RMS Voltage Control Circuit with Opto Triac MOC3021 and BT136

Triac is a power electronic component that conducts in both directions when triggered through gate. Figure below shows a generic working of triac.As it can be seen that at time t1, angle of sinusoid is 45' which means that if we triggered triac at this angle i-e at 45', only shaded blue area will pass through the triac and hence through the load. Observe that shaded blue are has RMS Voltage less than the pure sinusoid. This is the basic principle by which RMS Voltage control is accomplished. Firing needs a small pulse at gate that can be give through microcontroller also. Similarly at firing angle 90' (firing angle is an angle with reference zero crossing at which the triac is triggered using gate pulse) , only red part of sinusoid will pass through the triac giving us the RMS 110V for 220V.

 

MOC3021 is an optotriac (product of Motorola) that is used for isolation between power and driving circuitry. Note that when C828 on the base is applied voltage>0.7V, optotriac gets triggered. As the triac gets triggered now, the positive or negative voltage (whatever maybe) get pass through the gate of BT136 (triac) and hence triggered it. It should be noted here that by using above arrangement we can control the RMS voltage in both directions. What needs to be taken care of, is the triggering time or firing angle.

There is a need of a zero-crossing detector that will give us the reference for providing delay for desired firing angle. In above example, for firing angle to be 90' for 220V 50Hz AC signal, we need to have a delay of 2.5 ms (t1=2.5ms) right after each zero crossing. Usually MOC3021 is driven through microcontroller, which gives the firing pulse on the basis of interrupt generated by the zero-crossing detector.

The above circuit is mainly used as a dimmer and is often used speed controlling of AC motor. There are other versions of the above circuits available that caters for the inductive load which will be discussed later.