2.1 Optocoupler Circuit to Detect the Firing Angle

In a BLDC motor, the variation of RMS AC source does not change the output DC. The TIDA-00652 reference design (BLDC ceiling fan TI Design) is used to implement TRIAC-based fan control. A small optocoupler-based circuit is connected to the TIDA-00652 BLDC board as shown in Figure 5. The rectified AC is sensed using the optocoupler. As the phase angle of TRIAC is varied, a PWM signal is generated accordingly. The average of PWM is fed to the SPEED pin of the DRV10983 device.

The AC waveform chopped with a TRIAC at a certain firing angle generates a PWM signal corresponding to the firing angle as shown in Figure 6. This PWM signal is averaged using the RC low-pass filer. The variation in the PWM signal from 10% to 90% results in analog voltage from 0.8 V to 3.3 V correspondingly. The DRV10983 device can be used in Analog control mode and this output analog voltage can be connected to the SPEED pin of the DRV10983 device.