Using LM317 with Overvoltage Protection


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

Over voltages 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.

Circuit

Circuit Diagram

As an example using a 12V battery source, the LM317 voltage
regulator can be used to obtain 6 Volts. To protect any device from over
voltage, 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.


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