Overvoltage and overcurrent protector

When the power supply voltage or the current drawn by the load is too large, the circuit in the figure below can disconnect the load and give a fault indication. During normal operation, both Tr1 and Tr2 are cut off, 555 is reset, and the discharge transistor in 555 is turned on. It draws current from the base of Tr3, causing Tr3 to open to saturation, and the power supply 5~12V is directly sent to the main load. When the current drawn by the load exceeds the specified value, the voltage drop on Rsc increases, causing Tr1 to turn on, and 555 is triggered, so the internal discharge transistor is cut off, followed by Tr3, which is also cut off to isolate the power supply from the load. At this time, 555 is in a monostable state. Once the stabilization time is up, as long as the load overcurrent is not eliminated, 555 is triggered again, and Tr3 continues to isolate the load. If there is overvoltage in the load, Tr2 will be turned on after R4, R5, and D1, which will also trigger the 555. Tr3 will also isolate the load at this time. For overcurrent or overvoltage, pin 555③ will output a high potential, causing the LED to light up, indicating that the load is in an isolation state. Since Tr3 is either in saturation or cut-off, only one power transistor can work.