Triac regulator

A triac is a semiconductor that becomes conductive as soon as there is sufficient signal on its control electrode. After that, it remains conductive until the passing current falls below a minimum value, at an applied alternating voltage this happens near the zero crossing.
This means that a triac can be used to control the power of a load.
Load includes vacuum cleaners, washing machines, lighting and heating.
Where it concerns an ohmic load, this works fine, but with inductive loads the phase shift can cause problems.

There are many designs for controlling a triac, from very simple to complex.
In the simplest version, a diac and an RC time are used to control the ignition time.
The more elaborate circuits use a zero-crossing detector. This serves as the basis for controlling the triac.

A commonly used zero-crossing detector is formed by driving an optocoupler with the AC alternating voltage. Around the zero-crossing, the output of the optocoupler is high-impedance and can be read with a pull-up resistor.
There are optocouplers with 2 anti-parallel connected LEDs that can be connected directly (via a resistor) to the AC, if only 1 LED is present, a bridge circuit can be connected upstream.
For the optocoupler to function, sufficient current must always flow through the LEDs. This results in a less accurate detection of the zero crossing.

Where a control system is used to control the triac, a power supply will have to be present to feed that system.
You can of course use a standard USB power supply, safe and cheap, but the circuit proposed here uses another very standard product: a small power supply transformer.
In this way, a very accurate zero-crossing detector can be realized in a handy way.
   
fig.1

fig.2

The design is intended to be able to control the speed of a fan.
 
fig.3

The available transformer has a secondary of 24V-CT, so 2x 12V. This voltage is rectified with D1 and D2. The rectified voltage feeds an electrolytic capacitor via D3 and from there to a stabilizer that supplies the rest of the circuit with voltage.
The first opamp of an LM358 is used as a comparator. The + input is a voltage divider connected to the + power supply, level approximately 1%. The -input is connected with a voltage divider to the raw rectified voltage (so for D3), level approximately 90%. The result is a very narrow peak during the zero crossing at the output of the op amp.
A BS170 is connected with this peak, which discharges a capacitor that is charged by a current source.
In this way, a beautiful sawtooth is achieved that is exactly in phase with the AC alternating voltage.
The second opamp is also used as a comparator. This compares the level of the sawtooth with the level of the runner of the potentiometer. As soon as the voltage on the runner falls below the value of the sawtooth, the output of the opamp goes low and the MOC3023 receives control and thus the triac.
The potentiometer is also powered from a power source, but can if necessary be connected to the + power supply with a series resistor.
To ensure that the triac cannot ignite during the zero crossing, a transistor is also placed in series with the MOC, which is blocked during the zero crossing.
Furthermore, a relay is provided with which the triac is disconnected from the mains as soon as the switch (on the potentiometer) is in the zero position, which also records the initial state when switching on.
When mounting, make sure that the zero position of the potentiometer is not on the ground side but on the opposite side.

The controller described works fine, but not all loads respond to it properly. Especially the fan for which the control was built has problems with it. The motor therein is a type with a short-circuit armature in which the rotating field is generated by means of an auxiliary winding which is connected via a capacitor.
There are also short-circuit armatures where the rotating field is generated by an additional short-circuit winding, these types respond much better to the leading edge. Series motors, such as those used with vacuum cleaners and drills, also respond well to the leading edge.
 To properly regulate the speed of the fan used, a variac is used, an adjustable transformer that regulates the amplitude of the voltage. 

The MOC3023 is a triac driver equipped with a direct-acting circuit. There are also types that have a built-in zero-crossing detection that only turns the triac on immediately after a zero-crossing. A regulator for this has to be designed differently because only whole period halves of an alternating voltage can be passed. Regulation can then be done by allowing fewer period halves to pass through (whereby the number of positive and negative halves must then remain the same !!).