Latching current A. A SCR is connected in series with RL load and is fed from

a120V,60HzAC supply. The load resistance is15Oand inductance of0.75H. What is the maximum allowable latching current ofSCRif the gate trigger circuit output pulse is100s duration at a delay angle of45O.

B. If the latching current ofSCRis10mAwhat is the minimum delay angle.

Sol82:

(a) To calculate the maximum allowable latching current of the SCR, we need to determine the peak voltage and peak current of the circuit.

The peak voltage of the AC supply is:

Vp = Vrms x v2 = 120 x v2 = 169.7 V

The peak current through the circuit can be calculated as follows:

Ipk = Vp / RL = 169.7 / 15 = 11.31 A

The maximum allowable latching current can be calculated using the following formula:

IL(max) = (Ipk x T)/(2 x L x sin(p/2 - a))

where T is the duration of the gate trigger pulse (100s = 0.0001s), L is the inductance of the load (0.75H), and a is the delay angle (45).

Substituting the given values, we get:

IL(max) = (11.31 x 0.0001) / (2 x 0.75 x sin(p/2 - 45)) IL(max) = 0.00283 / 0.529 = 0.00535 A or 5.35 mA

Therefore, the maximum allowable latching current of the SCR is 5.35 mA.

(b) To calculate the minimum delay angle, we need to use the following formula:

IL = IL(max) x sin(p/2 - a)

where IL is the latching current of the SCR (10 mA).

Substituting the given values, we get:

10 = 5.35 x sin(p/2 - a) sin(a) = (5.35 / 10) x cos(45) = 0.378 a = sin^-1(0.378) = 22.9

Therefore, the minimum delay angle is approximately 22.9.