555 automatic start type over-current protection control cir

As shown in the figure 10-10, the over-current protection circuit is made as the core of 555 which is connected into R-S trigger mode. In the figure, M is the DC motor which is needed to limit current or regulate speed. 555's 6 foot(R terminal) is connected to high level, 2 foot(S terminal) is connected between M and sample resistor R4. RP1 is used to adjust the control terminal's(5 foot) voltage, namely, it adjusts the threshold voltage of R and S terminals. When it works normally, RP1 is adjusted, R4's sample voltage is lower than S terminal's threshold level, 555 is set, V1 is saturation conduction, M runs normally.