Push the problem to the extreme, and you will know that the thyristor can be completely turned on when the resistance is very small. When the resistance value increases, the charging speed of the capacitor will slow down. There must be a delay if the voltage on the capacitor rises enough to trigger the thyristor. The larger the resistance value, the more delay. Time delay transition to 10 ms (180) is the trigger angle of thyristor. This kind of circuit is obviously too primitive, which will lead to the longer the delay, the more difficult it is for the thyristor to turn on. Therefore, electronic circuits are needed for power amplification and shaping.