Triac controller

A compact electronic regulator that makes it possible to smoothly and over a fairly wide range to change the brightness of incandescent lamp filaments, the power of a household electric heater, or the rotational speed of an AC motor shaft can be built even by a not very experienced radio amateur. After all, the proposed device is based on a technical solution familiar to many from publications on earlier analogs and well proven in practice: a triac with economical phase-pulse control. In addition, the principal electrical circuit is complemented by a carefully developed printed-circuit-board topology with the exact placement of the mounted components. And the radio parts used in the design are quite common.

Among the advantages, one should also note the use of CMOS integrated circuits, which make it possible to reduce the current consumed by the control system in all modes to a minimum of 1.5 mA and therefore not to disconnect it completely from the mains. And replacing a typical toggle switch with a compact button located together with an LED indicator near the load increases the convenience of switching it on and off.

Of course, this is still not ideal. Not all logic elements of the integrated circuits are used in operation. Unused inputs have to be connected to the “common” wire.

Almost the entire circuit is powered from a DC source assembled on VD1-VD3, C2, C4, and C5. Moreover, capacitor C2 acts as a damping reactive impedance. Diodes VD1 and VD2 form a full-wave rectifier, whose voltage is maintained at 10 V by zener diode VD3 and smoothed by the total capacitance of C4 and C5. Capacitor C4 shunts mainly high-frequency interference coming from the household mains but not suppressed by the higher-capacitance “electrolytic” — because of its considerable parasitic inductance.

The next feature of this power supply is directly related to triacs. After all, most such characteristic semiconductor devices can be turned on (with “positive” voltage on the anode) by pulses of any polarity applied to the control electrode relative to the cathode, and with “negative” Ua — only by negative ones. Therefore, the positive terminal of the power supply under consideration is connected only to the cathode of the triac, and negative pulses will be formed on the control electrode at anode voltage of any polarity.

To clarify the essence, it seems worthwhile to recall that the phase-pulse method makes it possible to regulate power in the load by changing that part of the mains-voltage half-cycle during which the triac passes current. This means that for correct operation of the device it is first necessary to detect the beginning of each half-cycle (which corresponds to the instantaneous mains voltage equal to or close to zero), and then within 10 ms (the duration of a half-cycle of 50 Hz mains voltage) to form a pulse. And the earlier we open the triac, the greater the power dissipated in the load.

The 100 Hz pulse shaper is assembled on elements VT1, VT2, R3, R4, and R7. With the appearance of a positive half-cycle on the upper (in the diagram) mains wire, a voltage of the “opening” polarity is applied to the emitter junction of transistor VT1. The semiconductor triode indeed becomes open, and its Uc becomes close to Ue. The voltage drop across resistor R3 approaches 1 V of the open emitter junction of transistor VT1, so the “reverse-biased” emitter junction of transistor VT2 does not break down. During the negative half-cycle, the semiconductor triodes change roles.

Resistor R4 limits the current through the transistor bases. And R7, being the collector load of VT1 and VT2, sets a zero potential at input 1 of logic element DD1.1 (with the semiconductor triodes closed).

At moments when Umains is close to zero, no current flows through the above transistors, because the voltage drop across resistor R3 is insufficient for their turn-on. Therefore, Uc becomes equal to the voltage at the negative terminal of the power supply. As a result, short negative pulses are obtained, corresponding to the beginning of each mains half-cycle.

In the on state, input 2 of DD1.1 has a high voltage level. Therefore, the negative pulses arriving at the first input are inverted by the logic element and, through the emitter follower (transistor VT5), charge capacitor C8 practically to the power-supply voltage.

Discharge occurs through the R8R9 chain and VT4. When the voltage drops to the threshold level, elements DD1.2 and DD1.3 switch. The “falling edge” coming from element DD1.3 is differentiated by the C9R12 chain and, already as a pulse about 12 µs long, turns on (through inverter DD1.4 and transistor VT6, operating as a current amplifier) triac VS1.

Variable resistor R9 regulates the discharge duration of capacitor C8 and therefore changes the triac turn-on moment and the effective voltage across the load. The capacitance of capacitor C9 determines the duration of the triac turn-on pulse itself; resistor R12 sets the potential at the input of logic element DD1.4. As for zener diode VD6, it ensures reliable starting of the device.

The on-off node of the regulator is assembled on inverter DD2.1 and flip-flop DD3.1. Control signals to other parts of the circuit also come from this node. Transistor VT4 serves for smooth turn-on of the load, while elements DD2.2 and DD2.3 together with VT7 and VD5 provide button illumination.

During initial turn-on of the device or after loss of mains voltage, the C3R2 chain forms a positive pulse at the R input of logic element DD3.1, setting it to the zero state in which the load is off. Performing the functions of a T flip-flop, DD3.1 responds sensitively to positive voltage transitions at input C. With each appearance of such a transition, this logic element changes its state to the opposite one.

The R1C1 chain suppresses contact bounce, and resistor R1 included in it sets the required potential at the input of inverter DD2.1. Pressing any of buttons SB causes a positive voltage transition at the output of this element, switching flip-flop DD3 to the one state. The resulting high-level signal goes to DD1.1, enabling its operation. At the same time, favorable conditions are created for charging capacitor C6 to 10 V through resistor R6. The channel resistance of transistor VT4 smoothly decreases and after 5—7 s reaches its minimum.

But the channel of transistor VT4 is connected in series with resistor R9 in the discharge circuit of capacitor C8, and as the voltage on the gate of VT4 increases, the power in the load will smoothly rise to the level set by resistor R9.

Resistor R10 creates minimum negative bias on the gate for complete blocking of the regulator at zero resistance of resistor R9. The need for such bias voltage is due to the fact that after turn-on of the device there should be no time left for an abnormal situation to arise when the load is still de-energized, while capacitor C7, for AC voltage, acts as a shunt for resistor R10, excluding it from the discharge circuit of the above-mentioned C8.

A low level from the inverted output of the flip-flop closes VT3 and prohibits switching of inverters DD2.2 and DD2.3. A high level remains at the base of transistor VT7, and LED VD5 does not light.

The next press of any of buttons SB again switches the flip-flop to the zero state. Logical “0” from output 13 of the flip-flop will prohibit switching of element DD1.1, and a high level will be established at its output. Consequently, transistor VT6 will be constantly open, capacitor C8 will be charged, and the load itself (for example, an electric lamp) will be de-energized. The logical one coming from output 12 of the flip-flop through current-limiting resistor R6 will open transistor VT3, through which capacitor C6 will quickly discharge, and this will prepare the device for a new turn-on.

A high level at inputs 13 and 9 of logic elements DD2.2 and DD2.3 will allow them to pass negative pulses from transistors VT1 and VT2. These pulses briefly open transistor VT7, and the LED lights up. Resistor R13 limits the average current through VD5 (so as not to overload the power supply, otherwise the voltage it delivers will begin to fall).

Practically the entire homemade regulator (except for connectors, the fuse, the triac, and the LED) is mounted on a printed circuit board made of single-sided copper-clad fiberglass.

Transistors VT1, VT2, and VT7 may be low-power silicon devices, but they must be of p-n-p structure, with a current gain of more than 100. Almost the same requirements apply to the choice of VT3 and VT6, except for the structure itself. Here it is n-p-n. As VT5, a semiconductor triode of the KT201 series (with any letter index at the end) is acceptable. Low-power silicon transistors of n-p-n structure can also be used, securing such replacement by connecting VD4 (in the figure this is shown by a dashed outline). The diode will protect the emitter junction from breakdown by reverse voltage, which appears after transistor VT5 closes. In place of VT4, all field-effect transistors of the KP305 series work equally well.

The criteria are not very strict when selecting other radio parts either. Zener diode VT3 is no exception here — any one with a stabilization voltage of 10 V will do. Diodes from the KD509, KD510, and KD522 series. Capacitors: C5 of type K50-24, K50-29; C6, C7 — K53; C3 — any oxide type; C4, C9 — silicon; C1, C2, C8 — metal-film types K70-K78 (with C2 rated working voltage of at least 250 V). The variable resistor may be of any type; its body is connected for shielding purposes to the “positive” wire of the power circuit. Fixed resistors — type S2-33N, MLT. As for fuse FU1, it must, of course, match the current of the specific load.

Adjustment of the device comes down to selecting resistor R10 according to the following method (presented briefly).

Pin 2 of element DD1.1 is temporarily disconnected from the circuit and connected to pin 1. After installing a 100 kΩ variable resistor in place of R10, its resistance is reduced to zero. The triac regulator is connected to the mains and one waits a minute or two until electrolytic capacitor C5 is charged through “low-capacitance” C2 to the rated voltage of 10 V.

Monitoring the pulse shape in the load with an oscilloscope, the resistance of the variable resistor replacing R10 is increased until the triac stops opening. Then the load is switched on and off several times, using the available controls to ensure that transistor VT4, operating properly, reliably blocks VS1. After that, the variable resistor is replaced with a fixed one and the connection of pin 2 of DD1.1 is restored according to the circuit.

Practice shows: by installing and selecting resistor R11, one can achieve such a condition that the maximum resistance of resistor R9, operating as a rheostat, will correspond to zero voltage across the load. And to minimize the voltage drop across the triac when the load is fully on, it must be opened as soon as possible after the beginning of the half-cycle. Therefore, the zero-crossing pulse shaper of the mains voltage must produce sufficiently short pulses. To minimize them, the resistance of resistor R3 should be increased and R7 selected. Going the way of reducing the nominal value of R4 is undesirable — this is wasteful of energy.

And one more thing. During adjustment and practical use of the triac regulator, one must not forget that when the device is connected to the mains, everything, including the variable resistor, is under its high voltage. And one does not joke with 220 V AC, even if the housing of the electronic homemade device is made of good insulating material.

“Modelist-Konstruktor” No. 2’99, A. RUDENKO, Kharkiv