Thread Starter


Joined Dec 4, 2011
Hi guys, I was wondering what is the maximum firing angle that can be attained using a SCR. Firing it at 180 degrees wont turn it on because of no voltage (V=0 at x axis). So is the peak voltage (90 degrees) the maximum firing angle of an SCR for it to have maximum conduction? And also is current supply not interrupted to a load for the negative part of a sine wave through an SCR? correct me if am wrong. Thanks a lot. bless


Joined Mar 6, 2009
Firing angle beyond 90° usually means operation in inverter mode rather than in rectifier mode below 90°. One can envisage operation up to the 180° limit but this doesn't allow for the inevitable device commutation angle.


Joined Oct 26, 2011
DC gate current, extended gate pulse width, or repetitive gate pulses are all means to obtain full 180° conduction--if the phase control regulator goes into 'saturation' and fires before the SCR (thyristor) is forward biased, the gate current remains or reappears as soon as it is forward biased. Single, short-pulse triggering is troublesome when operating near this boundary. Of course, you must also factor in the slight junction voltage drop that becomes more of an issue in low voltage applications.

A similar thing happens at close to 90° when the load is capacitive or has counter electromotive force (CEMF) as in a motor, but may be solved via the same technique. In this case, maximum output voltage (peak voltage detection) is obtained rather than full 180° conduction.

In AC circuits, anti-parallel SCR devices may obtain full 360° conduction when properly triggered--and obtain higher current, lower conduction losses and superior thermal management than TRIACs.
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