Phase-fired controllers

The phase control and the (rarely used ) phase control are methods for power control of electrical loads that are operated with AC voltage. Typical applications include dimmers, speed control of AC motors (eg electric hand drills ), or the current setting in the resistance welding with alternating current. It represents a form of pulse width modulation ( PWM) dar.

Operation

Wherein the phase angle control of the current flow is generally controlled by a triac ( anti- parallel circuit of two thyristors). After the zero crossing of the AC voltage ( and current ), the triac conducts current as long as not until it receives a firing pulse; from this time ( this " phase " of the AC signal ), the consumer is supplied with energy (until the next zero crossing). The later the triac is fired, the lower the average power.

Phase angle and phase control

While is switched with a delay after the zero crossing of the AC voltage at the phase angle control of the current, and flows to the next zero crossing, the situation is reversed in the phase control: The power is switched on immediately after the zero crossing and is switched off again before the next zero crossing. The circuit complexity is higher; it must either turn-off thyristors ( GTO ) or Power- MOSFET, respectively. IGBTs are used, the control electronics is more complex.

For capacitive loads phase controls are due to the sudden voltage increase is not suitable (it would be an extremely high current flow ), this phase section controls are used. In contrast, phase section controls are not suitable for inductive loads ( voltage spike when switching off the current).

So-called universal dimmer automatically detect whether an inductive or capacitive load is present, and accordingly act as leading or - edge control.

Calculate rms voltage

The rms value of the voltage can be calculated as follows:

For example, the mains voltage of 230 V at a line frequency of 50 Hz ( the period is, in this case 20 ms ) at the time t0 = cut 3 ms after zero crossing, an effective voltage of about 212 V yields the consumer.

Advantages and disadvantages of phase control

The advantage of the phase control ( and phase control ) compared to circuits in which the voltage is controlled by a resistor is its very low power loss. Unlike complicated controllable switching power supplies, which also have low power dissipation, phase control is much simpler and less to build and less prone to failure. The variable transformers are also used for power control are much larger and heavier than phase controls.

The biggest disadvantage of phase control ( and phase control section ) is the non- sinusoidal shape of the current. Because the current and voltage not have the same shape, there occurs a distortion power. The time- lagging displacement of the current against the voltage profile acts like an inductive load, which is tolerated by the electricity supply companies at low power ratings. At high power, therefore, a cycle control should be used, causing no phase shift.

The consumer shall receive of the phase angle control or reverse phase control a non-sinusoidal voltage. While that is no problem for resistive loads such as incandescent lamps and heating resistors, electronic devices that are connected to a phase control, be damaged.

Even with transformers, eg for halogen lamps caution: It is on the one hand to ensure that the phase control (that is, the dimmer ) suitable for this application, on the other hand should not switching power supplies, as they are now used increasingly, be connected to phase control. However, there is phase portion controls which are suitable for electronic voltage transformers. Switching power supplies can be recognized because they are much smaller and lighter than the same power transformers. With universal dimmers with automatic load detection both transformers and switching power supplies can be used.

Most phase control ( dimmer ) are not suitable for fluorescent lamps or energy saving lamps.

Basic principle

The picture on the right shows the simplified circuit diagram of a phase control. The lamp La is controlled through the triac Tr, it is ignited by the diac Di. R1 and C1 reduce the hysteresis and R2 and C2 form two phase shifters, is carried out by a delay of the AC voltage, therefore the threshold voltage of the diac is reached long after the zero crossing and the triac remains off until shortly before the next zero crossing of the AC voltage. With the potentiometer P can be " added " a less delayed AC voltage. The smaller its resistance, the sooner ignite the diac and triac and the brighter lights the lamp.

Important circuit components (fuse, suppression choke and capacitor RC network across the triac to protect it ) have not been shown in practice to simplify. Today for the control of the triac (or thyristor ) and integrated circuits are often used, such as the TCA 785