Overcurrent protection circuit without current sampling

This overcurrent protection circuit without current sampling has the characteristic of automatically restoring output after the short-circuit point is removed. The current during protection is much smaller than that during operation. Even if the short circuit lasts for a long time, the power supply will not be damaged. The circuit is shown in the attached figure.     

Principle: When the circuit is normal, T3 is saturated and T1 is in the on state, so the voltage across C and E of T1 is low, the voltage regulator cannot be turned on, so T2 is cut off and the power supply output is normal. When the output end is overcurrent or short-circuited for some reason, the voltage difference between C and E of T1 is greater than the conduction value of the voltage regulator and LED, current flows through the base of T2, T2 turns from cut-off to conduction, T4 is turned on, T3 and T1 are cut off, and the power supply has no output. LED is an overcurrent indicator.     

After T1 is turned off, R7 charges C1, creating conditions for the next start of T3. However, when the short-circuit point has not been removed, the current flows into the ground through R7, R4, and T4, so T1 is still turned off and the circuit has no output. If the short-circuit point is removed at this time, the current flowing through R7 flows into the base of T3, T3 is turned on, T1 is closed normally, and the circuit output returns to normal. According to specific needs, different protection points can be obtained by replacing DZ with different voltage stabilization values.