Lighting fixture overload protector

A two-terminal A.C. device is described which can be series connected with a lighting fixture to prevent overheating of the fixture in an overload condition, namely, when a lighting element exceeding the power rating of the fixture is installed. In broad terms, the device includes a bidirectional, self-extinguishing switch which can be triggered to conduct current between the two terminals of the device, and control circuitry operable from the voltage difference and current between the two terminals occurring in use to regulate the triggering of the switch. The control circuitry includes triggering circuitry which generates triggering signals from the voltage difference across the terminals of the device and normally applies the triggering signals to the control terminal to permit a predetermined measure of conduction. In an overload condition detection circuitry detects an overload current and activates trigger signal suppressing circuitry which temporarily suppresses the application of triggering signals to the switch thereby reducing the mean level of the current delivered to the fixture in an overload condition. In an overload condition, the lighting fixture is operated in a dim or intermittent fashion thereby indicating the overload condition to the user.

FIELD OF THE INVENTION 
The invention relates generally to the regulation of power consumption by a 
lighting fixture or the like. 
BACKGROUND OF THE INVENTION 
The invention has particular though not exclusive application to recessed 
lighting fixtures which are commonly mounted in ceilings. Heat tends to 
build up in the interior of such a fixture, and manufacturers commonly 
specify with a prominent tag or label the maximum lamp wattage which is 
recommended for the fixture. Unfortunately, users will often overload a 
lighting fixture despite the manufacturer's warning, particularly when a 
lamp of lower, recommended wattage is not available. Such an overload 
condition can create serious risks of socket burn out and fire. 
A commonly adopted protection technique in the case of recessed lighting 
fixtures is to provide a thermal cut-off in the form of a bi-metalic 
strip. The bimetalic strip is fixed in thermal communication to the 
housing of the fixture and serves to discontinue application of line power 
to the fixture when overheating of the fixture is detected. Such a thermal 
cut-off is not entirely satisfactory as temperature gradients tend to form 
over the fixture which may depend in large measure on the surrounding 
materials. In some applications, the exact location of the bimetalic strip 
may become critical. Additionally, for many lighting fixtures the use of a 
bimetalic strip is not convenient, and to the inventor's knowledge there 
does not appear to be a device available for conveniently handling the 
problem of fixture overloading. 
BRIEF SUMMARY OF THE INVENTION 
The invention provides a two-terminal A.C. device which can be series 
connected with a lighting fixture operable from a line source of 
predetermined A.C. voltage to prevent overheating of the fixture in an 
overload condition. The device includes bidirectional self-extinguishing 
switching means for conducting a current between the two terminals of the 
device. The switching means have a control terminal at which a triggering 
signal can be received to initiate conduction. Control means are provided 
which operate from the voltage difference and current which occurs between 
the two terminals in use to regulate triggering of the switching means. 
The control means include triggering means which generate triggering 
signals from the voltage difference between the two terminals and which 
normally apply the triggering signals to the control terminals to permit a 
predetermined measure of conduction. For example, in the preferred 
embodiments the switching means is a triac which is normally triggered 
momentarily after the commencement of each half-cycle of the line voltage. 
The control means also include detection means which detect the magnitude 
of the current between the two device terminals and which cause trigger 
signal suppressing means to temporarily suppress application of the 
triggering signals to the control terminal of the switching means when the 
detected magnitude of the current exceeds a predetermined level. Because 
the lighting fixture is normally designed to operate from a predetermined 
line voltage (commonly 110-120 volts A.C.), the current delivered to the 
fixture is indicative of its power consumption, and consequently the 
predetermined level would normally be set to correspond to the 
manufacturers suggested power rating of the fixture. Thus, the mean value 
of the current otherwise delivered to the fixture in the overload 
condition is reduced. 
The temporary suppression of triggering signals is preferably a delay of 
triggering in each half-cycle of the A.C. line voltage whereby the 
lighting element is seen to glow only dimly, thereby indicating an 
overload condition. Alternatively, where for example a heat-sensitive 
current detecting device is employed, the triggering signals may be 
suppressed for an indeterminate period of time while the element cools. In 
such circumstances, a periodic or intermittent operation of the lighting 
element may be expected, also providing an indication of an overload 
condition, rather than burn out of the lighting element. In any event, the 
device will limit power consumption in an overload condition and restore 
itself for normal operation when a lighting element of proper wattage is 
provided.

In the schematic views of FIGS. 1-3 components performing an identical 
function in each of the three embodiments have been indicated with primed 
reference characters in FIG. 2 and double-primed reference characters in 
FIG. 3 unless otherwise indicated. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 schematically illustrates a first device 1 embodying the invention 
having a pair of terminals T1, T2 which are shown series connected with a 
lighting fixture L and a conventional on-off switch S for operation from a 
120 volt-60 Hz line supply. The components of the device 1 may normally be 
mounted in any suitable housing with the terminals T1, T2 exposed for 
connection to the wiring of the lighting system incorporating the fixture 
L. In such circumstances, the terminals T1, T2 may comprise conventional 
screw-down connectors. Alternatively, the device 1 may be incorporated in 
or may incorporate the switch S. As a further alternative, the device 1 
may be incorporated into a socket-type adaptor which can receive a pronged 
plug commonly provided on lamps and which itself has prongs insertable 
into a conventional wall power outlet. In such circumstances, the 
terminals T1, T2 would be connected between a prong of the adaptor which 
receives power from the wall outlet and a socket portion of the adaptor 
intended to receive a prong of the lamp plug. Such considerations apply 
equally to each of the devices described herein and are matters which will 
be readily apparent to one skilled in the art once the preferred 
embodiments have been described. 
The device 1 includes a bidirectional self-extinguishing switch, namely, a 
triac TR1 which serves to conduct current between the terminals T1, T2. 
Conduction in the triac TR1 self-extinguishes with each reversal of the 
polarity of the voltage difference between the terminals T1, T2, namely, 
at each half-cycle of the line voltage. The triac TR1 must consequently be 
re-triggered in each half-cycle of operation to ensure proper application 
of power to the lighting fixture L. To that end, a conductive loop 
consisting of a resistor R and a diac D is provided which connects the 
gate of the triac TR1 to the device terminal T1. In each half-cycle of 
operation the conductive loop effectively generates from the voltage 
difference across the terminals T1, T2 a triggering signal which is 
applied to the gate of the triac TR1. The diac D which is series connected 
with the gate of the triac TR1 simply provides a voltage threshhold which 
must be exceeded before a triggering current is received by the triac TR1 
and that threshhold voltage will normally be selected sufficiently low 
that the triac TR1 is conductive throughout almost the entirety of each 
half-cycle of operation. The threshold voltage provided by the diac D 
serves a function which is described more fully below in connection with 
the suppression of triggering signals in an overload condition. 
A manganin resistor R.sub.H series connects the triac TR1 to the device 
terminal T2. The manganin resistor is characterized by a substantially 
temperature-independent resistance value, and serves in this application 
as a heating element whose power output is directly proportional to the 
magnitude of the current conducted by the triac TR1. The manganin resistor 
is constructed in the form of a heating wire which is wound in thermal 
communication about a thermistor R.sub.T which has a negative 
resistance-temperature coefficient, that is, has a resistance value which 
drops as the temperature of the device rises. 
The thermistor R.sub.T connects the conductive loop at the junction of the 
resistor R and diac D to the device terminal T2. In normal operation, the 
resistance of the thermistor R.sub.T is sufficiently greater than that of 
the resistor R that the triggering function provided by the conductive 
loop is not impaired. However, in an overload condition, the resisted 
value of the thermistor R.sub.T drops in response to the heating effect of 
the resistor R.sub.H to an extent that the conductive loop is shorted at 
the junction of the resistor R and diac D to the device terminal T2 
thereby preventing triggering. The resistance characteristics of the 
thermistor R.sub.T are selected to ensure that a pronounced drop in the 
resistive value occurs when the current of a predetermined level is 
conducted by the heating resistor R.sub.C. This current level will 
normally be selected to correspond to the maximum power dissipation rating 
for the fixture L. 
The function of the diac D is to ensure suppression of the triggering 
signals in the overload condition. The thermistor R.sub.T has a finite 
impedance in the overload condition, albeit significantly lower than that 
of the resistor R and is not a true short. The resultant voltage divider 
effect produced by the series combination of resistor R and thermistor 
R.sub.T may result in a voltage being developed at the gate of the traic 
TR1 (absent the diac D) sufficient to deliver a current spike required to 
initiate conduction. The diac D effectively provides a voltage threshold 
which must be exceeded before triggering can occur. 
Thus, in an overload condition, the thermistor R.sub.T will effectively 
short the conductive loop to the second device terminal T1 thereby 
preventing further conduction by the triac TR1. Since the heating resistor 
R.sub.H no longer receives line current, the thermistor R.sub.T will no 
longer be heated and its resistance value will gradually rise. When the 
resistive value has increased sufficiently due to cooling, the conductive 
loop will once again trigger the triac TR1 for conduction. As a result, 
the lighting fixture L will be intermittently operative, indicating the 
existing overload condition. Once the lighting element in the fixture L 
has been replaced with one of proper wattage, normal operation can be 
resumed. 
FIG. 2 illustrates a device 2 which is an alternative embodiment of the 
invention. The device 2 differs from the device 1 essentially in the 
manner in which current detection and trigger signal suppression are 
effected, all of which will be described in greater detail below. 
Components of the device 2 which are essentially identical of those of the 
device 1 are labelled with the same but primed reference character and 
will not be described in detail in order to highlight the differences 
between the devices 1 and 2. 
In the device 2, the trigger signal suppressing means include a triac TR2 
which replaces the thermistor R.sub.T of the device 1. The triac TR2 when 
actuated effectively shorts the conductive loop consisting of the resistor 
R', capacitor C and diac D' to the device terminal T2'. A particular 
advantage of the triac TR2 is more predictable shorting of the conductor 
loop to divert a triggering signal. 
In the device 2, the means for detecting the current flowing between the 
terminals T1', T2' is a transformer. The transformer has a primary winding 
W1 which series connects the triac TR1' to the device terminal T2' thereby 
conducting the load current. The secondary winding W2 of the transformer 
is connected to the gate of the triac TR2. The transformer is preferrably 
formed as an iron rod on which a few turns of wire are wound to form the 
primary and secondary windings W1, W2. The windings ratio is so selected 
that when a predetermined load current is conducted by the triac TR1', the 
voltage developed across the secondary winding W2 is sufficiently large to 
produce the triggering current pulse required to initiate conduction in 
the triac TR2. A variable resistor R.sub.V which shunts the secondary 
winding W2 and which has a resistance value generally in the order of the 
secondary winding W2 effectively permits selection of the current level 
between the terminals T1', T2' which will trigger conduction in the triac 
TR2. 
The conductive loop of the device 2 used normally to trigger the triac TR1' 
includes a capacitor C, a phase-shifting impedance. The purpose of the 
capacitor C is to phase-delay conduction of the triac TR2 relative to the 
triac TR1 during an overload condition. The purpose of the phase shifting 
will be readily apparent from FIG. 4 in which startup of the device 2 in 
an overload condition is illustrated. The curve A represents current 
conducted by the triac TR1' (load current), and the current B, the current 
conducted by the triac TR2. No attempt has been made to indicate the 
relative scaling of the currents conducted by the triacs TR1', TR2'. 
Because of the phase delay, triac TR2 will still be conducting current in 
each cycle after the current in the triac TR1' has reached a 0 cross-over. 
Thus, the triac TR2 is conditioned to delay triggering of the triac TR1' 
in each half-cycle of the line voltage. This suppression of triggering in 
the triac TR1' will be apparent by comparing the solid sections of curve A 
(actual conduction in overload condition) with the stippled parts that 
indicate in a general way the conduction which would otherwise occur. It 
should be noted that no attempt has been made in view of FIG. 4 to 
indicate the effect of the diac D' which would also appear to delay 
triggering of the triac TR1' relative to zero cross-over in each 
half-cycle of the line voltage, although to a comparatively small degree. 
In normal operation, the conductive loop triggers TR1 in each half-cycle if 
the line voltage (substantially at a zero cross-over in the line voltage) 
thereby permitting substantially unfettered delivery of power to the 
fixture L'. In an overload condition, the load current is effectively 
detected in the transformer primary winding W1, and causes the secondary 
winding W2 to generate a voltage sufficient to produce the triggering 
current pulse required to fire the triac TR2. Once triggered, the triac 
TR2 shorts the conductive loop which normally initiates conduction in the 
triac TR1' for an interval in each half-cycle of the line voltage, thereby 
effectively phase-delaying conduction in the triac TR1' in each 
half-cycle. As a result, the mean value of the voltage applied to the 
lighting fixture L' (or alternatively viewed, the mean current delivered 
to the lighting fixture L') is significantly reduced. Thus, where an 
incandescent lighting element of excessive wattage in inserted into the 
lighting fixture L', the element is illuminating at a fraction of its 
normal level, indicating the overload condition. 
FIG. 3 illustrates a device 3 which is a third embodiment of the invention. 
Components of the device 3 which are identical to those of the device 1 
are indicated with the same reference character, double-primed. A triac 
TR2' serves substantially the same function as the triac TR2 of the device 
2 in suppressing triggering signals in an overload condition. 
Detection of the load current is effected by a temperature-sensitive barium 
titinate resistor R.sub.C. The resisitor R.sub.C series connects the triac 
TR1" to the second device terminal T2" thereby conducting the load 
current. The resistor R.sub.C is also connected to the gate of the triac 
TR2 so that the voltage difference developed across the resistor R.sub.C 
is applied to the gate of the triac TR2'. The gate of the triac TR2' is 
also connected by a second conductive loop containing a resistance R1 to 
the device terminal T1". The second conductive loop is capable, depending 
on the impedance of the resistor R.sub.C at any given time, of triggering 
the triac TR2' thereby depriving the triac TR1" of a triggering signal. 
The resistance characteristics of the resistor R.sub.C are as follows. The 
resistance has a relatively low, relatively constant value below a 
predetermined temperature which is commonly referred to as the Curie 
temperature. Above the Curie temperature, the resistor R.sub.C displays a 
very pronounced positive resistance-temperature coefficient, that is, the 
resistance increases in a relatively large fashion with increasing 
temperature. The characteristics of the resistor R.sub.C will be so 
selected that at a predetermined current level corresponding substantially 
to the maximum power rating of the fixture L" the resistor R.sub.C reaches 
substantially the Curie temperature. Thus any further increase in the load 
current causes a very dramatic increase in the resistance of the resistor 
R.sub.C. In the overload condition, the resistance of the resistor R.sub.C 
must be considerably larger than that of the resistor R.sub.1 so that 
triggering of the triac TR2' through the second conductive loop is 
enabled. 
In normal operation, the triac TR1" of the device 3 is triggered by the 
conductive loop consisting of the resistor R" and triac D" to permit 
conduction of load current throughout substantially the entirety of the 
half-cycle of the line voltage. In the overload condition, the overload 
current causes the resistance of the resistor R.sub.C to increase markedly 
thereby enabling triggering of the triac TR2' through the second 
conductive loop containing the resistor R1. Because of the presence of the 
diac D", in the overload condition, conduction of the triac TR2' preceeds 
in each half-cycle conduction by the triac TR" thereby inhibiting delivery 
of load current to the fixture L". This suppression of the triggering 
signal required by the triac TR1" continues until the resistor R.sub.C has 
cooled sufficiently. Thus, in the overload condition, the lighting element 
of the light fixture R" will be periodically or intermittently operated, 
thereby indicating the overload condition. 
Three embodiments of the invention have been illustrated. The device D1 is 
preferred for the low cost of construction. The device 2 is strongly 
preferred where a very precise detection and limiting of an overload 
condition is required. In particular, the device 2 is capable of reacting 
to an overload condition within 1 cycle of the line voltage. Additionally, 
in the overload condition, the device 2 will deliver a substantially 
reduced load current to add incandescent lighting element thereby causing 
the overload condition to be indicated by the relatively dim state of the 
lighting element. 
It will be appreciated that particular embodiments of the invention have 
been described and that modifications may be made therein without 
departing from the spirit of the invention and the scope of the appended 
claims.