the solid 
state relay can cause a short between the line voltage terminals, i.e., 
those connected to high voltage high current devices, and the input 
control terminals, i.e., those connected to low voltage low current 
devices. If this happens the line voltage can feed into the normally 
isolated control terminals to reach and destroy the control circuit. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the voltage between each input 
control terminal and ground is monitored and when this voltage exceeds a 
given voltage, conductivity between the control terminal and ground is 
established. The conductivity path includes a fusible link element which 
opens in response the abnormal current flow, thereby breaking the 
electrical connection between the control terminals and the control 
circuit. Accordingly, the control circuit is protected, in the event of 
relay failure, against damaging current flow originating from the line 
voltage terminals. 
According to a preferred embodiment of the present invention, a solid state 
relay including control terminals and line voltage terminals includes, for 
each control terminal, a series combination of a voltage responsive 
conductor and a fusible link. The fusible link connects to the control 
terminal of the relay and the voltage responsive conductor connects to the 
base of the solid state relay, or to earth ground. The interconnection of 
each fusible link and voltage responsive conductor is then available as a 
relay input terminal, i.e., in place of the corresponding control terminal 
of the solid state relay. Should the solid state relay fail and a large 
voltage develop at a control terminal, the associated voltage controlled 
conductor quickly establishes a conduction path to ground whereby 
sufficient current flows in the fusible link to blow the link and isolate 
the control circuit from damaging current.

DETAILED DESCRIPTION 
FIG. 1 illustrates schematically the context in which an isolated solid 
state relay in accordance with the present invention is utilized. The 
illustrated application of the present invention is a theater lighting 
control system, but it will be appreciated that the present invention 
finds application in a wide variety of control environments. In FIG. 1, a 
theater lighting system includes a control circuit 10 which may be a 
computer based programmable device having numerous operator controls (not 
shown) and programmable lighting sequences. Control circuit 10 selectively 
operates ones of a plurality of theater lights 12, individually numbered 
12a-12n in FIG. 1. Each theater light 12a receives power from a line 
voltage bus 14 comprising two electrical conductors 14a (power) and 14b 
(ground or neutral). The power terminals 16a and 16b of each light 12 are 
applied to respective ones of conductors 14a and 14b of line voltage bus 
14. Each power terminal 16a is coupled directly to the conductor 14a of 
line voltage bus 14. Accordingly, it may be appreciated that selective 
independent coupling of the other power terminals 16b of each light 12 to 
the conductor 14b of line voltage bus 14 provides selective actuation of 
each light 12. 
Selective coupling of conductor 14b of line voltage bus 14 to ones of the 
power terminals 16b is accomplished by isolated solid state relays 
20a-20n. For example, the isolated solid state relay device 20a operates 
the theater light 12a by selective coupling of the power terminal 16b of 
light 12a to the conductor 14b of line voltage bus 14. Each isolated solid 
state relay device 20 is coupled to control circuit 10 for independent 
operation thereof. In this manner, by virtue of the intermediate isolated 
solid state relay devices 20, independent control over each of theater 
lights 12a-12n is available. Each solid state relay device 20 comprises a 
conventional solid state relay 22 and an input stage protector circuit 24. 
In accordance with conventional solid state relay operation, each solid 
state relay 22 includes a pair of control input terminals, individually 
numbered C1 and C2, and a pair of line voltage terminals, individually 
numbered LV1 and LV2. Application of a control signal voltage across 
terminals C1 and C2, e.g., on the order of 0 to ten volts, results in 
selective conductivity between line voltage terminals LV1 and LV2, 
operating at AC voltages on the order of 120 to 240 volts. Thus, 
delivering a control voltage signal to terminals C1 and C2 selectively 
actuates the corresponding theater light 12. 
Conventional solid state relays 22 are, when exposed to excess current, 
subject to failure resulting in the possibility of a conductive path 
between the line voltage terminals and the control terminals. Heretofore, 
such failure potentially resulted in a conductive path between the control 
circuit, typically coupled directly to control terminals C1 and C2, and 
the line voltage bus 14. Such conductivity path, in most cases, results in 
severe damage to the control circuit 10. The present invention addresses 
the potential for such failure of the control circuit by provision of the 
input stage protective circuit 24 intermediate of the solid state relay 22 
and the control circuit 10. 
FIG. 2 illustrates details of the isolated solid state relay 20a and 
represents the other isolated solid state relays 20b-20n. In FIG. 2, the 
isolated solid state relay 20a includes the above-described solid state 
relay 22 coupled to theater light 12a. More particularly, line voltage 
terminal LV1 of relay 22 couples to conductor 14b of line voltage bus 14 
and line voltage terminal LV2 couples, by way of theater light 12a to 
conductor 14a of line voltage bus 14. Input stage protector circuit 24 
receives a control signal 26 from control circuit 10 and delivers the 
control signal to control terminals C1 and C2 of solid state relay 22. 
Under normal operating conditions, the control signal 26 received from 
control circuit 10 arrives unchanged at solid state relay 22. In the event 
of failure of solid state relay 22, however, input stage protector circuit 
24 isolates control circuit 10 from solid state relay 22 in order to avoid 
damage to control circuit 10. 
Input stage protector circuit 24 comprises a voltage responsive element 28 
and a current responsive element 30 for each of control terminals C1 and 
C2. Accordingly, a first voltage responsive element 28a and first current 
responsive element 30a are connected in series with the current responsive 
element 30a connected to control terminal C1 of relay 22 and the voltage 
responsive element 28a connected to a ground reference. A similar series 
combination of a second voltage responsive element 28b and a second 
current responsive element 30b are similarly coupled to control input C2 
of relay 22. The interconnection of voltage responsive element 28a and 
current responsive element 30a forms a relay 20a input terminal C1'. 
Similarly, the interconnection of voltage responsive element 28b and 
current responsive element 30b forms a relay 20 control terminal C2'. 
Optional resistors 32a and 32b may be serially interposed between the 
interconnection of corresponding elements 28 and 30 and the control 
terminals C1' and C2', respectively. Resistors 32 and 34 are used to 
prevent excess current flow from control circuit 10 for the case of a 
voltage spike at terminals C1' and C2' temporarily shorting element 28. 
Thus, the input stage protective circuit 24 replaces the control terminals 
C1 and C2 of relay 22 with control terminals C1' and C2'. Control circuit 
10 delivers to control terminal C1' and C2' the control signal 26 as it 
would for the solid state relay 22. 
Each series combination of voltage responsive element 28 and current 
responsive element 30 operates in a similar manner as follows. With 
respect to control terminal C1 of solid state relay 22, in the event of 
failure of solid state relay 22, a large voltage may develop at the 
control terminal C1 corresponding, potentially, to the voltage present on 
line voltage bus 14. Such excess voltage present at control terminal C1 
appears across voltage responsive element 28a. In response to this 
increased voltage across voltage responsive element 28, element 28 
provides a conductive path from control terminal C1 to ground by way of 
current responsive element 30a. In response to this large current passing 
through current responsive element 30a, current responsive element 30a 
opens the conductive path between control terminal C1 and control terminal 
C1' thereby isolating control circuit 10 from line voltage bus 14. In some 
cases, internal components of relay 22 may fail, before element 30a opens, 
due to the ground connection provided by element 28a. In either case, 
isolation from bus 14 is achieved. Accordingly, should excess voltage 
develop at either of control terminals C1 or C2, the input stage protector 
circuit 24 prevents damage to control circuit 10. 
In the illustrated embodiment of FIG. 2, the voltage responsive element 28a 
is a PN silicon transient voltage suppressor sold under the trade name 
Transzorb and is available from General Semi-Conductor Industries. In the 
preferred embodiment, the voltage responsive element 28a is a 
bi-directional element responding to excess voltages of either polarity. 
The current responsive element 30 is a fusible link.. Such fusible link 
may be provided by a conventional fuse or, preferably, a thin printed 
circuit board trace or integrated circuit trace adapted to open in 
response to a given threshold current. The Transzorb device was selected 
for its high speed reaction time to excess voltage. In selecting a 
threshold voltage for element 28, it need be greater than the normal 
operating voltages of the control signal 26. Thus, an acceptable threshold 
voltage for element 28 would be on the order of 20 volts for a control 
signal 26 operating at 0 to 10 volts. 
It may be appreciated that the faster the response time of the voltage 
responsive element 28, the less possibility of damage to circuit 10 in the 
event of relay 22 failure. Other devices which may be used as voltage 
responsive element 28 include metal oxide varistors, zener diodes, neon 
lamps, spark gaps, or any other device which has a high resistance until a 
specific voltage is created across the device. Again, because speed is 
critical in the operation of input stage protector circuit 24, the 
Transzorb device is the preferred embodiment of voltage responsive element 
28. 
Thus, an isolated solid state relay device has been shown and described. 
The isolated solid state relay device includes a conventional solid state 
relay device coupled in conventional manner to a line voltage bus and a 
controlled device, e.g., a theater light. In accordance with the present 
invention, an input stage protector device is provided at the control 
terminals of the solid state relay as an interface between a control 
circuit and the solid state relay. Should the solid state relay device 
fail and thereby provide a conductive path between the control terminals 
of the solid stage relay and the line voltage bus, the input state 
protective circuit 24 prevents excess current flow and/or voltage from 
reaching the control circuit. It may be appreciated that isolation of the 
control circuit is in effect accomplished by the voltage responsive 
element 28 providing a direct path to ground whereby little or voltage or 
current originating from the line voltage bus 14 may reach the control 
device 10. However, the current responsive element 30a is desirable as it 
prevents continued excess current flow into the ground connection provided 
by voltage responsive element 28. 
It may be appreciated that the present invention may be incorporated into 
pre-existing solid state relays by provision of a small circuit board 
mounted thereto and including the voltage responsive element 28 and 
current responsive element 30 as described above. Alternatively, the input 
stage protector circuit 24 may be incorporated integrally into the solid 
state relay at manufacture.