# Numerical - Maximum & minimum number of blades strokes per minute if UJT controls the motor of an automobile

**What will be the requirement of maximum & minimum number of blades strokes per minute if UJT controls the motor of an automobile with stand-off ratio of about 0.7, capacitance of 40 μF and the charging resistances of 40 k & 480 k potentiometer connected in series combination?**

**Options: **

a. 24 & 2.2 seconds respectively

b. 28 & 2.4 seconds respectively

c. 30 & 2.7 seconds respectively

d. 32 & 2.9 seconds respectively

Correct Answer: c. 30 & 2.7 seconds respectively

**Explanation:**

According to the given data,

Time constant has a least valure of about 50000 x 40 x10^{-6} = 2 seconds

Considering the utilization of an entire potentiometer, the maximum value of this time constant is given by,

= (40 + 500) x 10^{3} x 40 x 10^{-6}

= 21.6 seconds

Maximum number of blade strokes per minute = 60 / 2 = 30 seconds

Minimum number of blade strokes per minute = 60 / 21.6 = 2.7 seconds