Introduction to Optical Isolation (Opto-Coupler)
Opto-coupler is an optical-electrical device to provide electrically-isolated signal transfer by means of light. Inside the device, the signal source (from the low voltage side) is used to control a LED (light emitting diode) which is then detected by a photo-diac (two-way/bidirectional photo-diode) for the high-voltage side interface. Using this isolation, the low-voltage side is safe from hazardous high voltage electric shock while providing power control for high voltage devices (the soldering heater filament).
Triac and Zero Crossing Triggering
Triac is a generic trademark for bidirectional/bilateral triode thyristor, a three terminal electronic device that allow electrical current switching in alternating direction when triggered. When triggered, it becomes switched-on (conducting the current) immediately, but will remain switched-on until the the control voltage is deactivated and the load current falls to zero.
Driving a Triac with non-zero crossing triggering will produce AC voltage waveform like shown in the Figure 2 (B), where we can find immediate high voltage appears at triggering time, since the triggering is not done exactly when the voltage level cross the zero line (when it changes the polarity). The non-zero crossing triggering like this has drawback that switching the device from off-state to on-state produce a transition condition, a condition when the device is in “half-on” condition. This transition condition make the Triac dissipate the electric power into heat, lowering the overall efficiency of the system. By using a MOC3042 opto-coupler device (Figure 2 A), the AC voltage should have a perfect-full/half-wave waveform as shown in Figure 2 (C). The zero crossing detector inside the MOC3042 device automatically detect the zero crossing condition before activating the Diac when it receives the light signal from the LED. The limitation of using this zero-crossing control is that the smallest pulse width and the pulse width increment of the PWM control is the half-cycle period of the powerline frequency. This limitation should be considered when implementing the PWM control in the software.
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