Phase Control Circuit for Regulating Power

This invention relates generally to phase control circuits and, more particularly, to a digital circuit for providing an output pulse which is accurately positioned with respect to the zero crossover points of an a.c. line signal.

The use of analog and RC circuits for controlling the phase firing of thyristors in a.c. circuits is well known. It is intended that these circuits control how long an a.c. powered device remains "on" during a particular a.c. cycle. For example, U.S. Pat. No. 3,967,172 describes an RC control circuit which controls the speed of an alternating current motor in a refrigeration system. However, due to the inherent characteristics of the circuit components (resistors, capacitors and coils), continuous, accurate, and absolute control over the full a.c. cycle is impossible partially due to what may be referred to as "firing point drift". Thus, the disclosed circuit is of limited value. Further, the disclosed control circuit is complex and costly; factors which must be seriously considered since less than optimum control is being achieved.

U.S. Pat. No. 3,968,401 describes a circuit for controlling the intensity of a light source. While many of the components of this circuit are of a digital nature, an analog control signal is generated thus presenting those problems described above in connection with "firing point drift".

Another problem associated with known phase control circuits is their lack of interchangeability. The control circuit of U.S. Pat. No. 3,967,172, described briefly hereinabove, cannot be substituted for the control circuit described in U.S. Pat. No. 3,968,401.

It is an object of the present invention to provide an improved phase control circuit.

It is a further object of the present invention to provide an accurate, reliable and economical circuit for controlling the firing of thyristors in a.c. circuitry.

It is a still further object of the invention to provide a phase control circuit which can be used in conjunction with a large number of products, i.e. military, commercial, industrial and consumer oriented products and sub-assemblies.

It is a further object of the invention to provide a phase control circuit which is both simple and safe to operate. A safety feature provided by the present invention is total operator isolation from the a.c. line.

It is still another object of the invention to provide a phase control circuit which consumes little current (a few nanoamperes), operates at low voltage (typically 3 volts to 18 volts) and has high noise immunity.

It is a still further object of the present invention to provide a full range of phase control over an a.c. cycle, presently impossible using known circuits, by employing only digital techniques in the generation of the control signal.

According to a broad aspect of the invention there is provided a method for generating a pulse in synchronism with zero crossover points of an applied signal, comprising the steps of generating said pulse responsive to the zero crossover points at a predetermined time, and varying said predetermined time.

According to another broad aspect of the invention there is provided a method for varying the length of time during which a device has electrical power applied thereto comprising the steps of periodically generating a pulse for enabling the application of power to said device at a predetermined time, and varying said predetermined time.

According to still another broad aspect of the invention, there is provided a circuit for varying the length of time during which a device has electrical power applied thereto. This circuit comprises generating means for generating a pulse for enabling the power at a predetermined time, and means for changing the predetermined time.

According to a still further broad aspect of the invention, there is provided a circuit for generating a pulse in synchronism with zero crossover points of an applied signal. The circuit comprises means responsive to the zero crossover points for generating the pulse at a predetermined time and means for varying the predetermined time.

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a functional block diagram of a digital full wave a.c. phase control circuit according to the present invention;

FIGS. 2a-2f are waveforms for assisting in the explanation of the circuit in FIG. 1;

FIG. 3 is a more detailed schematic diagram of the zero crossover detector in FIG. 1;

FIG. 4 is a circuit diagram for generating the sync clock signal used by the apparatus of FIG. 1;

FIG. 5 is a schematic diagram of a lamp circuit for use as the load in FIG. 1;

FIG. 6 is a circuit diagram of an oscillator for generating the slew dock signal and apparatus for generating control signals used in the up/down counter of FIG. 1;

FIG. 7 is a block diagram illustrating in more detail the up/down counter and increment counter of FIG. 1;

FIG. 8 is a block diagram of an alternate means for controlling the up/down counter of FIG. 1;

FIG. 9 is a detailed block diagram of the full on/off decoder and counter enable of FIG. 1; and

FIG. 10 is a partial block diagram of a modification of the circuit of FIG. 1 to enable use of said circuit in a d.c. system.

FIG. 1 is a functional block diagram of a programmable and universal, digital full wave a.c. phase control circuit according to the present invention.

Load and load control 2, not considered a part of the invention, may be any device for which phase control is desired, e.g. a lamp, an electric motor, a heater, etc. To explain the inventive phase control circuit, it will be assumed that load 2 is an electric lamp having its power input coupled to the output of a triac.

In order to provide phase control over the entire a.c. cycle, it is necessary that the triac be triggered "on" by increment counter 4 at least once every one-half cycle. To accomplish this, a zero crossover detector monitors the a.c. line signal which is applied at terminal 8 and generates a load pulse each time the a.c. signal crosses zero. If the line signal is a standard 60 Hz signal, load pulses will be generated at a frequency of 120 Hz. Each load pulse applied at input 16 of increment counter 4 causes the contents of up/down counter 10 to be loaded into increment counter 4 over lines 12a-12h. The sync clock signal which is applied to increment counter 4 at input 14 is chosen such that a maximum count will be reached, each time resulting in a control trigger, every one-half cycle or 120 Hz for triggering the triac and turning on the lamp. The triac will of course be triggered "off" by the zero crossing of the a.c. line signal applied at input 18. Thus it can be seen that for each half cycle of line signal, load 2 will be energized for a fraction of the half cycle.

For an understanding of how the fraction of a half cycle during which load 2 is energized may be varied, attention is drawn to FIGS. 2a-2f. FIG. 2a shows a typical a.c. line signal and FIG. 2b shows a series of load pulses generated by zero crossover detector 6 and applied to increment counter 4. Each load pulse causes the contents of up/down counter 10 to be loaded into increment counter 4. Let us assume that the count loaded into increment counter 4 is approximately one half its maximum count capability. Since its maximum count capability corresponds to a full one-half cycle, half of its maximum count capability will result in a control trigger at approximately the middle of each half cycle as shown in FIG. 2c. Since the load 2 will be triggered "on" by the control trigger and "off" by the zero crossing of the a.c. line signal, the load energization curve will appear as in FIG. 2d. If now the count loaded into increment counter 4 is approximately one-fourth its maximum count capability, control triggers generated will be positioned as shown in FIG. 2e, i.e. approximately three-fourths of the distance between successive zero crossings of the a.c. line signal. This results in a load energization curve of the type shown in FIG. 2f. Clearly now, the greater the number loaded into increment counter 4, the sooner a control trigger will be generated. Since the above referred triac is turned "on" by the control trigger and "off" by the next zero crossing of the a.c. line signal, the greater the number loaded into increment counter 4, the longer the triac will be "on" during each half cycle thus resulting in a greater light intensity.

Referring back to FIG. 1, up/down counter 10 has a count capability no greater than that of increment counter 4 and may be made to count up or down by properly energizing an up/down input and enabling a slew clock signal to be applied at input 22. While the sync clock rate was determined by the a.c. line frequency, the slew clock rate is independent of all components and variables and can be varied to suit a particular application. The above referred to enabling of the slew clock is shown functionally in FIG. 1 as including an AND gate 24 having the slew clock signal as a first input and an enable signal as a second input.

Instead of allowing up/down counter 10 to operate as described above, a predetermined count may be loaded via lines 26a-26h if parallel load input 28 is enabled and the slew clock is disabled. In this manner, the inventive phase control circuit may be, for example, coupled to and used in conjunction with a microprocessors and advanced memory systems to create unlimited functional control via programmability.

Each of the outputs of up/down counter 10 are applied not only to increment counter 4 but also to a full on/off decoder and counter inhibit. This decoder determines if up/down counter 10 is in its minimum or maximum state and, if so, it halts up/down counter 10 to prevent run-over of data input or control signals into the next occurring a.c. half cycle. This is accomplished via inhibit line 32. Further, decoder 30 generates "full on" and "full off" signals corresponding to maximum count and minimum count respectively.

Now that the inventive phase control circuit has been described generally, certain portions of the system will be described in more detail. However, it is to be understood that the following is given by way of example only and is not intended to limit the scope of the invention.

Referring to FIG. 3, there is shown a zero crossover detector circuit incorporating an MC3370P zero voltage switch 34 manufactured by Motorola Semiconductors, Phoenix, Arizona. The a.c. line signal is applied to terminal b of switch 34 and terminal a of switch 34 after passage through resistor R1, which resistor acts as a voltage reducing resistor and by proper selection, may be used to derive a desired load pulse width. The series combination of resistor R2 and capacitor C1 provides noise rejection. Resistors R3 and R4 are reference resistors, and capacitors C2 and C3 provide for further noise rejection and ripple suppression. This circuit not only provides for the required load pulse at 36, and may provide the logic voltages Vdd and Vss at 38 and 40 respectively which are required by the remainder of the digital phase control circuit. Adjustable resistor R5 is provided for achieving a properly shaped load pulse.

In FIG. 1, the sync clock which is applied to increment counter 4 via terminal 14 was shown and described as merely an input of sufficient frequency to provide a control trigger at least once every half cycle. FIG. 4 shows one example of a digital oscillator which comprises two inverters 42 and 44, capacitor C4, resistor R6 and adjustable resistor R7. These components may be adjusted and selected to provide a sync clock signal of a desired frequency.

In the description of FIG. 1, it was stated that the load 2 could be a lamp having its power input controlled by a triac. FIG. 5 shows such an arrangement. A lamp 46 is coupled to the a.c. line signal and to the output of triac 48. The triac is likewise coupled to the a.c. line signal and to the collector of transistor 50. A control trigger from increment counter 4 is applied to the base of transistor 50 via resistor R8 rendering the transistor conductive and switching triac 48 "on". In this manner, a.c. power will be delivered to lamp 46 until the next zero crossing of the a.c. signal.

FIG. 6 shows a second digital oscillator for generating the slew clock signal and additional apparatus for generating other control signals used by the up/down counter. A first mechanical switch 52, when closed, generates a reset signal which is applied via debounce circuit 54 to up/down counter 10. A second switch 56 when closed causes a voltage to be applied to up/down counter 10 via debounce circuit 58 instructing it to count up. Capacitors C6, C7 and C9 and Resistors R10, R11 and R17 each coupled to Vss function together to eliminate voltage spikes which result from operation of the mechanical switches. Switch 60, while called a "down" switch enables the slew clock oscillator comprising gate 62, inverter 64, capacitor C8, resistor R12 and adjustable resistor R13 via debounce circuit 68. Since switch 56, when "open" instructs counter 10 to count down, switch 60 when "closed" will enable the slew clock oscillator. When switch 56 is closed and switch 60 open, the oscillator is enabled via diode 66. It should be apparant that the circuit is operational even if diode 66 is removed as long as switch 60 is closed to enable the slew clock oscillator. Capacitor C7 and resistor R11 eliminate voltage spikes resulting from the operation of switch 60. Components R12, R13 and C8 may be selected and adjusted to yield a slew clock signal of a desired frequency. It should be clear that switches 52, 56 and 60 may be mechanical, ultrasonic, optical, analog or RC switches.

FIG. 7 is a block diagram of a specific embodiment of the up/down counter 10 and increment counter 4 shown in FIG. 1. Two 4-bit up/down counters 70 and 72 are cascaded to form a single 8-bit up/down counter. Such 4-bit up/down counters are commercially available from, for example, Mortorola Semiconductor, Phoenix, Arizona, and bear the part number MC14516P. It can be seen that each 4-bit up/down counter has an up/down control input 74 and 76; a master reset input 78 and 80; four (P.sub.0 -P.sub.3) parallel load data inputs 82a-82h; a parallel load control input 84 and 86 each coupled to Vdd via R14, switch 88 and debounce circuit 90; a slew clock input 92 and 94; and four (Q.sub.0 Q.sub.3) output data bits 96a-96h.

The increment counter (4 in FIG. 1) comprises two 4-bit binary counters 98 and 100 cascaded together to form an 8-bit binary counter. Each 4-bit counter has four (P.sub.0 P.sub.3) parallel load date inputs 102a-102h for receiving data from outputs 96a-96h respectively each time a load pulse is received at 104 and 106. Such 4-bit binary counters are commercially available from Motorola Semiconductor, Phoenix, Ariz. and bear a part number MC14161P. The count enable 108 and master reset inputs 110 and 112 are tied to Vdd. The sync clock signal is applied to binary counters 98 and 100 at inputs 118 and 120. Finally, the control trigger is taken off counter 100 at 122. As a result of using an 8-bit counter, there are 256 trigger points in each half a.c. cycle yielding full "off" to full "on" control.

Since it is desired to have at least one control trigger generated for each half cycle of line signal or 120 triggers per second for a 60 Hz a.c. line signal the sync clock rate must be at least 120.times.16.times.16=30,720 Hz.

FIG. 8 illustrates an alternate means of controlling the counting operation of up/down counter 10. In this case, the output of counter 10 is applied via inverter 124 to the clock input of flip-flop 126. The Q output of flip-flop 126 is applied to the DATA (D) input, and the Q output is applied to the up/down control input of counter 10. Thus, when Q=1 and the counter begins counting up, the intensity of the lamp being controlled will increase until a maximum count has been reached. At this point, flip-flop 126 will be clocked causing it to change state; i.e. Q="0". The counter will now count down until a minimum condition has been attained. The flip-flop, at this point again changes state and the process is repeated. With this arrangement, an operator merely enables the slew clock signal and awaits the desired lamp intensity irrespective of whether this represents an increase or decrease in intensity.

FIG. 9 is a detailed logic diagram of the full on/off decoder and counter inhibit 30 shown generally in FIG. 1. The data outputs 96a-96h (FIG. 7) from the up/down counter are applied to the inputs of NOR gate 128 and NAND gate 130. The output of NOR gate 128 is applied to the input of invertor 132 output of which is applied to the input of OR gate 136 in conjunction with the up/down control signal. The output of OR gate 136 is applied to a first input of NAND gate 138. The output of NAND gate 130 is applied to inverter 134, the output of which is applied to NAND gate 140 in conjunction with the up/down control signal. The output of NAND gate 140 is applied to the second input of NAND gate 138. The output of NAND gate 138 is applied to a "counter inhibit" input of the up/down counter (FIG. 7). In this manner, whenever the slew pulse up/down counter is counting up and a maximum condition is realized, the up/down counter will be halted. Likewise, when the up/down counter is counting down and a minimum condition is reached, the up/down counter will be halted. This may be used to prevent run over of data input or control signals to the next occurring a.c. half cycle. Besides halting the operation of the up/down counter, light emitting diode (LED) 144 gives a visual indication when the up/down counter is in a maximum state. LED 144 is coupled between Vss and the output of inverter 134 via resistor R16, buffer gate 148 and invertor 150. Likewise, LED 142 gives a visual indication when the up/down counter is in a minimum state. LED 142 is coupled between Vdd and the output of inverter 132 via resistor R15 and buffer gate 146.

The above described control circuit can also be employed in a d.c. system, e.g. controlling the speed of a d.c. motor, by using an auxilliary clock system which may generate artificial load signals. Referring to FIG. 10, the sync clock signal is not only applied to increment counter 4 at 14, but is also applied to an auxilliary counter 152. A predetermined state of this counter may be decoded in accordance with known techniques to provide a load pulse for "LOADING" the data ON inputs 12a-12h into the increment counter 4 as described hereinabove. The resultant load pulse may be used to reset the load pulse generating apparatus.

Thus, there has been provided a versatile programmable digital full wave phase control circuit which is accurate, efficient, reliable and economical. It is simple to operate with total operator isolation from the line voltage and current. With such a suitable control circuit, wide use could result in considerable energy savings since electrically powered devices rarely require maximum power for satisfactory operation.

In the interest of completing the above description, the following is a table of component values used in the above description:

While the invention has been particularly shown and described with reference to the preferred embodiments above, it will be understood by those skilled in the art that the foregoing and other changes in the form and details may be made therein without departing from the spirit and scope of the invention.