Short Circuit Protection using Transistor
A short circuit is an electrical circuit that allows ideally infinite and practically very high amounts of current to travel along an unintended path with almost zero resistance and zero voltage. Short circuit protection using a transistor is a circuit that solves the short circuit problem.
Ideally, an infinite amount of current passes through the short circuit. The above figure shows the flow of current through the short circuit in a practical case.
A battery of 5V is connected in parallel with the two resistors of equal resistance. Hence the current that flows through is the same and is equal to half of the total current in the circuit. If the resistor R2 is shorted by some problem, the current chooses to go through the least resistance path i.e., the short. Hence, the total current present in the circuit goes through the short.
Imagine if there was an expensive IC connected to the nodes where the short circuit occurred. The IC would be damaged due to the excess current that would pass through it.
Short circuits are common while working with electricity. We often encounter short circuits in our daily life too! Electrical appliances stop working after a sudden spark sometimes, this is due to short circuits in their internal circuitry. A surplus of current then flows through the appliances and can damage all the other household appliances if not stopped. A fuse is used to stop the surplus current.
Similarly, while working with smaller circuits, we are always at the risk of a short circuit. We might make wrong connections or connect the wrong terminals i.e., anything can go wrong. A short circuit can cause significant damage to the components such as ICs, rendering them useless. Hence, using circuits that protect the circuit from short circuiting are important.
The short circuit protection circuit discussed below plays the same role of a fuse in the household electric circuits. There are many ways to override the short circuit. A short circuit protection circuit can be built using relays, transistors and even basic diodes.
The circuit discussed here uses two simple transistors to protect the circuitry from short circuit.
Components needed:
- NPN transistor – BC547 – 1 no
- PNP transistor – BC558 – 1 no
- Resistors – 10k Ω – 1 no
- Resistor – 1k Ω – 1 no
- Resistors – 330 Ω – 3 nos
- LEDs – 2 nos
- DC Power Supply – 1 no
Circuit explanation:
The circuit is powered by a 5V DC supply. LEDs are connected in order to identify the flow of current and the presence of a short circuit. A switch is used to replicate the shorting of the circuit. The circuit is shorted when the switch is closed.
Current chooses the path that offers the least resistance. When the power supply is ON, transistors Q1 and Q2 get biased and start conducting. The current flows through the green LED D1 and it turns ON. During this time Red LED D2 remains off as there is no flow of current through it. As long as the green light is on, there is no short circuit.
Now the switch is closed i.e The circuit is ‘shorted’, the output voltage drops to 0V as there exists no potential difference. Q1 stops conducting as its base voltage is 0V now and it is no longer biased. Transistor Q2 also stops conducting as its collector voltage also dropped to 0V and it too, is not biased anymore.
The current now flows through RED LED D2, which is now ON and passes through the short circuit path (the closed switch) and through the ground terminal.
The current is diverted through the RED LED D2 instead of damaging the entire circuit.
Outputs:
The above circuit was implemented in Proteus. And the following are the outputs observed:
- When the switch is open (There is no short circuit detected)
The current flows through the transistor Q2 → Resistor R2 → transistor Q1 → Resistor R3 → Resistor R4 → LED D1 → Ground terminal
- When the switch is closed (Short circuit is detected)
The current flows through the transistor Q2 → Resistor R5 → LED D2 → Shorted circuit → Ground terminal
The following circuit shows the current flow when the switch is closed.
This circuit can be made a part of any working circuit consisting of electronic components. It is ideal for mini projects with expensive components.
This circuit can be connected to any working circuit in the following way:
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