The basic concept of the fail-safe relay drive is that malfunctions of time delay lead to a changed of the timings signal t1 or t2, and as a result the delay time between M1 and M2, which should be t1 and t2 will become a fraction of the proper value, and they regulate circuit can protect the signals with false times. The output of the boost voltage is connected with the time-delay circuit, which is delayed with t1 and t2 by RC time constant circuit. In figure 4.18, illustrates the time-delay circuit produced during the operation of the relay drive circuit. The detailed condition for calculating delay time is as follows: