Question

A zener diode is rated as $1W$. If the given breakdown voltage is $6.2V$, then the max current it can hold is
     $A)\text{ }224mA$
     $B)\text{ 161}mA$
     $C)\text{ }239mA$
     D) None of the above

Answer 1

Hint: This problem can be solved by using the direct formula for the power consumed by a device in terms of the voltage across it and the current passing through it in an electric circuit. The power is the product of the voltage and the current.
Formula used:
$P=VI$

Complete answer:
The breakdown voltage is the voltage after which the reverse current in the zener diode increases almost infinitely for an infinitesimal increase in reverse voltage. Hence, the voltage remains constant but the magnitude of the current can be increased infinitely.
The maximum current that the zener diode can hold will be the current that is passing through it at the point of its breakdown voltage. We will find this current out from the power rating and the given breakdown voltage.
The power $P$ consumed by an electrical device in a circuit when a current $I$ flows through it and there is a voltage $V$ across it is given by
$P=VI$ --(1)
Now, let us analyze the question.
The power rating of the zener diode is $P=1W$.
The zener breakdown voltage is $V=6.2V$
Let the corresponding max current be $I$.
Therefore, using (1), we get
$1=6.2\times I$
$\therefore I=\dfrac{1}{6.2}=0.161A=161mA$ $\left( \because 1A=1000mA \right)$
Therefore, we have got the value of the max current that the zener diode can hold as $161mA$.

Therefore, the correct option is $B)\text{ 161}mA$.

Note:
This unique property of zener diodes to have a constant voltage (the breakdown voltage) even though a large amount of current can flow across it allows it to be used as a voltage regulator. The output voltage is taken parallel to the zener diode and hence, is equal to the breakdown voltage irrespective of the input voltage. Hence, even though there might be changes and irregularities in the input voltage, the output voltage is always regulated, uniform and steady.