Using LM317 with Overvoltage Protection

Figure 1 

The circuit diagram shows the overvoltage protection as a feature of LM317 linear voltage regulator.

Overvoltages come from several source or factors which are usually in the form of transients. Transients are represented in spikes which are short and fast disturbance or change in the voltage and current component. It only occurs in circuits containing conductance and capacitance. The causes are typically from power outages, short circuits, tripped circuit breakers, lightning spikes, inductive spikes and other malfunctions from power company.

As an example using a 12V battery source, the LM317 voltage regulator can be used to obtain 6 Volts. To protect any device from overvoltage, there are ways such as adding relays or a zener diode. A relay switch functions by opening or closing under the command of another circuit. But finding a relay that would limit the output from 6 V to 12 V is not easy. Fortunately, zener diodes are more abundant. A 6.2V zener diode rating can be used to surpass any excessive voltage set by the voltage regulator, to prevent more damage to the circuit. The components will be as follows:

ZD1 – 6.2 Volts
R1 – 1K ohms
R2 – 1K ohms
T1 – NPN Transistor (low power)
T2 – NPN Transistor (acts as a switch)

Every circuit design needs to be tested carefully as to avoid further damage to the equipment that will be connected. The trial can be done by using a multimeter, and gradually increasing the circuit voltage. Once the circuit turns off the supplied voltage, take note of the reading which will signify the threshold voltage of the zener diode.

Source:www.zen22142.zen.co.uk/Circuits/Power/car_conv.htm