Final answer:
To determine the power rating of a zener diode in a regulated supply, calculate the product of the zener voltage and minimum zener current. Using the provided specifications, a power rating of at least 0.25 W is advisable for the zener diode, taking into account various operational considerations for safety and reliability.
Step-by-step explanation:
The power rating required for a zener diode in a regulated power supply circuit can be calculated considering the worst-case scenario, which happens when the input voltage is at its maximum, and the load current is at its minimum. With an input voltage of +25 V and a zener voltage of +10 V, we must drop 15 V across the series resistor. The total current through the resistor is the sum of the maximum zener current and the minimum load current. Assuming a minimum load current of 0 mA and a minimum zener current of 10 mA, the resistor will have to carry only the zener current, i.e., 10 mA.
To calculate the power rating for the zener diode, we multiply the zener voltage (10 V) by the zener current (10 mA): Power P = V x I, which equals 0.1 W. However, considering power rating margins for safety and reliability, a zener diode with a power rating of at least 0.25 W (250 mW) would be advisable. This accounts for potential fluctuations in voltage, current as well as changes in ambient temperature and other factors that may affect the diode's operation.