Abstract:
A 2-terminal solid state electrical switch is provided which can be connected in series with a load device in a the same manner as a conventional mechanical-contact switch, does not leak current during an “OFF” state, and operates from a dynamic pulse run mode during an “ON” state. The two-terminal solid state electrical switch of the present invention requires neither a power supply to operate, nor any mechanical movement and contact points. Consequently, no spark, arc or any mechanical noise is created in the solid state electrical switch&#39;s operation, nor does it corrode, thus allowing it to be used in a hostile environment. The solid state switch of the present invention can be put to uses not practical for conventional mechanical-contact switches, such as to control multi-appliances, as static circuit breakers, contactors and relays for fire-proof, explosion-proof, water-proof, anti-chemical, anti-corrosion, humidity resistant, dust resistant, anti-vibrations and heavy duty frequently operations. Further, a unique initialization circuit in the solid switch of the present invention resets the switch intelligently to a suitable operating mode after a power interruption, thus avoiding accidents that may endanger property and lives. The present invention also provides a highly isolated multi-point random remote control switch/relay suitable for wide industrial and other applications.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This patent application is related to and claims priority of U.S. Provisional Patent Application Serial No. 60/130,919, filed on Apr. 22, 1999 entitled “Solid State Electrical Switch”, also owned by the inventor of this patent application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrical switch. In particular, the present invention relates to a single-pole solid state electrical switch which can be directly connected to the AC power lines. 
     2. Discussion of the Related Art 
     The basic electrical circuit includes a power switch, and a load connected in series with the power switch, connected across the output terminals of a power source (e.g., an AC outlet). Typically, the power switch is a mechanical device which makes or breaks an electrical contact. The electrical contact is made or broken by a mechanical force provided either manually or through a magnetic field (e.g., a relay). A manually operated conventional mechanical switch typically toggles between “ON” (conducting) and “OFF” (non-conducting) states by a mechanism of levers and springs. Such a power switch has typically a low reliability and a short life-time, especially when operating in a hostile environment (e.g., inflammable explosive, high temperature, high humidity, dusty or corrosive atmosphere, and severe vibrations). Such a power switch is susceptible to failures resulting from electrical arcing, sparks, mechanical wearout, corrosion, wetting, contact welding or contact miss. Under some circumstances, a failure of a power switch can lead to fires and other industrial accidents, endangering property and lives. To improve performance of these conventional mechanical switches, expensive modifications and use of precious metals are often required. Such improved switches remain susceptible to wearing out and frequent maintenance. Load damage can result from a defective mechanical switch. 
     Typically, in a conventional power switch, if the power switch is in the “ON” state when the interruption occurs, the power switch does not reset itself to the “OFF” state after a power interruption. Under such circumstances, accidents can often occur when the supply of power is resumed unexpectedly. For safety reasons, in many heavy machinery, magnetic contactors are provided to reset the power switch. Such magnetic contactors typically are cumbersome, expensive and complicated, dissipate power and create “low frequency hum” noise. In areas where the power supply is noisy, i.e., where there are frequent transient “snap-offs” and “power slots”, the corresponding frequent resets required for conventional mechanical switches create significant inconvenience. 
     In a conventional mechanical circuit breaker, the electromagnetic tripping or/and thermal tripping mechanism for overcurrent protection is not designed for frequent operations. When used with conventional mechanical switches in an electrical circuit with many branches, such a circuit breaker does not individually provide overcurrent protection to branch circuits and terminal loads. Typically, in most home or office applications, a master circuit breaker provides overcurrent protection to a large number of switches, so that overcurrent in one circuit results in shutting off a large number of circuits protected by the same overcurrent protection. 
     For over-voltage protection, a solid state device of the prior art (e.g., a solid state switch) is typically protected by a protective device which can be either a varister or a special thyristor connected in parallel with the solid state switch. In the “off” state of the solid state switch, when a substantial over-voltage occurs (e.g., when the voltage across the solid state switch exceeds the “breakover” voltage of the thyristor), the protective device becomes conducting to limit the voltage drop across the solid state device, thus protecting the solid state switch from damage by the over-voltage. However, if the over-voltage persists, the high current in the protective device can generate sufficient heat to irreversibly destroy the protective device over time. Thus, such an over-voltage protection scheme is expensive both because of the cost of the protective device and also for the cost of replacing the protective device. 
     In addition, conventional switches are not practical for implementing multi-point “random” control (i.e., to allow switching “ON” or “OFF” of a piece of machinery at any one of multiple locations) beyond three control points, because of the complex switch logic and the large number of wires that are required. 
     Because of the cost and the above disadvantages of the conventional switch, a solid state electrical switch is long desired. However, until now, one fundamental technical problem has not been solved—a solid state electrical switch is necessarily an electronic circuit. As an electronic circuit, a DC power supply is typically required. In most integrated circuits, such a DC power supply operates at one or more lower DC supply voltages, such as 2.7 v, 3.3 v, 5 v, or ±5 v, ±12 v, . . . , ±35 v, . . . between power pins V CC  (or V DD ) and ground (or V SS ). Thus, unless a battery provides the supply voltages, a power supply circuit is necessary to provide the operating voltages. As a power switch, which is typically connected in series with a load, such a power supply circuit necessarily draws a current through the load, in the form of a leakage current. Such a leakage current, even though from several milliamperes to tens of milliamperes, in fact, operates the load under an “undervoltage condition.” While such a switch may still be acceptable, for example, as a electronic dimmer in a lighting application, such a switch would be unacceptable, especially from a safety point of view, in applications such as fluorescent lights, AC motors, transformers or other appliances. For example, under the safety standards in virtually all countries and recognized safety organizations (e.g., the Underwriter Laboratories), a power switch which allows a leakage current in the milliampere range or higher is considered unsafe. In fact, for this reason, dimmers and electronic timers, even though connected serially with the lighting, are considered electronic appliances or loads rather than power switches. In many applications, where safety is a paramount concern, an additional conventional mechanical switch is often required to be provided in series with the electronic dimmer or timer. 
     In the prior art, without exception, the electronic circuit of a 2-terminal solid state switch is connected in parallel to the load current-conducting component (e.g., between the two anodes of a triode AC switch, or TRIAC). Examples of these switches can be found, for example, in U.S. Pat. Nos. 5,550,463 and 5,030,890. Thus, these switches draw a significant current during the solid state switch&#39;s “off” state. From a switch current requirement point of view, any one of such 2-terminal solid state switches is not different from a solid state switch which draws a current through a third terminal directly coupled to the power source. 
     In addition, in the “ON” state of a solid state switch of the prior art, the voltage drop across the solid state switch (e.g., across a TRIAC), V on-sat , is typically 0.8 to 1.8 volts AC. Thus, the electronic circuit of the solid state switch, which is connected in parallel with the switch terminals (i.e., across V on-sat ), does not receive sufficient rail-to-rail voltage for proper operation. Alternatively, for example in U.S. Pat. No. 3,660,688 and 4,289,980, to obtain the operating voltages from the two terminals of the solid state switch, the voltage drops V on-sat &#39;s across the solid state switch can be maintained at the higher voltage ranges of 2.4-4.0 volts and 12-14 volts, respectively. However, in those solid state switches, the power dissipation can be significant. For example, if one of the solid state switches of U.S. Pat. Nos. 3,660,688 and 4,289,980 is used in series with a 120 volts, 5-amp light fixture, the power dissipation in the solid state switch would reach 12-20 watts, in one case, and 60-70 watts, in the other case! To handle such severe power dissipation, not only are bulky heat sinks required, the resulting low performance and insufficient operating voltages across the load render such solid state switches impractical and undesirable. 
     In FIG. 1 of U.S. Pat. No. 4,703194 to Brovelli (“Brovelli”), a 2-terminal network is disclosed. However, as in the prior art solid state switches discussed above, the main switch formed by the rectifier bridge (i.e., diodes  1 - 4 ) and the silicon controlled rectifier (SCR)  6  provide an “ON” state voltage drop V on-sat  of 2.4-4.0 volts. Thus, as in the solid state switches discussed above, a load current of 6 amperes would result in a power dissipation of 13-24 watts across the solid state switch. Further, to avoid SCR  6  from switching off when the AC voltage crosses zero volts, the “ON” state of Brovelli&#39;s solid state switch is maintained by the charge stored in capacitor  7 . Capacitor  7  maintains a voltage (e.g., 0.7 volts) exceeding the trigger voltage of SCR  6 . However, polarized capacitor  5 , which Brovelli requires a 1-μf electrolytic capacitor and performs a low-pass filtering function for the load current, cannot be used to sustain a load current exceeding one ampere. Under normal “OFF”-state operation, an electrolytic capacitor working on high voltage and high ripple current conditions, or a harsh surge power line, the leakage current flowing into the load can cause a breakdown, leading to undesirable and unpredictable results. 
     Further, to ensure that the solid state switch has high sensitivity, SCR  6  must be of high sensitivity also. Typically, because of the high sensitivity required, the gain of SCR  6  is relatively low, and thus can carry only a relatively small current (e.g., TIC106D SCR is rated for a current of about 1 ampere). In order to provide a higher current, a high power component, such as a TRIAC, must be included in the solid state switch. However, such a TRIAC would short the anode and cathode terminals of SCR  6 , draining charge from capacitor  7  at the gate terminal of SCR  6 , so that the “ON” state of SCR  6  cannot be maintained when the input voltage crosses zero. Thus, Brovelli&#39;s design cannot be used with practical currents, and cannot be extended to handle a larger current by simply including a high power component. 
     Furthermore, Brovelli&#39;s solid state switch is turned on and off by triggering highly sensitive SCRs  6  and  9  through small currents created in touch plates  15  or  14  through resistors  8  and  11  respectively. Currently commercially available high-sensitivity SCRs (e.g., Mitsubishi&#39;s CR02AM and CR03AM, and Motorola&#39;s MCR100-8 and TIC106D) all require at least 200 μA to trigger. However, when a human body contacts a touch plate, such as touch plate  14 , the impedance between the touch plate and ground through the human body can often be as high as 100 megaohm, thereby providing a current much lower than 200 μA and insufficient to trigger SCR 6  or SCR 9  to effectuate turning Brovelli&#39;s solid state switch “ON” or “OFF”. At other times, the resulting impedance between the ground and the touch plate through the human body can cause a current exceeding 200 μA, thereby causing electric shocks or raising other safety issues. For these reasons, Brovelli&#39;s solid state switch is deemed impractical. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fully solid state 2-terminal electrical switch (referred to as the “Liu Switch”), which can be used with a single pole application (i.e., the load and the switch are coupled in series to an AC power line). The Liu Switch is a static switch which does not include any mechanical or moving component, and therefore is not susceptible to wear and tear. As the Liu Switch does not include mechanical contact points, it does not create a spark, an arc, corrosion, or mechanical noise, and can withstand operations in a hostile environment, such as a high temperature, high humidity, corrosive, dusty or intensely vibrating environment. 
     In one embodiment of the present invention, the Liu Switch, which can be directly connected in series with a load and an AC power outlet, includes (a) a semiconductor switch device controlled at a control terminal by a control signal which determines whether the semiconductor switch is in a conducting or non-conducting mode; (b) a rectifier receiving an AC signal from the terminals of the semiconductor switch device during the non-conducting mode of the semiconductor switch; and (c) a control circuit including a capacitor which (i) is coupled to receive the rectified signal of the rectifier during the non-conducting mode of the semiconductor switch and (ii) is discharged in response to an electrical signal from a gain circuit coupled in parallel to said capacitor. 
     In one implementation, during the non-conducting mode of the semiconductor switch, the rectified DC signal maintains the capacitor in a fully charged condition. The semiconductor switch remains in the non-conducting mode until the electrical signal which causes the capacitor to discharge is received. The electrical signal which causes the capacitor to discharge can be, for example, an electrical signal associated with a button being pressed. After the capacitor is discharged, the rectified DC signal provides a charging current to bring the capacitor back to a fully charged state. This charging current then provides the control signal, in the form of a trigger signal, to put the semiconductor switch into a conducting mode. The conducting semiconductor switch causes the capacitor to discharge. However, at each zero-crossing of the AC signal, the semiconductor switch device momentarily becomes non-conducting again, so as to allow the rectified DC signal to charge the capacitor. The charging current then regenerates the trigger signal to put the semiconductor switch device back into the conducting mode. Thus, once the semiconductor switch device is in a conducting mode, a regenerative or feedback process provides a control signal (e.g., the trigger signal) that ensures that the semiconductor switch device remains in the conducting mode. 
     In one implementation, the control circuit further includes a second gain circuit responsive to a second electrical signal. The second electrical signal causes a signal path to be provided between the control terminal and a common ground of the control circuit, thus interrupting the feedback process by shunting the control or trigger signal to ground. 
     The control circuit further includes an initialization circuit having a capacitor (the “second capacitor”) coupled between the control terminal and the common ground. The second capacitor has a capacitance larger than the capacitance of the capacitor of the control circuit (the “first capacitor”). A forward-biased diode couples the control terminal to the second capacitor. A resistor connected in parallel with the second capacitor, in combination with the rest of the control circuit forms a circuit (“Liu&#39;s Network”) with multiple time constants. In one the embodiment, Liu&#39;s Network serves as both a state memory and an initialization circuit. At initialization (e.g., when power is first applied), the second capacitor of Liu&#39;s Network provides a large capacitance which absorbs the initial charging current of the first capacitor. Thus, the trigger signal that places the semiconductor switch device into the conducting mode is prevented. As a result, the Liu Switch remains in a non-conducting mode upon initialization. 
     Further, upon a power interruption occurring when the Liu Switch is a conducting mode, the Liu Switch remains in the conducting mode if the power resumes after a time period less than a predetermined time interval, and becomes non-conducting when the power interruption lasts longer than the predetermined time interval. Within the predetermined time interval, Liu&#39;s Network serves as a state memory which retains the conducting or non-conducting mode of the Liu Switch prior to the power interruption. In one implementation, the second capacitor is realized by an electrolytic capacitor and an unpolarized capacitor coupled in parallel. 
     In one implementation of the Liu Switch, the control circuit further includes a second gain circuit having a terminal which receives an external signal. Absent the external signal (e.g., a trigger signal generated by a button being pushed), the second gain circuit does not draw any current and has a high output impedance. 
     According to one aspect of the present invention, the Liu Switch further includes a zero-crossing detection circuit coupled to receive the rectified signal and coupled to the control terminal. The zero-crossing detection circuit prevents the control signal from being asserted except when the instantaneous magnitude of the rectified signal is below a predetermined voltage. In one implementation, the zero-crossing detection circuit includes a transistor which shorts the control terminal to common ground when the instantaneous magnitude of the rectified signal rises above the predetermined value. In one implementation, the zero-crossing detection circuit is implemented by a transistor controlled by an output signal of a voltage divider between an output terminal of the rectifier and a common ground. 
     In addition, a light-emitting diode (LED) and a Zener diode connected in series with the voltage divider can be included. The LED can serve as a “night light” to allow the electrical switch to be visible for certain applications. 
     According to another aspect of the present invention, the Liu Switch includes a current detector coupled in series with the load and the semiconductor switch to provide a signal indicative of the current in the current detector. In one embodiment, the Liu Switch further includes an overcurrent protection circuit which forces the semiconductor switch into a non-conducting mode when the current detector indicates a current exceeding a predetermined value. The current detector can be implemented by a transformer. In one embodiment, the overcurrent protection circuit includes temperature-sensitive components, so that the threshold for overcurrent protection circuit can be self-tracking and adapted in accordance with the temperature of the environment and the temperature of the Liu Switch itself. 
     In one embodiment, the overcurrent protection circuit includes (a) a rectifier receiving a signal indicative of the current in the current detector to provide a voltage signal which represents the current in the current detector; and (b) a threshold component which becomes conducting when the magnitude of the current in the current detector exceeds a predetermined value. The threshold component can be implemented by a silicon diode, a Zener diode or a four-layer Shockley diode. The rectifier of the overcurrent protection circuit can be implemented by a Zener diode, or a diode bridge. Further, the overcurrent protection circuit can include a resistor network between the rectifier and the threshold component. This resistor network can include temperature-sensitive devices (e.g., thermisters or other thermal devices) which compensate and further fine-tune the overcurrent protection circuit&#39;s temperature response. By appropriately selecting the temperature characteristics of the temperature-sensitive devices, the tripping condition of the overcurrent protection circuit can be automatically adjusted according to the temperature of the operating environment and the temperature of the switch. 
     In accordance with the present invention, the Liu Switch further includes an optocoupler which controls a Liu Switch in response to any one of multiple external signals received at various points of a control bus, thus providing “multi-point random control” to the Liu Switch. 
     In one implementation, the Liu Switch is provided by a diode bridge and a silicon controlled rectifier (SCR). In a second implementation, the semiconductor switch is implemented by a TRIAC. In a third implementation, the semiconductor switch is implemented by antiparallel silicon controlled rectifiers. The rectifier circuit of the Liu Switch can be provided by a SCR controlled bridge rectifier. A low-pass filtering circuit can be coupled to a signal terminal of the semiconductor switch device to further protect the semiconductor switch, by absorbing any surge, shock or noise in the control input signal, thus keeping the system in steadily operations. 
     The touch panels can each include a metallic surface, or a metallic surface coated with a resistive material or an insulator. The touch panel can be mounted in a plane offset from a mounting plate (e.g., in a shallow depression or provided slightly protruding over the surface of the mounting plate). In one implementation, where two touch panels (one for the “ON” function and one for the “OFF” function) are provided, the touch panels are provided different colors or provided different tactile feels. 
     According to another aspect of the present invention, the Liu Switch includes a beep circuit for providing an audible response, in the form of a “beep” sound, to the external agent when the agent contacts a touch panel. The beep sound response can be provided by a Zener diode and a piezoelectric speaker connected in series across an output terminal of the rectifier of the Liu Switch and a common ground. The beep circuit can generate audible and distinguishable beep sounds to indicate which of the two touch panels is contacted. 
     In the control circuit of the Liu Switch, the various components (e.g., the gain circuits, the SCR controlled rectifier, the semiconductor switches, or the audio response circuit) are each selected such that, during the “off” state of the semiconductor switch, the leakage current in each component is negligible. Consequently, negligible power is drawn by the Liu Switch during its “off” state. 
     Another advantage of the present invention is a state-latched control contact panels which retain the “ON” or “OFF” state after contact by the external agent is broken. Based on this latch function, the Liu Switch of the present invention provides a multipoint random remote control system, including: (a) a 2-terminal Liu Switch coupled in series with a load circuit between two lines of an AC power outlet; (b) an optocoupler coupled to the Liu Switch, the optocoupler providing a very high electrical isolation between the AC power lines and an external remote control signal bus from which the optocoupler receives input signals; and (c) controllers (e.g., computers) coupled to the signal bus, each capable of asserting on the signal bus the control signals. The Liu Switch provides an “ON/OFF” latching feature which allows random control by an unlimited number of external controllers and computers. In one implementation, the signal bus include an independent external common ground to which both the “ON” signal and the “OFF” signal reference. In another implementation, separate independent external common ground references are provided for separate transmission and isolation between “ON”-channel and “OFF”-channel on a four-wire external signal bus. 
     One aspect of the present invention provides a Liu Switch with no current leakage to the load in the “OFF”-state. Another aspect of the present invention provides a Liu Switch operating under a fully dynamic run mode during the “ON”-state. The switch of the present invention operates from power received during the zero-crossing of each half-cycle of the input AC signal, thereby obviating a DC power supply. Thus, unlike any electronic switch in the prior art, the Liu Switch connects in series directly to the AC standard power line. 
     Another advantage of the present invention provides a static overcurrent tripping circuit and an automatic reset circuit in a Liu Switch. Thus, inexpensive independent overcurrent protection is provided at every load point. Independent overcurrent protection at every load point provides unsurpassed protection for property and lives. 
     Another advantage of the present invention is a universal Liu Switch capable of being directly connected to standard 120 volts, 220 volts or higher voltage AC power outlet. 
     According to another aspect of the present invention, an initialization and power recovery reset circuit (“Liu&#39;s Network”) is provided, including: a capacitor connected in series with a diode between a control terminal and ground, and a resistor connecting in parallel with the capacitor. 
     Another advantage of the present invention provides “optional functions” in a Liu Switch. Such optional functions include providing a night light or a visible indicator on the Liu Switch. The night light or indicator function can be achieved using passive fluorescent materials, such as some chemical compounds of phosphates or sulfurs. 
     In one embodiment, an efficient LED is incorporated into a zero-crossing detecting circuit of the Liu Switch. In that embodiment, the forward voltage of the LED provides a threshold level to the zero-crossing circuit, and the LED provides a night light to illuminate the switch. In “OFF” state, the LED draws a current drawing less than 200 μA. 
     Another advantage of the present invention provides a Liu Switch with a programmable dynamic threshold value for overcurrent tripping protection. The threshold value adapts to temperature, loading and environment conditions, and the configuration of the switch itself. LTS character especially can be use to create a smart terminals in electrical network. 
     The touch panels of the present invention is based on a discovery of an impedance effect related to the human body, called the “Liu&#39;s touch signal complementary effect” (referred below as the “LTS” effect). The LTS effect result from the impedance properties of the human body, i.e., acting both as an impedance to ground and as an equivalent inductive signal source, over a wide range of environmental conditions. The LTS effect allows the Liu Switch to be reliably switched “ON” and “OFF” over practically all environmental conditions by a human through the touch panels. 
     In that embodiment, a touch panel is electrically couple to the control circuit of the Liu Switch, such that when the touch panel is contacted by an external agent (e.g., a human operating the switch), an electrical path is created, and the LTS effect triggers the control circuit of Liu Switch. 
     According to on the aspect of the present invention, the “ON” and “OFF” touch panels of the Liu Switch can be created to be different in position, color, shape, or texture, so as to ensure safety and precision operations. Such touch panels can be made from metal, resistive non-metals or conductors plated with an insulating material. In a Liu Switch of the present invention, different contact durations at the touch panels are required to trigger the Liu Switch&#39;s “ON” and “OFF” operations. In some embodiments, the touch panels are designed to prevent triggering by inadvertent contacts. For example, each touch panel can be provided a contact surface in a shallow depression. 
     In accordance with on the aspect of the present invention, the touch panels can be operated even when an operator is wearing gloves. This capability can be important in certain applications which require a piece of machinery to be disabled during emergency and within a very short time period. In such applications, the delay caused by an operator removing his or her work gloves in order to operate the switch is very undesirable. In accordance with another aspect of the present invention, when the first and second touch panels are touched substantially simultaneously, the Liu Switch resolves to an “OFF” state, thus preventing operating a load device inadvertently. 
     The Liu Switch of the present can control a resistive load, an inductive load, and some special loads, such as a fluorescent light, and can also be used to control mixed loads. 
    
    
     The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a system block diagram of an electrical circuit  100 , including control circuit  200  in solid state switch (“Liu Switch”)  300  in accordance with the present invention. 
     FIG. 2 a  shows equivalent circuits  250  and  250 ′ each representing a human body in contact with a touch panel (i.e., touch panel  17  or  19 ). 
     FIG. 2 b  shows characteristic curves S a , S b , S c , S d , representing respectively, the resistive (leakage), distributed capacitive, and equivalent inductive signal source components, and the sum total of the Liu&#39;s touch signal complementary effect (the “LTS” effect) as a function of the impedance of the human body. 
     FIG. 3 shows waveforms  1 - 4 , representing (i) the voltage across load  2 , when solid state switch  300  is in the “ON” state; (ii) the voltage across terminal  83  and the common ground of trigger and control circuit  200 , when solid state switch  300  is in the “OFF” state, (iii) the voltage across load  2 , when solid state switch  300  is in the “OFF” state, and (iv) the voltage across terminals  01  and  02  , including events  310  and  310 ′ in the voltage waveform at zero crossings of waveform  1 , when Liu Switch  300  is in the “ON” state. 
     FIG. 4 shows on the embodiment of the present invention in a circuit  400 , in which SCR controlled bridge rectifier  8  and main semiconductor switch  1  are implemented into a combined SCR-diode bridge circuit ( 1 , 8 ), and first gain circuit  11  is implemented by transistor T 1 . 
     FIG. 5 shows one embodiment of the present invention in circuit  500 , in which SCR controlled bridge rectifier  8  and main semiconductor switch  1  are implemented into a combined SCR-diode bridge circuit ( 1 , 8 ), and first gain circuit  11  is implemented by transistor T 11 . 
     FIG. 6 a  shows on the embodiment of the present invention in circuit  600 , in which main semiconductor switch  1  is implemented by TRIAC  601 . 
     FIG. 6 b  shows one embodiment of the present invention in circuit  620 , in which main semiconductor switch  1  is implemented by antiparallel SCRs  602  (SCR 2  and SCR 3 ). 
     FIG. 7 a  shows another embodiment of the present invention in circuit  700 , in which is provided a realization of initialization circuit, includes a Liu&#39;s Network  13 , 15 . 
     FIGS. 7 b - 7   d  show embodiments of the present invention in circuits  720 ,  740 , and  760 , each of which being a modification of circuit  700  of FIG. 7 a.    
     FIGS. 8 a - 8   c  show, respectively, implementations of a current limiter (i.e., current limiter  18  or  20  of FIG. 1 in each of circuits  800 ,  810 , and  820 . 
     FIG. 9 a  shows a overcurrent protection circuit  900 , including a system block diagram of overcurrent signal processing circuit  16  of the present invention. 
     FIGS. 9 b - 9   d  show respectively circuits  910 ,  920  and  940  of Liu Switch  300 , showing implementations of current detector  5  and overcurrent protection circuit  16 . 
     FIGS. 10 a  and  10   b  illustrate micro-current zero-crossing circuits  1000  and  1050  respectively, suitable for implementing zero-crossing detection circuit  10  of FIG.  1 . 
       
     FIG. 10 c  shows waveforms  1  and  2 , being respectively the threshold voltage V thres , and the impedance at the gate terminal G 1  of SCR 1 , due to the action of transistor T 4 . 
     FIGS. 11 a  and  11   b  show implementations  1100  and  1150  of optocoupler circuits  22  and  23  of FIG. 1 coupled to Liu Switch  300 , in the manner discussed above with respect to FIG.  1 . 
     FIGS. 12 a  and  12   b  respectively show a 3-wire control bus  400  and a 4-wire control bus  410  which are used to provide multipoint random control of loads  2   a  and  2   b  by external equipment, through solid state switches  1200  and  1201 , and to provide high isolation between the control bus, the Liu Switch and the power line. 
     FIG. 13 shows a schematic diagram  1300  of one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a solid state switch (“Liu Switch”), which is illustrated by way of example in electrical circuit  100  of FIG.  1 . As shown in FIG. 1, electrical circuit. Circuit  100  includes, connected in series, main semiconductor switch  1 , load  2 , trigger and control circuit  200 , electromagnetic interference (EMI) suppressers  3  and  3 ′, fuse  4  (for short circuit current protection), current detector device  5 , terminals  6  and  7  for connection to AC power outlet, and touch panels  17  and  19  for conveying external input signals to trigger and control circuit  200 . In one embodiment, the Liu Switch (“Liu Switch  300 ”) incorporates main semiconductor switch  1 , current detector device  5 , and trigger and control circuit  200 . Together with load  2 , Liu Switch  300  forms electrical circuit  100 . 
     Trigger and control circuit  200  includes a silicon controlled rectifier (SCR) controlled bridge rectifier  8 , an attenuator  9 , first gain circuit  11 , second gain circuit  14 , current-limiters  18  and  20 , initialization and power recovery circuit  15 , diode gate  13 , state memory and trigger circuit  12 , and filter circuits  28  and  29 . In one implementation of trigger and control circuit  200 , for example that shown in FIG. 7 a , state memory and trigger circuit  12  includes a capacitor C 1  and initialization and power recovery circuit  15  includes capacitor C 4 . 
     Gain circuit  11 , which is coupled to control terminal  113  and terminals  121  and  122 , is designed such that, when no input signal is provided at control terminal  113 , a high impedance exists between terminals  121  and  122 , so that no current flows in gain circuit  11 . Conversely, when an input signal is provided at terminal  113 , a low impedance exists between terminals  121  and  122 . Gain circuit  14 , which receives a signal at control terminal  143  and controls the impedance between “executive” terminal  141  and terminal  142 , can be similar constructed. In this embodiment, however, gain circuit  14  includes additional control terminals  144  and  145 , each of which can also be triggered to provide a low impedance path between terminal  141  and common ground terminal  142 . Control terminal  144  is provided to allow triggering gain circuit  14  during an overcurrent condition, and one or more control terminals  145  are provided to respond to other conditions for which Liu Switch  300  is to be turned “OFF”. 
     Initialization and power recovery circuit  15  and diode gate  13  together provide a “Liu&#39;s Network” (described in further detail below) which includes a diode (D 3 ), an electrolytic capacitor (C 4 ) and a resistor R 8 . An unpolarized capacitor C 4 ′ can be connected in parallel to capacitor C 4  to allow a faster response, as explained below. Initialization circuit  15  ensures that Liu Switch  300  is in the “OFF” state when power is first applied across its terminals. 
     SCR controlled bridge rectifier  8  includes a triggerable circuit, such as an SCR (“SCR 1 ”), which becomes conducting when triggered by a control signal at terminal  84 . When SCR 1  conducts, main semiconductor switch  1  also conducts, thus resulting Liu Switch  300  to be triggered into a conducting mode. As explained below, once triggered into the conducting mode, Liu Switch  300  generates its own subsequent trigger signals at terminal  84  to maintain Liu Switch  300  in the conducting mode. Main semiconductor switch can be implemented, for example, by a TRIAC or SCRs. 
     Trigger and control circuit  200  also includes the following “optional” components: zero-crossing detector  10 , coupling diode  50  (replacing jumper  60 , when zero-crossing detector  10  is included), “beep” circuit  21 , current detector device  5  (for overcurrent protection), overcurrent protection circuit  16 , and optocoupler  22  with external control bus  400 , or an optocoupler  23  with external control bus  410 . These optional components allow the Liu Switch to be used in a multi-point random remote control scheme as an electrically highly isolated relay (“Liu Switch-Relay”). 
     SCR controlled bridge rectifier  8  includes AC terminals  81  and  82 , a DC output terminal  83 , trigger terminal  84 , executive terminal  87 , and terminal  85 , which is coupled to a common “ground” of control and trigger circuit  200 . Resistor  86  can be provided in series with AC terminals  81  and  82 . When main semiconductor switch  1  is non-conducting, terminals  81  and  82  receive an input AC signal from power terminals  01  and  02  of main semiconductor switch  1 . SCR controlled bridge rectifier  8  provides a rectified signal (shown in FIG. 3 as waveform  2 ) across terminals  83  and  85 . As shown in FIG. 3, the rectified signal across terminals  83  and  85  is a half-wave signal at twice the frequency of the input AC signal. 
     Liu Switch  300  takes advantage of an impedance effect of the present invention, referred below as “Liu&#39;s touch signal complementary effect” (or “LTS” effect), which is further explained below. In FIG. 1, Liu Switch  300  includes touch panel  17 , which is electrically coupled to trigger and control circuit  200 . When touch panel  17  is contacted by an external agent (e.g., a human operating the switch), an electrical path is created as the result of the LTS effect for triggering trigger and control circuit  200 . The LTS effect is explained with the aid of FIGS. 2 a  and  2   b.    
     FIG. 2 a  shows equivalent circuits  250  and  250 ′, each representing a human body in contact with a touch panel (i.e., touch panel  17  or  19 ). Under certain environmental conditions, equivalent circuit  250  is dominated by an equivalent resistance (i.e., resistor R 23 ) representing the leakage path to ground, varying from about 30 Mega-ohms down to 100 ohms. The electrical signal intensity resulting from the LTS effect of this resistive component is shown in FIG.  2   b  by curve S a . Similarly, under other environmental conditions, the human body provides a medium to high of impedance (i.e., approximate from 30 Mega-ohms to 300 Mega-ohms or higher). In this range, equivalent circuit  250  is dominated by equivalent capacitor C 24  with distributed reactance X c . The electrical signal intensity resulting from the LTS effect of this capacitive component is shown in FIG. 2 b  as curve S b . To properly harness this LTS effect, trigger and control circuit  200  provides an anti-parallel diode connected between a common ground and an emitting junction of an input transistor of gain circuit  11 . 
     Under even higher impedance conditions, the human body can reach an impedance of more then 300 Mega-ohms or higher. At such high impedance, equivalent circuit  250  is dominated by inductive signal source AT 26 , providing an electrical signal from the stray electromagnetic fields of the ambience. Such electromagnetic fields result from various signal sources in a wide spectrum, from low frequency hum signals (e.g., 50(100)Hz or 60(120)Hz hum) to signals in the VHF or UHF band, and perhaps from static electricity as well, under certain conditions. Inductive signal source AT 26  provides an electrical signal resulting from a superposition of such electromagnetic fields which can be detected by gain circuit  11  of trigger and control circuit  200 . The electrical signal intensity resulting from the LTS effect is shown in FIG. 2 b  as curve S c . 
     The combined LTS effect of resistor R 23 , capacitor C 24  and inductive signal source AT 26  of FIG. 2 b  is shown in FIG. 2 b  as curve S d . In FIG. 2 b , a region labeled “dull” represents the region in which the electrical intensity at panel  17  is insufficient to trigger reliably trigger and control circuit  200 . As shown by curve S d , the electrical signal strength of the combined LTS effect of equivalent circuit  250  is above the “dull” region over virtually all practical impedances. Significantly, where the resistive component (i.e., S a ) falls below into the dull region, the capacitive component (i.e., S b ) maintains curve S d  significantly above the dull region. Similarly, where both the resistive and capacitive components (i.e., curves S a  and S b ) fall into the dull region, the inductive signal source component (i.e., S c ) maintains curve S d  above the dull region. Thus, the combined LTS effect allows Liu Switch  300  to be reliably switched over practically all impedances of the human body, substantially regardless of the environmental conditions. 
     Equivalent circuit  250 ′ operates in substantially the same manner as equivalent circuit  250  described, and thus description here of its operations is omitted. 
     The operation of Liu Switch  300  is next discussed. Initially, i.e., when Liu Switch  300  is first powered up, both capacitor C 1  of state memory and capacitive triggering circuit  12  and capacitor C 4  (and capacitor C 4 ′) of the initialization and power recovering reset circuit  15  are in a discharged state. 
     When an AC power signal is impressed across main semiconductor switch  1  and load  2  (i.e., “power-up”), terminals  81  and  82  receive the AC signal. The twin half-wave DC signal resulting from rectification by SCR controlled bridge rectifier  8 —this half-wave DC signal has a frequency twice that of the AC signal-appear across terminals  121  and  122 . Capacitor C 1  of state memory and capacitive triggering circuit  12  becomes fully charged almost immediately. Initially after power-up, the charging current pulse is shunted through forward diode gate  13  to capacitor C 4  and C 4 ′ of initialization and power recovering reset circuit  15 . The voltage at control terminal  122  does not significantly rise to cause a signal to pass through attenuator  9  to trigger terminal  84  of SCR controlled bridge rectifier  8  to trigger semiconductor switch  1 . Thus, the twin half-wave DC voltage maintains capacitor C 1  of state memory and capacitive triggering circuit  12  at a fully charged state. In this “OFF” state, no current flows in trigger and control circuit  200 . 
     In the “OFF”-state, when a person touches touch panel  19 , however, an “OFF”-state feedback effect is created. Gain circuit  14  provides a low-impedance path between terminals  141  and  142  to discharge and to maintain discharged capacitors C 4  and C 4 ′ of initialization and power recovery circuit  15 . Discharged capacitors C 4  and C 4 ′ prevents any signal at terminal  122  to trigger main semiconductor switch  1 , thus main semiconductor switch  1  remains non-conducting The voltage across the two terminals of semiconductor switch  1  maintains capacitor C 1  in a fully charged state. As capacitor C 1  in state memory and capacitive trigger circuit  12  remains fully charged, no trigger signal is propagated to the trigger terminal  84  of SCR controlled bridge rectifier  84  and hence to main semiconductor switch  1 . Consequently, main semiconductor switch  1  remains stable in the “OFF” state. 
     When control and trigger circuit  200  is in the “OFF” state, first and second gain circuits  11  and  14  are both in a current cutoff state, since no signal input is received. (So long as they are not contacted, panels  17  and  19  do not provide electrical signals). In addition, SCR controlled bridge rectifier  8  is in a current cutoff mode when it is not triggered. Thus, Liu Switch  300  has no leakage current to load  2  in the “OFF”-state of Liu Switch  300 . 
     While in the “OFF” state, when an external agent (e.g., a human hand) touches touch panel  17 , an electrical signal is provided to Liu Switch  300  at input terminal  113  of first gain circuit  11 , in accordance with the LTS effect discussed above. This electrical signal causes gain circuit  11  to provide a low impedance path between terminals  121  and  122  of state memory and capacitive triggering circuit  12 . As a result, capacitor C 1  within state memory and capacitive triggering circuit  12  rapidly discharges, and the DC signal at terminal  121  is coupled to terminal  122 . Even if the electrical signal is removed, the charging current of capacitor C 1  creates a pulse that is shunted through gain circuit  11  and diode gate  13  to rapidly and fully charge capacitors C 4  and C 4 ′ of initialization and reset circuit  15  and triggers SCR controlled bridge rectifier  8 , thereby causing semiconductor switch  1  to become conducting. Thereafter, as explained below in the next half wave, the brief charging current of capacitor C 1  at zero crossing of the AC signal creates a trigger pulse which continues to be sufficient to trigger SCR controlled bridge rectifier  8  at trigger terminal  82  of main semiconductor switch  1 . (At this time, capacitor C 4  is fully charged, so that the triggering pulse is provided entirely to the SCR controlled bridge rectifier  8 .) This trigger pulse triggers SCR 1  to cause main semiconductor switch  1  to become conducting, thus placing Liu Switch  300  into an “ON” state, and providing power across load  2 . In addition, an “ON” state feedback effect is initiated. The “ON” sate feedback effect depends upon: (a) while SCR 1  and main semiconductor switch  1  are causing each other to conduct a low impedance path for discharging capacitor C 1  is provided between terminals  121  and  122 , thus maintaining capacitor C 1  in a discharged state; and (b) main semiconductor switch  1  remains in a conducting state, except at zero-crossings of the input AC signal. 
     At a zero-crossing, main semiconductor switch  1  becomes non-conducting, which allows the next half-wave of the twin half-wave DC signal to momentarily appear across terminals  121  and  122 , thus charging capacitor C 1  almost immediately. The charging current pulse is coupled to terminal  122  and through attenuator  91 , to trigger terminal  84  of SCR controlled bridge rectifier  8 . Thus, the charging current pulse causes main semiconductor switch  1  to become conducting again until the next zero-crossing of the input AC signal. 
     While in the “ON” state, when a human (e.g. a hand) contacts touch panel  19 , an electrical signal is provided to second gain circuit  14 , in accordance with the LTS effect discussed above. A low impedance path is provided across the “Liu&#39;s Network” in initialization and recovering reset circuit  15 . As a result, capacitor C 4  discharges through the low impedance path rapidly. Through diode gate  13 , the low impedance-path also interrupts the “ON” state feedback effect by shunting the trigger pulse in control terminal  122  to common ground. Thus, at the next zero-crossing, when main semiconductor switch  1  becomes non-conducting, the absence of the trigger pulse places Liu Switch  300  in an “OFF” state, while the twin half-wave DC signal appearing across terminals  121  and  122  recharges capacitor C 1 . A fully charged capacitor C 1  prevents a trigger pulse to be created at control terminal  122 , and hence prevents main semiconductor switch  1  from triggered into a conducting mode, until the next contact at touch panel  17 . 
     Thus, since Liu Switch  300  conducts no current during the “OFF”-state, and is short-circuited during the “ON” state (except at brief instances at zero-crossings of the AC signal), Liu Switch  300  does not require a power supply circuit for proper operation. Consequently, a two-terminal network solid state switch which operates in the same manner as a conventional mechanical single-pole switch is realized. 
     According to the present invention, the Liu&#39;s Network has is characterized by multiple time constants. For example, in the implementation shown in FIG. 7 a : (a) Capacitor  4  is discharged through resistor R 8  at a rate characterized by a large predetermined time constant; (b) Capacitor  4  is rapidly charged at a rate characterized by a small time constant (relative to the predetermined time constant above) when charged by a current from control terminal  122  through diode gate  13 ; and (c) Capacitor  4  is rapidly discharged at a rate characterized by a small time constant (relative to the predetermined time constant above) when discharged by the low impedance path of gain circuit  14 . 
     The small time constant in charging through diode gate  13  (i.e., in (b) above) allows the Liu&#39;s Network to serve as a dynamic energy absorber, to absorb shock, noise spike, and surge pulses which appear at control terminal  122 . With this quality, a very high stability in the “ON” and “OFF” states of Liu Switch  300  is achieved. The Liu&#39;s Network discharges through the low impedance path provided by second gain circuit  14  with a small time constant, so as to allow Liu Switch  300  to switch off quickly in response to contact at touch panel  19 . In addition, the Liu&#39;s Network can achieve a desirable safety function. For example, in a conventional mechanical power switch, when the switch conducts (turns “ON”), a load is running (e.g., a heater, or a electric tool, a machine, etc.). However, during a power interruption, the load may be left in an operating mode when the power was interrupted. Thus, when power resumes at a later time unexpectedly, the load continues the interrupted operation. The resumption of power to a load left in an operating mode is the cause of accidents, causing fires, bodily injury and property damages. 
     The Liu&#39;s Network includes resistor (R 8 ) to provide the predetermined discharge time constant. If the AC power is interrupted while Liu Switch  300  is in the “ON” state, capacitor C 4  discharges through resistor R 8 . The predetermined time constant maintains the charge on capacitor  4  for a predetermined time period. If the AC power recovers during this predetermined time period (e.g., from a few to tens of seconds), capacitor  4  is unable to completely absorb the charging current of capacitor C 1 , thereby allowing a trigger pulse to reach terminal  84  (FIG. 1) to trigger SCR 1  and hence allowing Liu Switch  300  to resume its former “ON” state. However, if the AC power recovers only after the predetermined time period has elapsed, capacitor C 4  of the Liu&#39;s Network is substantially discharged through resistor R 8 , such that the charging pulse of capacitor C 1  is shunted through diode gate  13  to capacitor C 4 . As a result, when AC power recovers after the predetermined period has elapsed, Liu Switch  300  recovers into the “OFF” state and remains stable in the “OFF” state indefinitely. Thus, the Liu&#39;s Network avoids load  2  resuming operation unexpectedly when power recovers, thus avoiding accidents and harm. 
     When included in trigger and control circuit  200  (FIG.  1 ), “beep” circuit  21  indicates by sound successful switching of Liu Switch  300 . 
     When included in trigger and control circuit  200 , overcurrent protection circuit  16  monitors the current in current detector  5 , and provides an output control signal at input terminal  144  of gain circuit  14  when the current in current detector  5  exceeds a predetermined threshold. As discussed above, the control signal at terminal  144  interrupts the “ON” state of Liu Switch  300 . 
     When included in trigger and control circuit  200 , optocoupler circuit  22  or  23  allow Liu Switch  300  to be used in a multipoint random control system. As shown in FIG. 1, optocoupler circuit  22  includes input terminals  223 ,  224  and  225 , and provides output terminals  221 ,  222 ,  226  and  228 . Terminal  225  is coupled to an external ground reference, terminals  221  and  226  are coupled respectively through terminals  121  and  122  to gain circuit  11 , and terminals  222  and  228  are coupled respectively through terminals  141  and  142  (also common ground) to gain circuit  14 . When a signal (AC or DC) appears across terminals  223  and  225 , an optically isolated low impedance signal path is provided between terminals  221  and  226 , thereby triggering the “ON” state of Liu Switch  300 , in the manner explained above. Similarly, when a signal appears across terminals  224  and  225 , an optically isolated low impedance path appears between terminals  222  and  228 , thereby triggering the “OFF” state of Liu Switch  300 . Using optical isolation, the output terminals  221 ,  222 ,  226  and  228  are each isolated by very high impedance from input terminals  223 - 225 . 
     Optocoupler circuit  23  is similar to optocoupler circuit  22 , except that an additional external ground signal  237  is provided so that highly isolated electrical paths are provided between terminals  223  and  235 ,  234  and  237 ,  231  and  236 , and  232  and  238 . When a signal (AC or DC) appears across terminals  233  and  235 , an optically isolated low impedance signal path is provided between terminals  231  and  236 , thereby triggering the “ON” state of Liu Switch  300 , in the manner explained above. Similarly, when a signal appears across terminals  234  and  237 , an optically isolated low impedance path appears between terminals  232  and  238 , thereby triggering the “OFF” state of Liu Switch  300 . 
     Several implementations of Liu Switch  300  are described below. To facilitate cross-referencing among the diagrams, reference numerals used in FIGS. 4-13 that correspond to reference numerals used in FIG. 1 indicate elements or terminals identified in FIG. 1 in these implementations. 
     FIG. 4 shows one embodiment of the present invention in circuit  400 , in which SCR controlled bridge rectifier  8  and main semiconductor switch  1  are implemented into a combined SCR-diode bridge circuit ( 1 , 8 ). As shown in FIG. 4, a diode bridge BZ 1  and an SCR 1  realize the functions of SCR controlled bridge rectifier  8  and main semiconductor switch  1  of FIG.  1 . State memory and trigger circuit  12  is realized by series-connected resistor R 3  and capacitor C 1 . Current-limiters  18  and  20  are realized by resistors R 1  and R 2 . Gain circuits  11  and  14  are realized by NPN bipolar transistors T 1  and T 2 . Attenuator  9  is realized by resistor R 4 . 
     During the “OFF” state, a rectified signal appears across terminals  83  and  85  and capacitor C 1  is charged. When a human person contacts touch panel  17 , a contact signal results from the LTS effect which turns on NPN transistor T 1 . Conducting transistor T 1  discharges capacitor C 1  and couples the twin half-wave DC signal at terminal  87  to terminal  92 . Through resistor R 4 , the twin half-wave DC signal is provided to gate terminal G 1  of SCR 1 , thus triggering SCR 1  into conduction. Conducting SCR 1  provides a short circuit between terminals  01  and  02  (i.e., main semiconductor switch  1 ), thus providing an AC signal across load  2 , maintaining capacitor C 1  in a discharged state, and incurring a voltage drop (V SAT-ON ) equal to the sum of two forward-biased drops in diode bridge BZ 1  and the voltage drop across forward-biased SCR 1  (i.e., 2.4-4.0 volts). When the AC signal crosses zero, SCR 1  shuts off, so that capacitor C 1  is charged by the next twin half-wave of the AC signal, thus providing a charging current which triggers SCR 1  back into conduction. Conducting SCR 1  provides the AC signal across load  2  once again. This regenerating feedback process maintains Liu Switch  300  in the “ON” state. When a human contact touch panel  19 , a contact signal from the LTS effect turns NPN transistor T 2  into conduction, thus shorting terminal  92  to common ground, interrupting the feedback process described above, and setting Liu Switch  300  to the “OFF” state. 
     FIG. 5 shows another embodiment of the present invention in circuit  500 , which is substantially identical to circuit  400 , except that PNP transistor T 11  in circuit  500  replaces NPN transistor T 1  of circuit  400 . Otherwise, operation of circuit  500  is virtually identical to circuit  400  described above. 
     Circuits  400  and  500  have several disadvantages. First, a relatively large “ON” state voltage drop (2.4-4.0 volts) results in relatively large power dissipation in Liu Switch  300 , requiring a heat sink for proper high-current operation, and reducing the voltage seen across load  2 . Second, circuits  400  and  500  provide a Liu Switch which initializes in the “ON” state when coupled in the first instant to an AC signal source. 
     To minimize the “ON” state voltage drop (V SAT-ON ) semiconductor switch  1  can be implemented by a TRIAC or anti-parallel SCRs, as shown in FIGS. 6 a  and  6   b . FIG. 6 a  shows one embodiment of the present invention circuit  600 , in which main semiconductor switch  1  is implemented by a triode AC switch (TRIAC)  601 . As shown in FIG. 6 a , terminals B 13  and B 14  (i.e., terminals  81  and  82  of FIG. 1) of diode bridge BZ 1  are coupled to gate terminal G and a second anode MT 2  respectively. Current-limiting resistor  86  can be inserted in series between diode bridge BZ 1  and TRIAC  601  at either terminals B 13  and B 14  to protect the gate terminal of TRIAC  601 . During the “ON” state, the “ON” state voltage drop (V SAT-ON ) is between 0.8 volt to 1.6 volts. TRIAC  601  turns “OFF” at the AC signal&#39;s zero-crossing, after a high impedance is imposed across terminals G and MT 2  (SCR 1  non-conducting). 
     FIG. 6 b  shows on the embodiment of the present invention in circuit  620 , in which main semiconductor switch  1  is implemented by circuit  602  which is implemented by SCRs (i.e., SCR 2  and SCR 3 ) in an antiparallel configuration. Each SCR in circuit  602  is capable of carrying higher current than SCR 1  of SCR controlled bridge rectifier  8 . As shown in FIG. 6 a , terminals B 13  and B 14  (i.e., terminals  81  and  82  of FIG. 1) of diode bridge BZ 1  are coupled to gate terminals G 3  and G 2 , respectively. As in circuit  600  of FIG. 6 a , current-limiting resistor  86  can be inserted in series between diode bridge BZ 1  and antiparallel diodes SCR 3  and SCR 2  at either terminals B 13  and B 14  to protect the gate terminals G 2  and G 3 . During the “ON” state, a trigger current is created between cathodes K 3  and K 2  of SCR 3  and SCR 2 , respectively, which flows through the path formed by gate terminal G 3  of SCR 3 , bridge rectifier BZ 1 , SCR 1  and gate terminal G 2  of SCR 2 . The trigger current alternately triggers SCR 2  and SCR 3  into conduction mode. During the “ON” state, the “ON” state voltage drop (V SAT-ON ) is thus approximately 1 volt. Main semiconductor switch  1  turns “OFF” at the AC signal&#39;s zero-crossing, after a high impedance is imposed across terminals G 2  and G 3  (SCR 1  non-conducting), thereby setting SCR 2  and SCR 3  in non-conducting states. 
     By using power components (e.g., TRIAC  601 , and SCR 2 , SCR 3  of switch devises  602 ,) in circuits  600  and  620 , the current that can be provided to load  2  is easily expanded over the corresponding currents deliverable by circuits  400  and  500  of FIGS. 4 and 5. In addition, because of the low “ON” state voltage drop across Liu Switch  300 , the power dissipation is also diminished, as compared to circuits  400  and  500  discussed above, for the same current load. 
     FIG. 7 a  shows another embodiment of the present invention in circuit  700 , in which is provided a realization of initialization circuit and power recovering reset circuit  15  and diode gate  13 . Initialization circuit and power recovering reset circuit  15  and diode gate  13  is implemented by diode D 3 , resistor R 8 , and capacitors C 4  and C 4 ′, referred below as “Liu&#39;s Network”. In addition, circuit  700  implements attenuator  9  by a voltage divider formed by resistors R 4  and R 6 . In circuit  700 , gain circuit  14  is provided by a complementary cascaded amplifier including resistor R 15 , PNP transistor T 3 , and NPN transistor T 2 . SCR controlled bridge rectifier  8  and main semiconductor switch  1  are implemented by a circuit similar to circuit  600 , except that a bypass filter formed by capacitor C 6  and resistor R 7  is provided at the gate terminal of TRIAC  601 . Gain circuit  11  and state memory and trigger circuit  12  are provided as in circuit  400  above. Filter circuits  28  and  29  are each implemented by a capacitor (C 2  or C 3 ) connected in parallel with a diode (D 1  or D 2 ). In each of filter circuits  28  and  29 , the diode (e.g., D 1 ) is connected in an antiparallel fashion to the emitter junction of the gain transistor (e.g., transistor T 1 ). In this configuration, the diode performs at least three functions: (a) providing a negative path for to increase the distributive capacitive component (S b ) of the LTS effect; (b) protecting the emitter junction of the gain transistor; and (c) detecting a peak in the equivalent inductive signal source component (S c ) of the LTS effect. 
     In circuit  700 , capacitor C 4  provided in Liu&#39;s Network has a much larger capacitance that capacitor C 1  of state memory and trigger circuit  12 . In this embodiment, capacitor C 4  can be implemented by an polarized electrolytic capacitor. Since a large electrolytic capacitor has a parasitic inductance, an unpolarized capacitor C 4 ′ is provided in Liu&#39;s Network to provide a fast response time to the Liu&#39;s Network. 
     Initially, i.e., when an AC power signal is first imposed across Liu Switch  300 , both capacitors C 4  and C 1  are uncharged. As described above, capacitor C 1  is fully charged within a very short period of time after the rectified signal across terminals  83  and  85  crosses zero. However, as capacitor C 4  remains relatively uncharged, the charging current of capacitor C 1  is shunted to charge capacitor C 4  through forward-biased diode D 3 , so that a sufficient triggering current pulse is not seen at input terminal  92  of attenuator  9 , thus preventing triggering Liu Switch  300  into the “ON” state. During the “OFF” state of Liu Switch  300 , no current thus flows in trigger and control circuit  200 , as described above. (Leakage currents in gain circuits  11  and  14  are in the nano-ampere range, and thus negligible.) Resistor R 8  maintains capacitor C 4  in a substantially discharged state during the “OFF” state of Liu Switch  300 . Thus, initialization circuit  15  provides additional stability in the “OFF” state, maintaining input terminal  122  of attenuator  9  no higher than one diode drop (i.e., about 0.7 volts) above the common ground, thereby preventing triggering of Liu Switch  300  by a momentary surge in the AC signal or by inadvertent momentary contact of touch panel  17 . 
     As described above, when a person touches touch panel  17 , capacitor C 1  is discharged and the rectified signal is transmitted through the low impedance path through transistor T 1  across terminals  121  and  122 . This signal, even though attenuated by attenuator  9 , is sufficient to trigger SCR 1  of SCR controlled bridge rectifier  8 , which in turn, triggers main semiconductor switch  1  to a conducting state. As the “ON” state feedback process described above maintain Liu Switch  300  in the “ON” state, a steady state is reached whereby capacitor C 4  is maintained at a steady state voltage. At this time, if the AC power signal across lines  6  and  7  is suddenly interrupted, capacitor C 4  discharges through resistor R 8  according to the time constant determined by the capacitance and resistance values of capacitor C 4  and resistor R 8 . Thus, by a judicious choice of these values, Liu Switch  300  can return to the “ON” state (i.e., an “ON” state memory is achieved), if the power resumes within the predetermined period of time. However, if the power resumed after the predetermined time period, capacitor C 4  becomes discharged. Since discharged capacitor C 4  can absorb the charging pulse of capacitor C 1 , as described above, a stable “OFF” state condition is created. Thus, Liu Switch  300  remains in the “OFF” state, until contact is made at touch panel  17  again. 
     Maintaining the “ON” state for a time period and then resetting to the “OFF” state thereafter can be exploited to provide a significant safety advantage (referred below as “Liu&#39;s function”). Many accidents with tragic consequences have been caused by electrical appliances or machinery left unattended and in the powered mode by a power outage. When the power recovers, these appliances or machinery resumes operation unattended, often leading to electrical appliance damages, accidents, fires or bodily injuries. However, with Liu&#39;s Network, Liu&#39;s function allows the electrical appliances or machinery to resume operation from a power interruption (i.e., “latched-ON”), only if power recovers within a time period during which continued operation can proceed safely, but resets to the “OFF” state beyond such time period. Liu&#39;s function thus maintains a stable “ON” state even during frequent or short-duration power interruptions. 
     Capacitor C 2  or C 2 ′ of filter circuit  28 , in conjunction with resistor R 1  and the operator&#39;s equivalent impedance, forms a low-pass filter which eliminates high frequency electromagnetic interference from touch panel  17  and noise in parallel feedback from gain circuit  11  due to a surge in the rectified signal. Diode D 1  serves three functions: (a) to provide an RF peak detector which enhances the inductive component (S c ) of the LTS effect; (b) to rectify the negative half-cycle signal current, thus improving the touch sensitivity of the touch panel towards the capacitive component (S b ) of the LTS effect; and (c) to protect the input terminals of the gain circuit. Capacitor C 3  of filter circuit  29  performs a similar function as that performed by capacitor C 2  or C 2 ′ described above. 
     In conjunction with resistor R 5 , resistor R 7  and capacitor C 6 , which are connected in parallel at the gate terminal G of TRIAC  601 , provide surge protection and suppress high frequency noise to gate terminal G of TRIAC  601  during operation. Further electromagnetic interference and noise filtering can be provided by series-connected EMI suppressers  3  and  3 ′ (see FIG.  1 ), which can be provided by many forms of low-pass filters, including LC circuits formed by solenoids with ferric alloy, ferrites cores with capacitors, or commercial monolithic EMI filters. 
     FIGS. 7 b - 7   d  show embodiments of the present invention in circuits  720 ,  740 , and  760 , each of which represents a modification of circuit  700  of FIG. 7 a . Circuit  720  includes a realization of filter circuit  28  by capacitor C 2 ′ and diode D 1 ′, and a realization of gain circuit  11  by PNP transistor T 11 . Circuit  740  of FIG. 7 c  is substantially identical to circuit  700  of FIG. 7 a , except that antiparallel SCR 2  and SCR 3  are provided for realizing main semiconductor switch  1 , as in circuit  620  of FIG. 6 b . Circuit  760  of FIG. 7 d  is substantially similar to circuit  720  of FIG. 7 b , except that antiparallel SCR 2  and SCR 3  are provided for realizing main semiconductor switch  1 , as c in circuit  620  of FIG. 6 b.    
     FIGS. 8 a - 8   c  show respectively circuits  800 ,  810  and  820  of a limiter (e.g., limiter  20 ). In circuit  800 , a resistor R 40  is provided. In circuit  810  a capacitor C 42  is provided as a limiter. In circuit  820 , a capacitor C 45  and a resistor R 44  are provided in series. Capacitors C 42  and C 45  can be achieved by coating a dielectric or a resistive material on the metallic or conductor surface of the touch panel. 
     FIG. 9 a  shows a system block diagram of circuit  900 , representing overcurrent signal processing circuit  16  of the present invention. Circuit  900  includes a current-to-voltage conversion device (AVC)  5 , an AC-DC converter or a rectifier  904 , an amplitude limiter with a regulator  905 , a ripple filter  906 , an auto-temperature compensation and overcurrent tripping value following temperature character circuit  907 , a overcurrent tripping preset circuit  908 , a threshold circuit, and an OR gate  903 . 
     AVC  5  can be provided by a current transformer or a shunt element, or another type of current-voltage conversion devices. The current-side terminals of AVC  5  are connected in series with Liu Switch  300 , load  2  and fuse  4  across AC power lines  6  and  7 . The output signal of AVC  5 , which is representative of the current in load  2  (“main current”), is coupled at to a AC-DC converter or rectifier  904  (e.g., a half-wave rectifier, a full-wave rectifier, or a full wave bridge rectifier). The rectified signal is then regulated by regulator and amplitude limiter  905  and filtered by ripple filter  906 . The resulting DC signal remains representative of the main current. Temperature automatic compensation circuit  907  is provided to adjust the resulting DC signal according to temperature-sensitive current-tripping values that follow a set of characteristics curves (i OT -T curves) The temperature-compensated DC signal is then attenuated by a voltage divider  908 , and coupled to a threshold element set for a predetermined overcurrent tripping value. When an overcurrent condition occurs, an overcurrent signal is provided at terminal  144  to “OR” gate  903  to trigger second gain circuit  14 , to force Liu Switch  300  into the “OFF” state for protection. Backup input terminals  145  receive one or more actuating signal for turning off Liu Switch  300 . Additional backup signal inputs, if needed for performing automatic shut-off in additional protection functions, can be provided as additional input terminals of “OR” gate  903 . 
     FIGS. 9 b - 9   d  show respectively realizations of three static overcurrent tripping circuits  910 ,  920  and  940  which can be used in Liu Switch  300 . By providing overcurrent protection at Liu Switch  300  in the manner illustrated by circuits  910 ,  920  and  940 , overcurrent protection is extended to a branch single pole switch at very low cost, thus enhances safety significantly and avoiding the inconvenience of prior art centralized overcurrent protection schemes, which results in multiple electrical circuits being affected by an overcurrent in a single electrical circuit. 
     In circuit  910 , a small current transformer TF is provided as current detector  5 . For current detection, rather than a current transformer, a shunt element or another current-voltage conversion element can also be used. In current transformer TF, the primary coil TF 1  conducts the current in main semiconductor switch  1 , while Secondary coil TF 2  provides an AC current signal proportional to the current in coil TF 1 . Zener diode Z 4 , capacitor C 7  and R 12  form a voltage multiplier and half-wave rectifier. Zener diode Z 4  regulates and limits the amplitude of resulting half-wave rectified signal, which is transmitted by rectifier D 10  to provide a DC voltage signal at terminal  902  representative of the current through main semiconductor switch  1 . This DC voltage signal, which is ripple-filtered and attenuated by capacitor C 8  and resistor network  907  (consisting of resistors R 9  and R 11 , and temperature-sensitive resistors R 10  and R 10 ′) is provided through silicon diode D 5  as an input signal to gain circuit  14  of Liu Switch  300 . By selecting appropriate resistive values for resistors in resistor network  907 , an appropriate overcurrent tripping value for Liu Switch  300  can be selected. 
     When the overcurrent threshold is reached, resistor R 14  and diode D 4  (see FIG. 13) provide a positive feedback which allows gain circuit  14  to rapidly set Liu Switch  300  into the “OFF” state. Circuit  910  enhances sensitivity to overcurrent. 
     Significantly, silicon diode D 5 &#39;s forward-bias voltage drop has a negative temperature coefficient (i.e., diode D 5 &#39;s forward-bias voltage drop across its terminals decreases as temperature increases). Thus, diode D 5  provides a lower tripping threshold at a higher temperature. Temperature-sensitive devices (e.g., thermisters R 10  and/or R 10 ′) provide dynamic characteristics of temperature-dependent overcurrent tripping points, and thus can be used to further compensate and fine-tune the tripping threshold in relation to temperature changes. In this manner, a temperature-compensated or adaptive overcurrent protection is achieved. When environmental temperature and the switch temperature changes, Liu Switch  300  can adapt to a new tripping value automatically. Thus, Liu Switch  300  provides an advanced protection function to protect human lives, the electrical network or the electric appliance in which Liu Switch  300  is installed, and Liu Switch  300  itself. 
     Of course, other thermal or temperature-sensitive elements can be used in Liu Switch  300  to achieve the same results discussed above. 
     FIG. 9 c  shows circuit  920 , which is a variation of circuit  910  of FIG. 9 a . In FIG. 9 c , circuit  920  uses a center-tapped secondary coil TF 2  in current detector  5 , and provides full-wave rectification by diodes D 6  and D 7 . In circuit  920 , the threshold element is provided by reverse-biased Zener diode D 5  and a load resistor R 13 . 
     FIG. 9 d  shows circuit  940 , which is a variation of circuit  910  of FIG. 9 a . In FIG. 9 d , circuit  940  uses a diode bridge rectifier BZ 2  to provide full-wave rectification of the voltage signal in Secondary coil TF 2  of current detector  5 . In circuit  940 , the threshold element is provided by element TE 1 , which can be, for example, a PNPN four-layer Shockley diode and a load resistor R 13 . 
     FIGS. 10 a  and  10   b  illustrate optional zero-crossing circuits  1000  and  1050  respectively. As shown in FIG. 10 a , circuit  1000  includes a voltage divider formed by resistors R 16  and R 15 , which are connected in series between terminals  83  and  85  (i.e., also coupled to the common ground of Liu Switch  300 ) of SCR controlled bridge rectifier  8 , and NPN transistor T 4 , which base terminal is controlled by output signal V b4  of the voltage divider. During the “OFF” state of Liu Switch  300 , NPN transistor T 4 , when conducting, provides a low impedance path to short the trigger signal at the gate terminal G 1  of SCR 1  to common ground. Voltage V thres , which is the minimum voltage above which NPN transistor T 4  conducts, is thus a threshold voltage above which G 1  is not triggered. FIG. 10 a  also shows state memory and trigger circuit  12 , attenuator  9 , SCR controlled bridge rectifier  8  and main semiconductor switch  1  being implemented in the manner shown in  7   a . Coupling diode D 50  couples terminal  83  of SCR controlled bridge rectifier  8  to terminal  121  of trigger and control circuit  12 . FIG. 10 c  shows waveforms  1  and  2 , being respectively the threshold voltage V thres  and the output impedance of transistor T 4  at the gate terminal G 1  of SCR 1 . 
     In circuit  1050  of FIG. 10 b , the voltage divider of resistors R 15  and R 16  of circuit  1000  is replaced by series-connected Zener diode Z 1 , light-emitter diode (LED) LED 1  (optional) and a voltage divider formed by resistors R 18  and R 17 . Because of the relatively constant voltage drop across Zener diode Z 1 , when conducting, a more precise zero-crossing threshold voltage V thres  can be selected from the voltage range of 9 volts to 19 volts. LED 1  is included to allow Liu Switch  300  to be visible in the dark. This “night light” function, which requires less than 200 micro-amperes of current (still insufficient to trigger conduction in an implementation of main semiconductor switch  1  by a TRIAC or antiparallel SCRs), is particularly useful when Liu Switch  300  is used as a light switch. 
     FIGS. 10 a  and  10   b  also show a simple, small, inexpensive and, in the steady state, current leakage-free implementation of “beep” circuit  21  by series connected Zener diode Z 2  and a piezoelectric speaker element PE between node A and common ground. When node A rises from substantially common ground (i.e., the “ON” state of Liu Switch  300 ) to a high DC voltage (i.e., the “OFF” state of Liu Switch  300 ), a breakdown current passes through Z 2  and rapidly charges the capacitive piezoelectric element PE, thereby generating an audible sound. This audible response can also be provided by coupling the Zener diode Z 2  and piezoelectric element PE across any two nodes which experience a sudden change voltage. An audible “beep” sound is generated by piezoelectric speaker element PE when Liu Switch  300  switches. Thus, beep circuit  21  can be used to provide an audible sound response to the operator when Liu Switch  300  switches. 
     FIGS. 11 a  and  11   b  show implementations  1100  and  1150  of optocoupler circuits  22  and  23  coupled to Liu Switch  300  used as a multi-point random remote control solid state switch/relay, in the manner discussed above with respect to FIG.  1 . Using optical isolation, input control signals at terminals  223  and  224  (relative to common external ground terminal  225 ) of circuit  1100  are isolated from output signals across terminals  221  and  226  (“ON”-channel), and across terminals  222  and  228  (“OFF”-channel), by up to thousands of volts. Similarly, input control signals at terminals  233  and  234  of circuit  1150  (relative to two individual, highly isolated external ground terminals  235  and  237 , respectively) are isolated from output signals across terminals  231  and  236  (“ON”-channel), and across terminals  232  and  238 (“OFF”-channel), by up to thousands of volts. Thus, signals at terminal  223  and  224  and at external common ground terminal  225  can be provided from a 3-wire bus  400  on which any number of devices each capable of providing the control signals can be connected. FIG. 12 a  shows 3-wire control bus  400  used to provide multipoint random remote control by external equipment to load  2   a  through solid state switch  1200 . Similarly, control signals at terminals  233 ,  234  and at external ground terminals  235  and  237  can be provided from a 4-wire bus  410  on which any number of devices each capable of providing the control signals can be connected. FIGS. 12 b  shows 4-wire control bus  410  used to provide multipoint random remote control to the load  2   b  through solid state switch  1201 . 
     FIG. 13 shows an embodiment of the present invention in circuit  1300 , which combines various implementations of the various functional circuits of FIG.  1 . 
     Liu Switch  300  has a built-in over-voltage protection and thus does not require protection by a protective device. This built-in over-voltage protection can be illustrated, for example, by reference to FIG. 7 a . During the “OFF” state, if an over-voltage condition occurs (e.g., during a large voltage surge in the AC signal), such that the rectified twin half-wave DC signal across the collector and emitter terminals of transistor T 1  exceeds the breakdown voltage of transistor T 1 , transistor T 1  conducts. Conducting transistor T 1  discharges capacitor C 1 . The current of conducting transistor T 1  provides a triggering pulse, which triggers SCR 1  through attenuator  9 . Conducting SCR 1  shorts diode bridge BZ 1 , which in turn triggers TRIAC  601  into a conducting state. Liu Switch  300  is thus switched into the “ON” state, so that the voltage across Liu Switch  300  drops to the voltage across conducting TRIAC  601 . From this point, the regenerating feedback process described above sustains Liu Switch  300  in the “ON” state until Liu Switch  300  is switched into the “OFF” state by, for example, an external agent contacting touch panel  19 . As the breakdown of transistor T 1  and the conduction of TRIAC  601  occur within microseconds, because conduction of TRIAC  601  shorts the two terminals of semiconductor switch  1 , thus substantially eliminating the voltage drop across Liu Switch  300 , and also because the breakdown current of transistor T 1  is limited by the resistance in attenuator  9 , the over-voltage appearing across the terminals of Liu Switch  300  is too brief to cause damage to Liu Switch  300 . Thus, Liu Switch  300  does not require protection by a protective device, even if the over-voltage condition persists in the AC signal. By eliminating the need for a protective device and by avoiding the cost of replacing damaged protective devices, Liu Switch  300  achieves substantial cost savings over the solid state switches of the prior art. 
     Thus, the present invention provides a switch which draws substantially no current during the “OFF” steady state, and requiring no power supply to operate the switch itself. In addition, the present invention provides a solid state switch that does not require a protective device for its over-voltage protection. 
     Numerous variations and modification within the scope of he present invention are possible. For example, gain circuits of Liu Switch  300  (e.g., gain circuits  11  and  14 ) may be implemented by another gain device, such as certain J-FETs, or MOS-FETs, Darlington bipolars, or another integrated circuit or operational amplifier. Main semiconductor switch device  1  can also be implemented by, for example, some IGBTs, GTOs, MCTs, V-MOSs, D-MOSs, power bipolars, or another bilateral triode thyristors. 
     The above detailed descriptions provided to illustrate the specific embodiments of the present invention described above, and is not intended to be limiting of the present invention. The present invention is set forth in the following claims.