Zero Crossing Phase Fired Controller

Difficulties are often encountered in electrical circuits because of the variation in line voltage and errors due to the tolerance levels of the various components. This is particularly true in phase fired control circuits when there are large line voltage variations and many components involved. Variations caused by these factors lead to large shifts in the firing angle of the control circuit. Additionally, the setting of a control resistor can present problems if setting of the optimum operating point is required at nominal line voltage at the center of the variable control resistor travel.

A phase fired control system is provided which includes a clamping and zero crossing detector circuit that controls the charging and discharging of a capacitor. The capacitor is discharged when a low signal is detected by the zero crossing detector and charged when a high signal is detected. A comparator receives the output voltage from the capacitor and a variable standard voltage, compares the two and sends a signal to an inverting driver in response thereto. The inverting driver enables a light emitting diode that controls a gate triac that in turn controls a load triac for serving a given function.

FIG. 1 shows a circuit of a phase fired controller that incorporates the instant invention.

FIGS. 2a-2e show voltage wave forms at various locations within the circuit shown in FIG. 1.

A control system is generally shown at 10 in FIG. 1 and includes a source of power 12 such as a conventional 110 volt line circuit that is in connection with a transformer T1. The output end of the transformer T1 is in connection with a grounded bridge having four diodes D1, D2, D3 and D4 and which acts as a power supply RECTIFIER. Another diode D5 receives the output signal from the bridge 14 and blocks d.c. voltage from the power supply filter. A power supply 16 supplies a fixed voltage level VA, such a twelve volts, through a lead 18 to the balance of the circuit where required and also supplies a reference signal REF through a lead 20. The leads 18, 20 are grounded through a pair of resistors 22. A grounded capacitor C1 is located between the diode D5 and the system power supply 16. This combination filters the output from the diode D5.

A lead 26 is connected to the output of the bridge 14 and a clamping and zero crossing detector circuit 30 which includes a pair of resistors R3 and R5. The voltage on this lead 26 will have the wave form shown in FIG. 2a. A grounded comparator U1-A receives a signal from the clamping and detection circuit 30 having the wave form shown in FIG. 2b and also receives the reference signal REF from the lead 20 and the fixed voltage VA from the lead 18. A grounded RC circuit 32 is connected between the two resistors R3, R5 and includes a capacitor C2 and a resistor R4. This RC circuit is used to filter the control signal supplied to the positive terminal of the U1-A comparator. The fixed voltage VA is also supplied to the lead 26 intermediate the resistors R3, R5, there being a diode D6 on the lead 18 intermediate the resistors R3, R5. The voltage output of the comparator U1-A is carried by a lead 38 and will have the wave form shown in FIG. 2c. The output lead 38 of the comparator U1-A is coupled to another lead 40 through a diode D7 and protective resistor R7. The lead 40 is connected to an on/off switch 42. A resistor R6 is included in the lead 40 between the on/off switch 42 and the connection with the output lead 38. A grounded capacitor C3 is connected to the lead 40. The lead 40 is coupled through another lead 44 to a comparator U1-B, the lead 44 serving as the positive input for the comparator U1-B. A variable input voltage is supplied to the negative input of the comparator U1-B through a circuit generally shown at 46, and consists of resistors R8, R9, R10, and R11, the resistor R10 being a variable control resistor serving to control voltage inputted to the voltage comparator U1-B. The voltage supplied by the lead 18 through the resistor R8 sets the maximum power that can be supplied. The comparator U1-B also receives the fixed voltage VA and has a connection with ground. A resistor R12 is on the lead 40 downstream from the junction with the lead 44 and is connected to the output lead 48 of the comparator U1-B. An inverting driver U2 receives the output 48 of the U1-B comparator, the inverting driver also being connected to the fixed voltage VA and to ground. The output lead 52 of the inverting driver U2 is coupled to a light emitting diode 54 whose anode is connected to a circuit that includes a resistors R13 and R14 and a capacitor C4 and the fixed voltage VA through a resistor R14. A gate triac 56 is coupled to the LED 54. The gate triac 56 is connected to a load triac 58 which has an output lead 60 that is connected to the lead 62. The load TRIAC 58 is also coupled to an induction coil L1 which is in parallel with a capacitor C5 to act as a suppressor to slow down the rate of rise of the current supplied to the lead 62.

In operation, the clamping and zero crossing detection circuit 30 discharges the capacitor C3 every cycle that is used to generate the firing angle delay. The comparator U1-B then compares the capacitor C3 charging voltage to the set voltage received from the circuit 46. The output of the comparator U1-B goes to the inverting driver U2 that is used to drive the LED 54 which will enable the optically coupled gate triac 56. The optically coupled gate triac 56 will enable the load triac 58 thereby supplying an RMS voltage to the load 62.

The transformer T1 and diodes D1-D4 are included for continuity. The diode D5 blocks DC from the system power supply 16 to the lead 26. The voltage at the anode of the diode D5 will look like the wave form of FIG. 2a. The wave form of voltage at the output of comparator U1-A is shown in FIG. 2c, the output being low when the sine wave input at the positive comparator input is less than the reference voltage at the reference input 20. The resistors R3, R4 divide off a portion of the sine input so that the output of the comparator is low for about 1 ms after zero crossing on the sixty cycle sign wave with a total period of 8.33 ms. The diode D6 assures that the maximum voltage supplied to the comparator U1-A is only slightly greater than the fixed voltage VA, as for example, 0.7 volts greater than the fixed voltage. In this way, the magnitude of the voltage supplied by the circuit 30 may be controlled to a selected maximum.

The low output on the comparator U1-A discharges the capacitor C3 when the on/off switch 42 is on. The capacitor C3 is kept discharged until the output of the comparator U1-A goes high through the diode D7 and resistor R7. After the output of the comparator U1-A goes high, the capacitor C3 starts to charge through resistor R6 at a time constant equal to C3(R6.times.R12)/(R6+R12) maximum value equal to (R12.times.VA)/(R12+R6). The resistor R12 also sets the compare point hysteresis. The wave form at the output of capacitor C3 is shown in FIG. 2d. The output of the comparator U1-B goes high when the voltage on the capacitor C3 equal the value of voltage from the circuit 46 at the minus input of the comparator U1-B. This voltage is set by the resistors R8-R11, and is adjustable through varying the resistor R10. The minimum voltage at this point is set by the resistors R8 and R11 to a value greater than the highest value to which the capacitor C3 will be discharged plus an amount that will increase the turn on angle to the minimum desired. The maximum voltage is set by the resistors R8-R11 and is set for the maximum turn on angle desired. The resistor R9 is used to the set the center travel of the variable resistor R10 equal to the center turn on angle required to obtain the RMS voltage required. The inverting driver U2 goes low when the output of the comparator U1-B goes high and discharges the capacitor C4 through resistor R13 and the LED 54. The resistor R14 also supplies some current through the LED 54. The resistor R14 could supply all the current if 15 ma of current is available.

The gate triac 56 will turn on and conduct current through the resistor R15 into the load triac 58 which supplies an RMS voltage to the load 62 that is variable from a large to a small portion of the total RMS voltage available, as shown in FIG. 2e. The shaded portion of FIG. 2e represents the minimum RMS voltage that will be available with R10 set at maximum resistance. It will be appreciated that the load 62 may be a lamp dimmer of an electrostatic copier.

The lamp dimmers that are presently in use have some problems that the control system 10 will solve. These problems include sensitivity of the setting variable resistor, usable range of the setting variable resistor, component tolerance variations, line voltage variations, on/off control, and voltage isolation. The setting of the control resistor in a dimmer can present problems if setting of the optimum operating point is required at nominal line voltage at the center of variable control resistor travel. The tolerance of the parts makes this difficult to accomplish. The values and ratings on the parts cannot be made tight enough without excessive cost, and even then some parts cannot be held to the tolerance required.

The prior systems also start charging from the time of zero crossing and a fixed resistance or part of the variable resistor must be used to set the minimum angle. The RMS voltage does not drop linearly. As the time increases from zero, the RMS voltage has dropped less than 1.5% at 16.67% of total cycle time. The controller system 10 can set this value much more accurately because of part tolerances without using a part of the variable resistor, so that the useful range of the potential is increased.

The loss of variations due to part tolerance, line voltage and minimum angle reduce the amount of the potential used to compensate for such loss and increases the effective potential range without making it too sensitive.