# what is firing angle with respect to SCR?

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An SCR passes current after it receives an input on its gate terminal until the voltage across its cathode and anode falls to or below zero. Think about the positive part of a sine wave of AC power. The wave begins to rise at zero degrees, peaks at 90 degrees and crosses thru zero again at 180 degrees.

So if you gate the SCR at 10 degrees (10 degree firing angle), it passes almost the whole wave. If you gate it at 90 degrees, it conducts for the second half only until it crosses zero. This resulting waveform is commonly called "Sawtooth."

You need a second SCR to deal with the bottom half of the AC waveform since its polarity is opposite.

An SCR connected to the mains ac can have a maximum conduction time of 180 degrees before the voltage has opposite polarity. An SCR is usually configured to be switched on at some point after the voltage has started to rise, this is the firing angle. When the voltage drops back to zero, the SCR will turn off.

I have seen manufacturers (incorrectly) call the conduction angle, firing angle so be wary of this.