Question

(a) A student wants to use two p-n junction diodes to convert alternating current into direct current. Draw the labelled circuit diagram she would use and explain how it works.
(b) Give the truth table and circuit symbol for NAND gate.

Answer 1

(a) Since two diodes are to be used, she can make a full wave rectifier as shown:

Explanation : In positive half cycle of AC, end A becomes positive and $D_1$ becomes forward biased and $D_2$ is reverse biased, so $D_1$ conducts and $D_2$ doesn't. So conventional current flows through $D_1$,$R_L$, and upper half of secondary winding. Similarly, during negative half cycle of AC, diode $D_2$ becomes forward biased and $D_1$ is reverse biased, current flows through $D_2,R_L$, and lower half of secondary winding. Thus, current flows in same direction in both half cycles of input AC voltage.

(b) Circuit symbol :


Truth table for NAND gate:

$\begin{array}{cccc}\text{Input}& & \text{Output}& \\ A& B& Y^{\prime }& Y\\ 0& 0& 0& 1\\ 1& 0& 0& 1\\ 0& 1& 0& 1\\ 1& 1& 1& 0\end{array}$