Abstract:
An electromechanical/solid-state AC relay has an electromechanical winding coil that moves an armature to force mechanical contacts to open or close. Electrical arcing across the mechanical contacts that occur as the contacts are opening or closing can damage and severely reduce the lifetime of the relay. Contact arcing is prevented by pulsing a triac on for a short period of time just before and after the mechanical contacts make or break contact. The triac limits the voltage difference across the mechanical contacts to less than one volt to prevent arcing. The triac is turned off after the mechanical contacts finish moving, reducing the heating and average power through the triac. A zero-sampling circuit that detects when the AC input voltage switches across 0 volts and activates a control integrated circuit to switch on the triac during zero-crossings to minimize power surges.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 USC § 119 of the co-pending application for “AC Relay”, China App. No. 200620045110.0, filed Aug. 25, 2006. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to alternating-current (AC) relays, and more particularly to AC relays with both mechanical and solid-state contacts. 
       BACKGROUND OF THE INVENTION 
       [0003]    A relay is a type of electronic control device that often used as an automatic control circuit. It has a control system (also known as the input circuit) and a controlled system (also known as the output circuit). A smaller current in the control system can control a larger current of the controlled system using such an “Automatic Switch.” 
         [0004]    While simple direct current (DC) relays are common, alternating-current (AC) relays may be somewhat more complex since the AC current switches direction. However, many commercial appliances use AC and thus could benefit from an AC relay to control the AC current that powers the appliance. Electric irons, toasters, and other small electronic appliances can benefit from the use of improved AC relays. The improved AC relays can increase reliability and precision of temperature control for such appliances. 
         [0005]    Current products commonly use electromagnetic relays (EMR) and solid state relays (SSR). Electromagnetic relays (EMR) use a mechanical contact switch. The contact resistance and power dissipation of an EMR is very small. However, EMR&#39;s have some drawbacks. 
         [0006]    The process of switching an EMR may take a few milliseconds to a few tens of milliseconds. When AC current is used, it is difficult to turn the EMR on and off at the zero-crossing of the AC current, when the AC current switches direction. Also, the mechanical contact in the EMR might arc, producing a reduced contact lifetime. EMR&#39;s also have a large electromagnetic interference (EMI). 
         [0007]    The stochastic operation time causes the EMR to be incapable of turning on or turning off at exactly the load&#39;s AC zero crossing. Instead, the electromagnetic relay&#39;s switching action brings a surge current. This surge current creates interference in the electrical grid system, producing an inextricable EMI problem for the electromagnetic relay. 
         [0008]    Usually an arc discharge phenomena appears at the moment of switching, when the electromagnetic relay controls a high voltage and a large current flows. An electric spark (arc) appears, and the arc discharge creates electrical wear. This electrical wear is much worse than the mechanical wear on the EMR, producing an electrical lifetime that is far less than the mechanical lifetime of the EMR. Usually the electrical lifetime is about fifty to one hundred thousand times, but the mechanical lifetime is over one million times. 
         [0009]    Solid state relays use a solid-state semiconductor such as a Silicon-Controlled-Rectifier (SCR) for the switch function. SCR&#39;s produce no arcing and no large electromagnetic interference. However, SCR&#39;s operate with about a 1-Volt voltage drop. This 1-volt drop through the SCR is a serious problem, especially for high-power control applications. 
         [0010]    Simply combining an electromechanical EMR relay with a solid-state relay (SSR) would likely produce the disadvantages of both. The combined device could have low reliability due to arcing of the EMR, and have the voltage-drop problem of the SSR, along with a high cost. 
         [0011]    What is desired is an AC relay that combines the benefits of an electromechanical relay and a solid-state relay while reducing or eliminating the disadvantages of each. A relay that switches AC currents near the AC zero-crossing point is desirable. An AC relay that solves the problems of electromagnetic interference (EMI) and high power dissipation when switching large AC currents is also desirable. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a side view of an electromechanical/solid-state AC Relay that combines the benefits of an EMR and a SSR while reducing the drawbacks of each. 
           [0013]      FIG. 2  shows a bottom view of the electromechanical/solid-state AC relay of  FIG. 1 . 
           [0014]      FIG. 3  is a block diagram of the electromechanical/solid-state AC relay. 
           [0015]      FIG. 4  is a circuit schematic diagram of the electromechanical/solid-state AC relay. 
           [0016]      FIG. 5  is a timing diagram of operation of the electromechanical/solid-state AC relay of  FIGS. 1-4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The present invention relates to an improvement in AC relays. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. 
         [0018]    The inventors have realized the desirability of a solution to the technical problem of the low reliability of electromechanical relays caused by arcing of the mechanical contacts, and the 1-volt drop of the solid-state relay. 
         [0019]    The electromagnetic relay usually has an iron core, a winding coil, an iron armature, a movable spring with movable contacts, and so on. As long as both ends of the coil are within a certain voltage, the coil will conduct a certain flow of current, resulting in electromagnetic effects. The iron armature is attracted by this electromagnetic force from the coil. The electromagnetic force overcomes a return force of a spring, or the force of air suction in a core. 
         [0020]    Static or dynamic contacts can be normally opened or normally closed by a spring or by air suction. When the coil voltage is removed, the electromagnetic force disappears, and the iron armature is returned to its original position by a spring or air suction. A pull-release thus turns on or cut offs the circuit. 
         [0021]    The contact resistance of the metallic contact points is very small. Therefore the electromagnetic relay often is suitable for high-power control applications. The electromagnetic relay depends on a mechanical operation, so it needs some time to turn on or off. This action time is the relay&#39;s operation time. Usually the electromagnetic relay operation time for turning on or off is about 5 ms to 30 ms. 
         [0022]    The solid state relay (SSR) is a type of switch component with no moving parts, having all solid-state electric parts. The SSR uses the switching characteristic of a bidirectional thrysistor to turn on and off currents without mechanical contacts that physically touch and spark. An AC solid-state relay can use a zero-crossing trigger, producing little EMI interference, and can be used safely for computer output interface. 
         [0023]    However, when the solid-state relay turns on, there is a voltage drop of about 1 Volt. When large currents pass through the solid-state relay, internal heating and power dissipation of the solid state relay can be large, producing a very serious problem. The price of a large, high-power solid-state relay is very high. 
         [0024]      FIG. 1  shows a side view of an electromechanical/solid-state AC Relay that combines the benefits of an EMR and a SSR while reducing the drawbacks of each. Shell  108  encloses both electromechanical and solid-state relay elements. 
         [0025]    Mechanical contact  102  is part of an electromechanical device that opens and closes in response to movement of iron armature  104 . Winding coil  106  creates the electromagnetic force that moves iron armature  104 , causing mechanical contact  102  to make or break electrical connection. Thus an EMR relay coil and contact are enclosed by shell  108 . 
         [0026]    Control circuit  110  is also integrated within relay Shell  108 . Control circuit  110  contains much of the circuitry shown later in  FIGS. 3-4 . 
         [0027]      FIG. 2  shows a bottom view of the electromechanical/solid-state AC relay of  FIG. 1 . Overall, the AC relay is a 6-port device with four input ports A, B, C, D and two output ports E and F. The load is connected at the two output ports E and F. The load has it&#39;s AC current switched by the AC relay. 
         [0028]    A control signal is applied to the A, B ports. This control signal on the A, B ports controls the E, F output ports. 
         [0029]    An AC input is applied to the C, D port. This AC input on the C, D port is connected to the E, F output port when the control signal indicates “turn on”. Thus the AC output load on the E, F ports is driven on from power supplied by the C, D ports. 
         [0030]    The control signal applied to the A, B ports, controls connecting together the D and F ports to “turn on” or “turn off”, and achieve the “On/off” function. When the control signal indicates “turn on”, port D is connected to port F. When the control signal indicates “turn off”, port D is disconnected from port F. 
         [0031]    The function of the coupling circuits is to form a conducting channel between input ports C, D and output ports E, F in response to the “turn on” input control signal at the A, B ports, and to cut off the electrical contact between input D and output port F when the control signal is in the “turn off” state. In the “turn off” state, output ports do not affect input ports. 
         [0032]    An AC ground or other AC common voltage can be connected to ports C and E. Thus ports C, E can be the same port. The AC relay could be regarded as a 5-port component rather than a 6-port component. 
         [0033]      FIG. 3  is a block diagram of the electromechanical/solid-state AC relay. Components on control circuit  110  ( FIG. 1 ) include coupling circuit  302 , control IC  304 , zero sampling circuit  306 , power supply circuit  308 , driving circuit  310 , and triac  314 . Triac  314  can be a pair of back-to-back SCR&#39;S, a TRIAC (TRIode for Alternating Currents), a thyristor, or similar components. 
         [0034]    The control signal is applied to control input terminals A and B. An AC input is applied to the two AC input terminals C and D. The AC output terminals E and F are connected to an AC load, such as the AC circuit to be switched on and off by the control signal. 
         [0035]    Coupling circuit  302  is connected to control input terminals A, B and drives the input terminal of control IC  304 . Zero-sampling circuit  306  and power-supply circuit  308  are connected to the two AC inputs C, D in parallel. The zero-crossing output signal terminal from zero-sampling circuit  306  is connected to the zero-crossing input signal terminal of control IC  304 . 
         [0036]    Control IC  304  generates a control signal to driving circuit  310 , which drives winding coil  312 . Winding coil  312  then moves the iron armature to force mechanical contact  316  into the open or closed position. Winding coil  312  connects to both of the output terminals of driving circuit  310 . 
         [0037]    Triac  314  and mechanical contact  316  connect in parallel to AC input terminal D and to AC output terminal F. Triac  314  and mechanical contact  316  switch on and off to selectively connect and disconnect terminal F from terminal D. The gate terminal of Triac  314  is directly controlled by control IC  304  while mechanical contact  316  is indirectly controlled by control IC  304 , through driving circuit  310  and winding coil  312 . 
         [0038]    The AC input terminal C connects to the AC output terminal E. Terminals C, E can carry a common AC ground or other common AC signal. 
         [0039]    Under normal working conditions, a control signal applied to the A, B ports can control the D, F ports of “turn on” or “turn off”, achieving the “On/off” function of the AC relay. 
         [0040]      FIG. 4  is a circuit schematic diagram of the electromechanical/solid-state AC relay. Optoelectronic-coupler  402  optically connects the control signal on inputs A, C to input terminal IN of control IC  426 . Optoelectronic-coupler  402  is sensitive, has a high response speed, and provides excellent voltage isolation. The input port of optoelectronic-coupler  402  has a good match with the input signal because the load of the input port is often an LED. Thus optoelectronic-coupler  402  can connect to a computer&#39;s output interface directly. 
         [0041]    AC input L connects to optoelectronic-coupler  402 , to the Vcc power-supply input of control IC  426 , to the emitters of transistors  428 ,  430 , to triac  424 , and to capacitors  404 ,  410 . Coil  432  controls connection of mechanical contacts between terminals L and L-OUT (D and F of  FIG. 3 ) in response to the RELAY output from control IC  426 , which turns on the base of transistor  430  to drive current through coil  432 . Diode  420  protects coil  432  from damage. 
         [0042]    Triac  424  is turned on and off by the TRIAC output from control IC  426 , which is directly connected to the gate of triac  424 . When triac  424  turns on, current can flow between L and L-OUT in parallel with the currents through the mechanical contacts controlled by coil  432 . Thus both mechanical contacts and triac  424  provide the relay&#39;s switched currents. 
         [0043]    The power circuits include simple resistance/capacitance drop voltage circuits and a power regulator built-in to control IC  426 . The power circuits are simple and dependable. Resistors  414 ,  416 , capacitor  406 ,  408 ,  410  and diode  422  perform power-supply and other functions. Resistor  418  limits power current through transistor  428  to maintain a constant voltage difference across capacitor  410  when control IC  426  switches its discharge DISC signal. Terminal N is an AC ground or other common AC voltage line. 
         [0044]    When the control signal is detected from optoelectronic-coupler  402 , control IC  426  samples its zero-crossing (ZC) input, which is a voltage generated by capacitor  402  and resistor  412  that are in series between the AC input terminal L and the AC common terminal N (ports D, C of  FIG. 3 ). 
         [0045]    When the AC zero-crossing is detected by control IC  426 , control IC  426  drives its TRIAC output to triac  424  to turn it on during the zero-crossing of the AC voltage. Then control IC  426  drives its RELAY output, turning on transistor  430  which turns on winding coil  432  which closes the mechanical contact points, connecting L to L-OUT. 
         [0046]    Control IC  426  later turns off its TRIAC output and triac  424 , depending on the mechanical contact controlled by coil  432  to drive the load current between L and L-OUT. 
         [0047]    When the control signal at optoelectronic-coupler  402  is de-asserted into the “turn off” state, control IC  426  again turns on its TRIAC output, turning on triac  424  and turning off winding coil  432  to open the mechanical contact point. Then control IC  426  turns off its TRIAC output and triac  424  during the zero-crossing of the AC voltage. Thus control IC  426  produces a series of actions. 
         [0048]      FIG. 5  is a timing diagram of operation of the electromechanical/solid-state AC relay of  FIGS. 1-4 . The control input signal on ports A, B switches low to signal the electromechanical/solid-state AC relay to “turn on” and close the mechanical contacts. Later in this diagram control input signal on ports A, B switches high to signal the electromechanical/solid-state AC relay to “turn off” and open the mechanical contacts. 
         [0049]    When the control input INPUT goes low, the control IC needs to switch on the load. However, rather than immediately switch on the triac and the electromechanical winding coil, the control IC waits for the next zero-crossing of the AC input on ports C, D, which is indicated by the pulses of zero-crossing signal ZC-OUT. When the ZC signal pulses high, the control IC turns on both the triac and the winding coil. Thus both the triac and the winding coil are turned on at the next AC zero-crossing by the control signal activating (driving low) its RELAY and TRIAC outputs. 
         [0050]    The triac switches on immediately, but the winding coil is a mechanical device that needs time move the armature, about 5 ms to 15 ms. Once the mechanical contact point closes, the triac has already been operating for several milliseconds and has reduced the voltage difference between the D, F terminals to about 1 volt, the voltage drop through the triac. 
         [0051]    Thus when the mechanical contacts eventually close, the voltage between the mechanical contact points is limited to about 1 Volt, which is the voltage drop of the triac. Dangerous surge currents cannot be produced and the electric arcs cannot appear, since the voltage difference is limited to only 1 volt. After the mechanical contact point closes, there is no voltage drop between the terminals of the triac, so the triac has no power dissipation. Thus the problems of the solid-state relay—the power dissipation and 1-volt drop of the triac—are eliminated by the inventors&#39; timing of operation of the triac and the winding coil. 
         [0052]    When the control input INPUT goes high, the control IC needs to turn off the load. The control IC immediately drives its RELAY output high, but there is a delay of 5-15 ms for the driver circuits to turn off the winding coil and physically move the armature and open the mechanical contacts. The control IC also immediately drives its TRIAC output low to turn on the triac immediately. Both the triac and the mechanical contacts now conduct the AC current. The triac turns on at the same time that the mechanical contact points are opening. 
         [0053]    The control IC keeps the triac on for 20 ms before turning off the triac. The mechanical contact points move apart within 15 ms, so the triac carries the AC current once the mechanical contacts move apart. 
         [0054]    When the mechanical contact points open, the voltage between the contacts is just the voltage drop of the triac, which is limited to 1 volt. The arc discharge phenomena cannot be produced. After 20 ms the control IC drives its TRIAC signal high to turn off the triac. The control IC can wait to turn off the triac until the next AC zero-crossing, so the surge currents cannot be produced and interference with the electricity grid system is eliminated. The system achieves the zero-cross triggering. 
         [0055]    The RELAY and DISC signals can be pulsed at 32 KHz with a 25% duty cycle as shown for power savings or other purposes. Pulsing the RELAY signal reduces current through winding coil  426  to prevent burnout or other damage to the coil. {What is the reason for this?} 
       ALTERNATE EMBODIMENTS 
       [0056]    Several other embodiments are contemplated by the inventors. For example additional resistors and capacitors and other components may be added at various locations for various purposes. Various sizes or values of the components may be substituted. The time periods may differ from the examples shown. 
         [0057]    Optoelectronic-coupler  402  could be a PC3H2 type; control IC  304  could be an integrated circuit of the PS8A0201 type; zero sampling circuit  306  may use a triode of the 2N5401 type; driving circuit  310  may use a triode with a 2N5401 type; triac  314  may be a BT134 type. Optoelectronic-coupler  402  can be universal optoelectronic-coupling component. Transistors or triodes can be universal triodes; they also can be integrated into the control IC. Other component types could be substituted. 
         [0058]    The winding coil and mechanical contacts may be normally open or normally closed. A spring or an air cushion or suction may be used for the recoil force. The movable arm or armature can have many shapes and may move in a straight line or in an arc or pivot or in other degrees of motion. 
         [0059]    The triac can be activated during AC zero-crossings when both making and breaking contact, or only when making contact. When breaking contact, the triac can be turned on at any time since the mechanical contacts are initially closed and carry most of the AC current. The triac may have an on voltage of 1 volt or less, such as 0.7 volt or 0.5 volt, depending on the technology and construction of the triac. 
         [0060]    The AC “ground” or common may be any reference voltage and does not necessarily have to be zero volts. For example, a high voltage may be designated as an AC common signal, and the sine waves or other AC waves have voltages below ground. The zero crossing refers to a middle voltage between the high and low peaks and troughs of the AC wave when the direction of current flow reverses. The middle voltage could be exactly midway between the highest and lowest voltages, or could be some other intermediate voltage. Zero-detection does not have to be exact to be effective, but could have some margin of error such as 10%. The AC waves could be sine waves or could have other wave shapes, and could operate at 60 Hz or at some other value. 
         [0061]    The background of the invention section and other sections may contain background information about the problem or environment of the invention rather than describe prior art by others. Thus inclusion of material in the background section and other sections is not an admission of prior art by the Applicant. 
         [0062]    Any advantages and benefits described may not apply to all embodiments of the invention. When the word “means” is recited in a claim element, Applicant intends for the claim element to fall under 35 USC Sect. 112, paragraph 6. Often a label of one or more words precedes the word “means”. The word or words preceding the word “means” is a label intended to ease referencing of claim elements and is not intended to convey a structural limitation. Such means-plus-function claims are intended to cover not only the structures described herein for performing the function and their structural equivalents, but also equivalent structures. For example, although a nail and a screw have different structures, they are equivalent structures since they both perform the function of fastening. Claims that do not use the word “means” are not intended to fall under 35 USC Sect. 112, paragraph 6. Signals are typically electronic signals, but may be optical signals such as can be carried over a fiber optic line. 
         [0063]    The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.