This is a fully solid state fan speed regulator suitable for any fan up to a load of 150 watts. Unlike the conventional resistor type fan regulator, it does not warm up and please note that it does not consume any additional electrical power. The control of fan speed provided by it is smoothly adjustable between minimum and maximum speed whereas the speed adjustment in steps provided by the conventional resistor type fan speed regulator. This solid state fan regulator can be quite small.
The device uses a DIAC and a TRIAC or a single QUARDAC and few other components. To understand the operation of this simple unit it is very much essential to understand the basic characteristics of DIAC and TRIAC.
The circuit diagram shown above basically consists of three separate portions. (1) RFI [Radio Frequency Interference] suppression circuit[RFC and C1]; (2) Phase triggered variable AC power control circuit [R1,Rx,VR-POT, C2, DIAC, TRIAC] and (3) dv/dt protection circuit [R2,C3] of the TRIAC.
The RFI suppression that reduces the high rate of change of current, di/dt provided by the TRIAC while controlling the power supplied to the fan. If the di/dt shall not been reduced it can cause interference in amplitude modulated radio broadcast and television picture transmission.
The basic circuit of the device is the phase triggered variable AC power control circuit. The potentiometer used as a variable resistor that controls the rate of charging of the timing capacitor and as such it provides variable phase delay adjustable between almost the entire duration of the half cycle period, which is 10 millisecond of the mains supply. Here the DIAC acts as a trigger device. When the timing capacitor charges to the break over voltage of the DIAC, the DIAC comes into conduction and starts discharging the timing capacitor through GATE-MT1 terminal of the TRIAC. When the timing capacitor discharges to the value which cannot sustain the avalanche action of the DIAC the DIAC becomes non-conductive. This happens within a few microseconds and the GATE-MT1 circuit of the TRIAC receives a triggering pulse sufficient to bring it in conducting state.
Once brought into conduction, the TRIAC remains conduction for the rest half cycle. Now though the DIAC has become non-conductive the timing capacitor cannot start charging through the series Resistor-Potentiometer network connected across the TRIAC, as the TRIAC in conduction has negligible voltage drop across it. When the zero crossing of the mains alternating supply occurs, the TRIAC become non-conducting state and the capacitor starts charging again. This process repeats twice in every cycle, i.e. in each positive and negative half-cycle of the mains supply. However the load connected to the device receives a part of every half-cycle.
If the resistance increased using resistance and potentiometer network the TRIAC shall get triggered late in the half cycle and shall remain conduction for a comparatively short duration. Thus the load will receive a small part of every half-cycle and vice versa. Thus with the solid state regulator one can control the average power fed to the load from minimum to maximum.
To adjust the solid state regulator the lowest useful speed provided by the fan it is very much important to add a resistor across the 1 Meg potentiometer as shunt with a suitable value. The resistor has been shown in the diagram as Rx. To select the value of the Rx, another one 1 Meg potentiometer may be connected temporarily across the 1 Meg potentiometer of the device. Set the 1 Meg potentiometer in full anti-clockwise position (maximum resistance value) gradually increase the value of the temporary 1meg potentiometer from the low ohmic setting till the fan runs at the required minimum speed. Remove the temporary 1 Meg potentiometer without disturbing its setting and measure the resistance. Put a fixed ½ watt resistance with a closest value as Rx. Please find the Schematic in a separate page.