HK1113454A - Load control circuit and method for achieving reduced acoustic noise - Google Patents

Description

Load control circuit and method for obtaining reduced noise

Technical Field

The present invention relates to load control circuits, such as lamp dimming circuits, and more particularly to an improved load control circuit for reducing noise, particularly in connection with dimming control of transformer-powered lighting loads. The invention may also be used to control the speed of motors such as fans, motorized window treatments, and electrical tools such as drills, grinders, and sanders.

Background

Low voltage lighting devices, such as halogen lamps, have been increasingly used in recent years. These lamps operate at low voltages, for example 12V or 24V, and a transformer is used to reduce the normal line voltage to the low voltage required to operate the lamp.

Complaints about noise from users when operating such electric lamps are increasing. This noise is believed to be due to a number of factors, including: the use of low-profile transformers in the same space as the luminaries, the increase in the use of toroidal transformers (as opposed to "coil and core" transformers, such as transformers with EI cores, having laminated cores made of E-shaped and I-shaped sheets), and the increase in the use of open-wire or track low-voltage lighting in residential applications. Primarily, this increase appears to be due to the use of large VA (volt-ampere) toroidal transformers (typically in the range of 150-600 VA).

Noise has been a problem with Magnetic Low Voltage (MLV) loads. A lamp de-buzzing coil or choke placed in series with the transformer primary winding reduces or eliminates noise by increasing the rise time of the current. Nevertheless, this solution has proved to be insufficient in view of the fact that the above-mentioned factors are now often present in the embodiment of low-voltage luminaires. It appears that one cause of noise is that transformer saturation is made easier by Direct Current (DC) components in the input waveform. This is especially a problem when the transformer has little or no air gap, as is the case for example with toroidal transformers.

There is therefore a need for an improved load control circuit, and in particular, a dimming circuit for use in low voltage lighting and applications with MLV loads, to reduce noise generation.

Fig. 1 illustrates a typical prior art two-wire open-phase (sometimes referred to as "phase-control") dimming circuit 100. The dimmer circuit 100 is considered a two-wire dimmer because the only connections required are a HOT terminal (HOT terminal)102 and a DIMMED HOT terminal (DIMMED HOT terminal)106, the HOT terminal 102 being connected to a first terminal of a source of line frequency Alternating Current (AC) voltage 104, the DIMMED HOT terminal 106 being connected to a first terminal of a load 108. A second terminal of load 108 is connected to a second terminal of Alternating Current (AC) voltage source 104 to complete the electrical path. The dimmed hot output voltage comprises an open-phase AC voltage waveform, as is well known to those skilled in the art, wherein current is only supplied to the lamp load after a certain phase angle of each half-cycle of the AC waveform.

To accomplish this, a triac 110 is employed to control the amount of voltage delivered to the load 108. The timing circuit 120 includes a dual phase-shift resistor-capacitor (RC) circuit having a resistor R122, a potentiometer R124, and capacitors C126, C128. To turn on the triac 110 after a selected phase angle in each half cycle, the timing circuit 120 sets a threshold voltage, which is the voltage across the capacitor C128. The charge time of the capacitor C128 is varied in response to the change in resistance of the potentiometer R124 to change the selected phase angle at which the triac conducts. A diac 130 is connected in series with the control input or gate of the triac 110 and is used as a triggering device. The diac 130 has a breakover voltage (e.g., 30V) and passes current to the gate of the triac only when the threshold voltage exceeds the breakover voltage of the diac plus the gate voltage of the triac. The prior art circuit also employs an input noise/EMI filter stage comprising an inductor L142, a resistor R144, and a capacitor C146.

Another prior art circuit 200 is shown in fig. 2A. The circuit employs a voltage compensation circuit 250, the voltage compensation circuit 250 including a diac 252 and a resistor R254 for adjusting the voltage of the potentiometer R224 to compensate for line voltage amplitude variations. It is well known that diacs have a negative impedance transfer function, so that as the current through the diac decreases, the voltage across the diac increases. As the voltage across the dimmer decreases, the current through the diac 252 also decreases. As a result, the voltage across the diac 252 increases, causing the current through R224 to C228 to increase, thereby causing the capacitor C228 to charge to the threshold voltage very quickly. This results in an increase in the on time of the triac 210 to compensate for the reduced voltage across the dimmer, thereby maintaining the set light level.

In addition, the prior art circuit shown in fig. 2A includes a DC voltage correction circuit 260, the DC voltage correction circuit 260 including a capacitor C264 and a resistor R262 for maintaining a net average output voltage of zero volts DC. The operation of the DC voltage correction circuit is described in U.S. patent 4,876,498, which is incorporated herein by reference in its entirety and therefore will not be described further herein.

The prior art arrangements of fig. 1 and 2A are believed to result in the generation of excessive noise in a load, such as an MLV lamp load comprising a transformer-powered low voltage bulb, when the load is coupled to the output of a dimmer.

Fig. 2B shows the voltage waveform across a 600VA toroidal transformer provided by the prior art circuit of fig. 2A. The waveform exhibits asymmetry in both half-cycles. Asymmetric, as used herein, means that the positive half cycle t_2(POS)Conduction time and negative half period t of middle trigger triode_2(NEG)The conduction time of the middle trigger triode is different. As a result, the area under the voltage curve across the load during the positive half cycle (measured in volt-seconds) is different from the area under the voltage curve across the load during the negative half cycle (measured in volt-seconds). This asymmetry results in an output voltage having a net DC component. It is generally believed that this asymmetry causes the transformer to saturate, thus increasing noise. In part mark a, the overshoot voltage shown in fig. 2B indicates that the transformer is saturated due to asymmetry in the output voltage waveform. In this case, the lamp de-buzzing coil or choke will not be able to eliminate the noise from the transformer, which originates from the asymmetry in the output voltage, because the coil or choke does not eliminate the net DC component.

Fig. 3A shows a schematic diagram of another prior art circuit comprising a three-wire dimmer 300, said three-wire dimmer 300 having a neutral terminal for direct connection to the neutral line of the AC voltage source. The circuit has a similar structure to the prior art circuit of fig. 2A and includes a triac 310, a timing circuit 320, a trigger circuit 330, a voltage compensation circuit 350, and a DC correction circuit 360. The timing circuit 320 includes a potentiometer R324 for setting the desired on time of the triac 310 and thus the desired output voltage of the dimmer 300, and a capacitor C328 that charges to a threshold voltage. The trigger circuit 330 includes a current amplifier composed of diodes D331, D332 and transistors Q333, Q334, a full wave bridge rectifier composed of a bridge BR335, resistors R336, R337, and a threshold device composed of a silicon bi-directional switch 338, an optocoupler 339, and resistors R340, R341. The optocoupler 339 provides electrical isolation between the NEUTRAL terminal (NEUTRAL) and the triac 310. The bridge BR335 allows current to flow through the photodiode 339A of the optocoupler 339 from the same direction during both half cycles of the AC line voltage. The silicon bi-directional switch 338 allows current to flow through the photodiode 339A only when the voltage across the capacitor C328 reaches a threshold.

It has been found that the circuit of fig. 3A results in less noise than the circuits of fig. 1 and 2A. Fig. 3B shows the output waveform of the circuit of fig. 3A, showing how the waveform is made more symmetrical with a smaller DC component. The three-wire dimmer in fig. 3A has a more symmetrical output waveform because the presence of the neutral connection allows the timing circuit 320 to be disconnected from the load. The timing circuit 320 of the three-wire dimmer charges from the HOT terminal (HOT terminal) through the timing circuit 320 to the NEUTRAL terminal (NEUTRAL terminal). In contrast, timing circuit 220 of the two-wire dimmer of fig. 2A charges from the hot terminal through timing circuit 220 to the dimmed hot terminal, and then through the load to the neutral connection of the AC voltage source.

It has been recognized that if the conduction times of the bidirectional switches of the two-wire load control circuit are the same in both the positive and negative half-cycles, the output voltage waveform exhibits greater symmetry and, therefore, a reduced DC component. It is believed that the asymmetry of the voltage and current characteristics of the diac and triac in their respective modes of operation contributes to the asymmetry and DC component of the output waveform. Specifically, three causes of asymmetry have been identified: (1) the breakover voltage of the diac in a first direction is different from the breakover voltage of the diac in a second (opposite) direction; (2) the v-amp curve of the diac when conducting in the first direction is different from the v-amp curve of the diac when conducting in the second direction; and (3) the current into the gate of the triac when turned on in a first direction is different from the current out of the gate of the triac when turned on in a second (opposite) direction.

Referring to FIG. 3C, the voltage-current (V-I) characteristic of the diac can be seen. It has been found that the V-I characteristic of a diac operating in the first quadrant is rarely, if ever, symmetric with the V-1 characteristic of the same diac operating in the third quadrant. For example, V_BO+Is the breakover voltage of the diac in the first (or forward) direction of conduction, and cannot be equal in magnitude to V_BO-Said V is_BO-Is the breakover voltage of the diac in the second (or reverse) direction of conduction. The inequality in the magnitudes of the breakover voltages affects, among other things, the charging time of capacitor C228 shown in the two-wire dimmer of fig. 2A.

Shape of the V-I characteristic in the first (I) and third (III) quadrant operation, in particular, magnitude of breakover voltage, V_BB+And V_BB-Affecting the level at which capacitor C228 ultimately discharges. If these V-1 characteristics are not perfectly symmetrical, then at the end of each half-cycle of the line cycle, the capacitor C228 cannot discharge to the same point. This may result in the initial condition of the capacitor C228 not being the same at the beginning of each half cycle. Thus, the capacitor C228 will not always charge to the desired threshold voltage in the same amount of time from half-cycle to half-cycle.

Referring to FIG. 3D, therein may be seen the waveform, -V_C228The voltage across capacitor C228, and the waveform of the gate current of the triac of the two-wire dimmer of FIG. 2A, I_GATE. In FIG. 3D, the vertical voltage scale is 20V/div, the vertical current scale is 0.5A/div, and the horizontal time scale is 2 ms/div. In the figureIn order to facilitate observation, the capacitor voltage V_C228Has been reversed. It will be appreciated that at this point the triac begins conducting and when the triac begins conducting in a first (or positive) direction (corresponding to conduction in quadrant I), a current spike, S_I(about 0.65A) into the triac gate lead and when the triac begins conducting in the second (or negative) direction (corresponding to conduction in quadrant III), a peak current, S_III(about 1.1A), the outflow triggers the triode grid lead. Thus, it can be seen that the current flowing out of the triac gate during the negative half-cycle is almost twice as large as the current flowing into the triac gate during the positive half-cycle. The inequality in the magnitude of the spike currents in the two directions causes the capacitor C228 to discharge to different levels at the end of each half cycle, which in turn causes the initial condition of C228 to be different at the beginning of the subsequent half cycle. The difference in initial conditions of the capacitor C228 causes the conduction time of the triac to be different for one half-cycle than for the next.

Therefore, there is a need for a two-wire load control circuit that provides a symmetrical voltage waveform for an MLV load, such as a transformer-powered lamp load, that has substantially no DC component. In particular, there is a need for a two-wire dimmer having a diac and a triac wherein asymmetry in the diac and triac is substantially reduced or eliminated.

Disclosure of Invention

It is an object of the present invention to provide an improved load control circuit, e.g. a dimmer circuit that reduces noise, especially when used with MLV lamp loads.

It is another object of the present invention to provide a load control circuit to provide a voltage output waveform that is substantially free of a DC component.

The object of the present invention is achieved by a load control circuit comprising: a bidirectional semiconductor switch for switching at least a portion of both positive and negative half cycles of an alternating current source waveform to a load, the bidirectional semiconductor switch having a control electrode; the load control circuit further includes a phase angle setting circuit including a timing circuit that sets a phase angle during each half-cycle of the AC source waveform when the bidirectional semiconductor switch is turned on; the phase angle setting circuit comprises a voltage threshold trigger device connected in series with a control electrode of the switch; the phase angle setting circuit further comprises a rectifier bridge connected in series between the output of the timing circuit and the control electrode of the semiconductor switch, the rectifier bridge having a first pair of terminals connected in series between the output of the timing circuit and the control electrode of the semiconductor switch and a second pair of terminals connected to the voltage threshold triggering means, whereby noise generated in a load connected in series with the load control circuit is reduced.

The object of the present invention is also achieved by a method for reducing noise generated in an electrical load driven by an open-phase load control circuit from an Alternating Current (AC) source waveform, the method comprising: setting a phase angle during each half cycle of an AC source waveform when a bidirectional semiconductor switch is conducting, providing a voltage threshold trigger connected in series with a control electrode of the switch, whereby a control electrode current is supplied to the switch when a threshold voltage is exceeded; further comprising providing a control electrode current to the switch such that the control electrode current flows through the voltage threshold trigger device in only one direction, thereby reducing asymmetry in the control electrode current and helping to reduce noise in the load.

The object of the invention is also achieved by a load control circuit having first and second terminals for connection in series with a controlled load, comprising a bidirectional semiconductor switch for switching at least a portion of both positive and negative half cycles of an alternating current source waveform to the load, the bidirectional semiconductor switch having a control electrode; the load control circuit further comprises a phase angle setting circuit comprising a timing circuit that sets a phase angle during each half cycle of an AC source waveform when the bidirectional semiconductor switch is turned on, the phase angle setting circuit comprising a voltage threshold trigger device connected in series with a control electrode of the switch; the phase angle setting circuit further comprises a first circuit connected between the timing circuit and the control electrode of the semiconductor switch for ensuring that current flowing through the voltage threshold trigger device flows in only one direction, wherein the first circuit has a first pair of terminals connected in series between the output of the timing circuit and the control electrode of the semiconductor switch and a second pair of terminals connected with the voltage threshold trigger device, whereby noise generated in a load connected in series with the load control circuit is reduced.

The object of the present invention is also achieved by a two-wire dimmer for transferring electrical energy from an alternating current, i.e. a line voltage source, to a load, comprising: a bidirectional semiconductor switch for being coupled between the power source and the load, the semiconductor switch having a control input and being operable to provide an output voltage to the load; a timing circuit for being coupled between the power source and the load and having an output, the timing circuit for generating a signal indicative of a desired on-time of the bidirectional semiconductor switch; a trigger device having a first terminal electrically connected in series with the output of the timing circuit and a second terminal electrically connected in series with the control input of the bidirectional semiconductor switch, the trigger device having a first volt-ampere characteristic curve when current flows from the first terminal to the second terminal and a second volt-ampere characteristic curve when current flows from the second terminal to the first terminal, wherein the first volt-ampere characteristic curve substantially coincides with the second volt-ampere characteristic curve; and an impedance electrically connected in series between said output of said timing circuit and said control input of said semiconductor switch, said impedance ensuring that the magnitude of current flowing into said control input is substantially equal to the magnitude of current flowing out of said control input.

Other objects, features and advantages of the present invention will become apparent from the following detailed description of the invention which refers to the accompanying drawings.

Drawings

The invention is described in further detail below, in which:

FIG. 1 illustrates a prior art two-wire dimmer circuit;

FIG. 2A shows another prior art two-wire dimmer circuit;

fig. 2B shows an output voltage waveform of the dimmer circuit of fig. 2A;

FIG. 3A shows a prior art three-wire dimmer circuit;

FIG. 3B shows an output waveform of the dimmer circuit of FIG. 3A;

FIG. 3C shows the V-I characteristic of a typical diac;

fig. 3D illustrates a triac gate current and timing circuit capacitor voltage waveforms of the dimmer circuit of fig. 2A;

FIG. 4A shows an improved load control circuit in accordance with the present invention;

FIG. 4B shows an output voltage waveform of the load control circuit of FIG. 4A;

FIG. 4C shows a triac gate current and timing circuit capacitor voltage waveform of the load control circuit of FIG. 4A;

FIG. 5 illustrates a load control circuit for fan motor speed control in accordance with the present invention;

FIG. 6 illustrates a circuit of the present invention employing a voltage compensated diac; and

fig. 7 shows a plot of the Direct Current (DC) component of the output voltage waveform versus the effective value (RMS) value of the output voltage for various embodiments of load control circuits with and without the elements of the present invention.

Other objects, features and advantages of the present invention will be apparent from the following detailed description.

Detailed Description

Referring now to the drawings, fig. 4A illustrates an improved load control circuit, and in particular, a dimmer circuit 400 for reducing noise, in accordance with the present invention. The high side of the ac power source 404 is typically connected to the hot terminal 402 and one side of the primary winding of the transformer driving the lamp load is typically connected to the dimmed hot terminal 406. The dimmer circuit includes a noise/EMI filter circuit including an inductor L442, a resistor R444, and a capacitor C446. Resistor R422, potentiometer R424, and capacitors C426, C428 form a double phase-shifted RC timing circuit 420, wherein the time constant is variably set by potentiometer R424, thereby varying the time that capacitor C428 charges. Once the threshold of the triggering device (diac 430) is exceeded, the charging rate of capacitor C428 will in turn change the phase angle of the Alternating Current (AC) waveform when the bidirectional semiconductor switch (triac 410) is turned on.

In accordance with the present invention, to reduce noise, diac 430 is coupled into rectifier bridge 470, which rectifier bridge 470 includes diodes D472, D474, D476 and D478. The first pair of terminals AC1, AC2 of the rectifier bridge are connected in series with the output of the timing circuit (the junction of R424 and C428) and the gate of the triac 410, preferably in series with another resistor R480, the function of which will be explained later. The diac 430 is connected in parallel to a pair of terminals DC +, DC-of the second or DC output of the rectifier bridge.

The purpose of the rectifier bridge 470 is to ensure that the current through the diac 430 always flows in the same direction. This eliminates any asymmetry between forward and reverse conduction through the diac 430 since the current flow through the diac is always in the same direction for both the positive and negative half cycles. With the convention of positive current flow, the current flowing through diac 430 is for two half cycles in the direction shown by arrow 432. During the positive half-cycle, current flows through diode D472, diac 430 in the direction of arrow 432, and then through diode D476. For the negative half-cycle, current flows through diode D474, diac 430 in the direction of arrow 432, and then through diode D478. Thereby, any asymmetry caused by the flow of current in the opposite direction in the diac is eliminated.

Thus, the diac 430 and the rectifier bridge 470 constitute a trigger device having a first terminal AC1 electrically connected in series with the output of the timing circuit 420 and a second terminal AC2 electrically connected in series with the control input of the bidirectional semiconductor switch 410. Further, the trigger device has a first volt-ampere characteristic curve when current flows from the first terminal AC1 to the second terminal AC2, and a second volt-ampere characteristic curve when current flows from the second terminal AC2 to the first terminal AC 1. Because the rectifier bridge 470 inhibits current flow through the diac 430 in the same direction during both positive and negative line half cycles, the first volt-ampere curve substantially coincides with the second volt-ampere curve.

In addition, the compensating diac 252 in fig. 2A has been eliminated from the circuit in fig. 4A, thereby eliminating another source of asymmetry. Nevertheless, the bridge rectifier 470 shown in FIG. 4A can also be used in the circuit of FIG. 2A to reduce the asymmetry. This is shown in fig. 6, which shows the circuit of fig. 4A, but with a voltage compensating diac 652. The load control circuit of fig. 6 can be further modified by incorporating a compensating diac 652 within the rectifier bridge in a manner similar to the manner in which bridge 670 is incorporated into diac 630.

Resistor R480 serves as a gate current limiting impedance. The gate resistor limits the gate current so that the initial condition of the trigger capacitor C428 is substantially the same in successive positive and negative half cycles. The gate resistor R480 balances the gate current in both half-cycles to compensate for the discharge of the timing circuit capacitor C428 so that the initial conditions at the beginning of each successive half-cycle are substantially the same. Preferred values for resistor R480 range from about 33 ohms to about 68 ohms. The resistor R480 preferably has a value of about 47 ohms.

Although the gate current limiting impedance R480 has been shown as being located between the triggering device (including the diac 430 and the rectifier bridge 470) and the control conductor of the bidirectional semiconductor switch 410, the impedance R480 may be located anywhere in series electrical connection with the control conductor of the bidirectional semiconductor switch 410. For example, an impedance R480 may be located between the output of timing circuit 420 and the input of the triggering device (diac 430 and bridge 470). As another example, the impedance R480 may be located within the bridge 470 in series with the diac 430.

Fig. 4B shows the output voltage waveform of the circuit in fig. 4A. As shown, the waveform shows greater symmetry, as shown by the conduction time t of the triac in the positive half-cycle_4(POS)Substantially equal to the conduction time t of the triac in the negative half-cycle_4(NEG). In fig. 4B, the absence of a portion of waveform label a in fig. 2B indicates that the transformer load is no longer saturated and that the waveform in fig. 4B has a reduced DC component. The DC component of the waveform in fig. 4B was observed by placing an RC low pass filter between the output of the dimmer and the neutral line, and then measuring the Direct Current (DC) voltage at the output of the dimmer with a multimeter. With the circuit in FIG. 4A, at 120V_RMSThe dc component on the line typically measures from about 40mV to about 60 mV.

Turning now to fig. 4C, the triac gate current and timing circuit capacitor voltage waveforms of the load control circuit of fig. 4A can be seen. In FIG. 4C, the vertical voltage scale is 20V/div, the vertical current scale is 50mA/div, and the horizontal time scale is 2 ms/div. A current spike of about 150mA flows into the gate of the triac when the triac begins conducting in the positive half-cycle, and a current spike of about 150mA flows out of the gate of the triac when the triac begins conducting in the negative half-cycle. (in the graph of fig. 4C, the polarity of the output voltage has been reversed for ease of viewing.) not only is the relative difference between the triac gate currents reduced from about 70% (i.e., a difference between about 1.1A versus about 0.65A) to almost zero, but the absolute amount of the triac gate current is also reduced to about 14% of the previous level (i.e., from about 1.1A to about 150mA) as compared to the prior art.

Although the embodiment of fig. 4A shows diacs in the bridge as triggering devices, other triggering devices may be used. For example, the triggering device may be a Silicon Bilateral Switch (SBS) inside the bridge, a bilateral trigger switch inside the bridge, or a zener diode inside the bridge.

Fig. 5 and 6 show two further embodiments of the invention. Fig. 5 shows an embodiment suitable for controlling the speed of a motor, such as a fan motor. The main difference between the embodiment in fig. 5 and the embodiment in fig. 4A is the elimination of capacitor C426. Capacitor C426 helps to eliminate "pop on" in dimmers for lamp loads. This is a hysteresis in that when going from the off state to the desired low brightness, the user must first increase the brightness to a level above the desired brightness before the lamp is turned on, and then dim the brightness back to the desired low brightness. However, for motor loads, the voltage used to drive the motor rarely drops below 60 volts even at the lowest speeds, which is the voltage at which dimmers typically "pop on" conditions. Therefore, the hysteresis eliminating capacitor may be generally omitted from the motor control load circuit. Nevertheless, the embodiment of fig. 5 can be used with a lamp load when the "pop on" phenomenon is not an issue.

Fig. 6 shows the prior art dimmer circuit of fig. 2A modified in accordance with the present invention by placing the triggering device, diac 630, inside the rectifier bridge 670 and placing the gate current limiting impedance, resistor R680, in series electrical connection with the gate of the bidirectional semiconductor switch, triac 610.

Fig. 7 shows a plot of the DC component of the output voltage waveform versus the RMS value of the output voltage for various embodiments of load control circuits with and without the elements of the present invention. The values shown in fig. 7 were obtained by measuring the DC output of different two-wire load control circuit configurations connected to a line voltage source to drive a 120V incandescent lamp load.

In fig. 7, the curve labels diac + and diac-indicate the DC component of the output voltage waveform of the prior art dimmer circuit of fig. 2A traversing substantially the entire dimming range, so that no significant amount of light is emitted by the lamp (about 20V)_RMS) Low end to when substantially all of the available line voltage (about 115V)_RMS) Are supplied to the high end of the lamp.

The plot labeled diac + represents the output of a prior art two-wire dimmer circuit having a trigger device diac mounted in a first direction, and the plot labeled diac-represents the output of the same dimmer circuit having a trigger device diac mounted in a second, opposite direction. The curve labels diac +/47ohm and diac-/47ohm represent the output of the prior art two-wire dimmer circuit with the addition of a 47 Ω triac gate current limiting resistor. The plot labeled diac w/bridge represents a prior art two-wire dimmer circuit with the addition of a triggering device, i.e., a diac, within the full-wave rectifier bridge. Finally, the curve labels diac w/bridge&47 ohms represents the output of the load control circuit embodiment in FIG. 4A. Thus, it can be seen that the DC component of the output voltage is preferably below 0.2V_Dc, more preferably less than 0.1V_Dc, throughout substantially the entire dimming range of the load control circuit.

Although the present invention has been described with reference to particular embodiments, many variations and modifications and other uses will become apparent to those skilled in the art. Accordingly, the invention is not to be limited by the specific disclosure herein, but only by the appended claims.

Claims (52)

1. A load control circuit having first and second terminals for connection in series with a controlled load, the load control circuit comprising a bidirectional semiconductor switch for switching at least a portion of both positive and negative half-cycles of an ac source waveform to the load, the bidirectional semiconductor switch having a control electrode, the load control circuit further comprising:

a phase angle setting circuit including a timing circuit that sets a phase angle during each half cycle of an AC source waveform when the bidirectional semiconductor switch is turned on; the phase angle setting circuit comprises a voltage threshold trigger device connected in series with a control electrode of the switch; said phase angle setting circuit further comprising a rectifier bridge connected in series between the output of said timing circuit and the control electrode of said semiconductor switch, the rectifier bridge having a first pair of terminals connected in series between the output of said timing circuit and the control electrode of said semiconductor switch and a second pair of terminals connected to said voltage threshold triggering means;

whereby noise generated in a load connected in series with the load control circuit is reduced.

2. The circuit of claim 1, wherein the voltage threshold trigger device comprises a diac, a silicon diac, a diac, or a zener diode.

3. The circuit of claim 1, wherein the semiconductor switch comprises a triac.

4. The circuit of claim 1, wherein the timing circuit comprises a resistor-capacitor time constant circuit.

5. The circuit of claim 1, wherein the rectifier bridge comprises four diodes connected in a bridge rectifier configuration.

6. The circuit of claim 4, wherein the resistor-capacitor time constant circuit includes a potentiometer for adjusting a phase angle when conduction of the semiconductor switch occurs.

7. The circuit of claim 1, further comprising a filter comprising an inductor coupled in series with the load control circuit.

8. The circuit of claim 1, further comprising a filter comprising a resistor-capacitor circuit coupled across the load control circuit terminals.

9. The circuit of claim 1, wherein the load comprises a step-down transformer having a primary winding coupled in series with the load control circuit and having a secondary winding connected to a low voltage lamp load.

10. The circuit of claim 9, wherein the transformer comprises a toroidal transformer.

11. The circuit of claim 1, further comprising a resistor coupled in series with a control electrode of the switch.

12. The circuit of claim 1, wherein the rectifier bridge ensures that current in the voltage threshold trigger device flows in only one direction.

13. The circuit of claim 1, wherein the load comprises a lamp load.

14. The circuit of claim 1, wherein the load comprises a motor.

15. The circuit of claim 1, further comprising a voltage compensation circuit coupled to the time constant circuit to vary the voltage provided at the output of the timing circuit to compensate for the voltage across the load control circuit.

16. The circuit of claim 15, wherein the voltage compensation circuit comprises a diac.

17. A method for reducing noise generated in an electrical load driven by an open-phase load control circuit from an ac source waveform, the method comprising:

setting a phase angle during each half-cycle of the AC source waveform when a bidirectional semiconductor switch is conductive;

providing a voltage threshold trigger device connected in series with a control electrode of the switch, whereby a control electrode current is provided to the switch when a threshold voltage is exceeded; the method also includes providing a control electrode current to the switch such that the control electrode current flows through the voltage threshold trigger device in only one direction, thereby reducing asymmetry in the control electrode current and helping to reduce noise in the load.

18. The method of claim 17, wherein the step of providing a control electrode current to the switch comprises: providing a rectifier bridge in series between the output of the phase angle setting circuit and the control electrode of the switch, the rectifier bridge having a first pair of terminals connected in series between the output of the phase angle setting circuit and the control electrode of the switch and a second pair of terminals connected to the voltage threshold trigger means.

19. The method of claim 17, further comprising providing a resistor in series with the control electrode to balance the current flowing to the control electrode in each half-cycle.

20. A load control circuit having first and second terminals for connection in series with a controlled load, the load control circuit comprising a bidirectional semiconductor switch for switching at least a portion of both positive and negative half-cycles of an ac source waveform to the load, the bidirectional semiconductor switch having a control electrode, the load control circuit further comprising:

a phase angle setting circuit including a timing circuit that sets a phase angle during each half cycle of an AC source waveform when the bidirectional semiconductor switch is turned on;

said phase angle setting circuit including a voltage threshold trigger device connected in series with the control electrode of said switch, said phase angle setting circuit further including a first circuit connected between said timing circuit and the control electrode of said semiconductor switch for ensuring that current flowing through said voltage threshold trigger device flows in only one direction, the first circuit having a first pair of terminals connected in series between the output of said timing circuit and the control electrode of said semiconductor switch and a second pair of terminals connected to said voltage threshold trigger device;

whereby noise generated in a load connected in series with the load control circuit is reduced.

21. The circuit of claim 20, wherein the first circuit comprises a rectifier bridge.

22. The circuit of claim 20, wherein the voltage threshold trigger device comprises a diac, a silicon diac, a diac, or a zener diode.

23. The circuit of claim 20, wherein the semiconductor switch comprises a triac.

24. The circuit of claim 20, wherein the timing circuit comprises a resistor-capacitor time constant circuit.

25. The circuit of claim 21, wherein the rectifier bridge comprises four diodes connected in a bridge rectifier configuration.

26. The circuit of claim 24, wherein the resistor-capacitor time constant circuit includes a potentiometer for adjusting a phase angle when conduction of the semiconductor switch occurs.

27. The circuit of claim 20, further comprising a filter comprising an inductor coupled in series with the load control circuit.

28. The circuit of claim 20, further comprising a filter comprising a resistor capacitor circuit coupled across the load control circuit terminals.

29. The circuit of claim 20, wherein the load comprises a step-down transformer having a primary winding coupled in series with the load control circuit and having a secondary winding connected to a low voltage lamp load.

30. The circuit of claim 29, wherein the transformer comprises a toroidal transformer.

31. The circuit of claim 20, further comprising a resistor coupled in series with a control electrode of the switch.

32. The circuit of claim 20, wherein the load comprises a lamp load.

33. The circuit of claim 20, wherein the load comprises a motor.

34. The circuit of claim 20, further comprising a voltage compensation circuit coupled to the time constant circuit to vary the voltage provided at the output of the timing circuit to compensate for the voltage across the load control circuit.

35. The circuit of claim 34, wherein the voltage compensation circuit comprises a diac.

36. A two-wire dimmer for delivering electrical energy from an ac line voltage source to a load, the two-wire dimmer comprising:

a bidirectional semiconductor switch for being coupled between the power source and the load, the semiconductor switch having a control input and being operable to provide an output voltage to the load;

a timing circuit for being coupled between the power source and the load and having an output, the timing circuit operable to generate a signal indicative of a desired on-time of the bidirectional semiconductor switch;

a trigger device having a first terminal electrically connected in series with an output of the timing circuit and a second terminal electrically connected in series with a control input of the bidirectional semiconductor switch, the trigger device having a first volt-ampere characteristic curve when current flows from the first terminal to the second terminal and a second volt-ampere characteristic curve when current flows from the second terminal to the first terminal, wherein the first volt-ampere characteristic curve substantially coincides with the second volt-ampere characteristic curve; and

an impedance electrically connected in series between the output of the timing circuit and the control input of the semiconductor switch such that the impedance ensures that the magnitude of current flowing into the control input is substantially equal to the magnitude of current flowing out of the control input.

37. A dimmer according to claim 36, wherein the triggering means comprises:

a rectifier bridge having a first pair of terminals for receiving an alternating voltage and a second pair of terminals for outputting a direct voltage, wherein said first pair of terminals are said first and second terminals of said triggering device; and

a diac coupled between the second pair of terminals of the rectifier bridge.

38. The dimmer of claim 37, wherein the impedance comprises a resistor.

39. A dimmer according to claim 38, wherein the timing circuit comprises a double phase-shifting resistor capacitor circuit having a potentiometer.

40. The dimmer of claim 38, wherein the timing circuit further comprises a voltage compensation circuit comprising:

a second rectifier bridge having a first pair of terminals for receiving an alternating voltage and a second pair of terminals for outputting a direct voltage; and

a second diac coupled between the second pair of terminals of the rectifier bridge;

whereby the voltage compensation circuit is operable to vary the desired on-time in inverse relationship to the effective voltage of the power source to substantially maintain the electrical energy delivered to the load at a desired level.

41. The dimmer of claim 40, wherein the timing circuit further comprises a DC compensation circuit, the DC compensation circuit comprising:

a DC compensation capacitor electrically connected in series between the voltage compensation circuit diac and the load; and

a DC compensation resistor electrically connected in series between the power supply and a junction of the DC compensation capacitor and the voltage compensation circuit diac;

whereby the dc compensation circuit is operable to reduce the dc component of the output voltage by causing the conduction time of the bidirectional semiconductor switch to increase in an ac half-cycle and decrease in a complementary ac half-cycle, thereby substantially rendering the conduction time of the bidirectional semiconductor switch equal in each half-cycle.

42. A dimmer according to claim 36, wherein the timing circuit comprises a single phase-shift resistor-capacitor circuit.

43. A dimmer according to claim 42, wherein the timing circuit comprises a double phase-shifting resistor-capacitor circuit.

44. The dimmer of claim 43, wherein the timing circuit further comprises a potentiometer.

45. The dimmer of claim 42, wherein the timing circuit further comprises a potentiometer.

46. The dimmer of claim 36, wherein the timing circuit further comprises a voltage compensation circuit; the voltage compensation circuit is operably coupled to vary a conduction time of the bidirectional semiconductor switch inversely related to an effective voltage of the power source to substantially maintain the electrical energy delivered to the load at a desired level.

47. A dimmer according to claim 46, wherein the voltage compensation circuit comprises a diac.

48. A dimmer according to claim 47, wherein the voltage compensation circuit further comprises a rectifier bridge having a first pair of terminals for receiving an AC voltage and a second pair of terminals for outputting a DC voltage; wherein the diac is coupled between the second pair of terminals of the rectifier bridge.

49. The dimmer of claim 48, wherein the output voltage comprises an alternating current component and a direct current component; the dc component has a net value of less than 0.1 volts.

50. The dimmer of claim 36, wherein the impedance is coupled between the second terminal of the triggering device and the control input of the bidirectional semiconductor switch.

51. The dimmer of claim 36, wherein the impedance is coupled between the output of the timing circuit and the first terminal of the triggering device.

52. A dimmer according to claim 37, wherein the impedance is coupled between the second pair of terminals of the rectifier bridge in series electrical connection with the diac.