Surge protection with partial voltage stabilization

This simple circuit protects the device against overvoltage.

The requirements for the district were as follows: simplicity, minimum number of components, at least minimal stabilization, minimum current consumption and minimum voltage drop in the forward direction.

How surge protection works?

Principle: at a voltage not exceeding the zener voltage of the ZD diode, the circuit is at rest and does not draw any current. If this voltage is exceeded, the zener diode opens the transistor, which stabilizes this voltage.

If the transistor is not able to stabilize this voltage /e.g. also because the voltage will be destroyed and will continue to increase, after exceeding the voltage by approx. 2V /threshold voltage of the light-emitting diode/ will open thyristor /which is capable of carrying a high current/. This exceeds the max. permissible current for the included fuse, the fuse has blown, the LED connected above it lights up, and the powered device is disconnected from the power supply.

If the fuse does not blow due to the short duration of the surge, thyristor by stabilizing the voltage stabilizes the voltage even at higher consumptions. The resistor in the thyristor control electrode automatically turns off this thyristor after a voltage drop and the device continues to operate / but with an output voltage higher by 2V if the transistor is destroyed /.

A capacitor connected in parallel to the tripping resistor of the thyristor removes random interfering pulses, which could trigger you and adversely affect the power supply.

If Darlington is used as the transistor, stabilization will be more accurate. The resistor at the base of this transistor only serves to close it faster, thereby improving the stabilization characteristics. A diode connected at the end of the circuit protects the supplied circuit from polarity reversal and removes any interfering negative voltage pulses.

Components

Resistors: 10k – the value is not critical
100nF capacitor – the value is not critical
Ty – any (functional!), s max. current exceeding the fuse current
she paused – best darlington, NPN
ZD – to the required voltage reduced by a decrease e.g.. on transistors, about 0.7V
LED – any, best with the lowest possible threshold voltage
note: threshold nap. The LED is approx: for red 1.8V, orange 2.0V yellow 2.1V and green 2.2V. Of course, it would be possible to use two silicon diodes.
The first complements the drop on the transistor, the second increases the switching voltage Ty by approx. 0,7V.