![]() In other words, for the NPN transistor the collector voltage is generally higher than the emitter voltage.įollowing the same convention, V BE is the voltage between the Base and the Emitter. With the voltages, V CE is the voltage between the Collector and the Emitter, and is generally a positive value for NPN transistors. The same arguments apply for I C and I E, with I E shown leaving the transistor. Starting with the current, I B is the base current, and is shown entering the base of the NPN. For low-side switching, NPN transistors are used, and for high-side switching a PNP is used.īefore getting into the actual methods, let’s define some nomenclature that are used when dealing with NPN transistors.įigure 2 shows the pertinent voltage and current naming conventions. BJT Low-Side SwitchĪ BJT can be used as a load switch and come in two flavors: NPN and PNP. If this type of switching arrangement is acceptable, then the low-side switch is usually the cheapest way to achieve load switching. This means that when the switch is open, the load is essentially floating with respect to the negative of the power supply, which is usually the ground reference in most designs. The switch controls the negative side of the load. Figure 1 shows this type of load switching. The actual electronic switch element comes in two variants: Bipolar Junction Transistors, or BJTs, and MOSFETs.īefore getting to the actual switch itself, let’s define what is meant by low-side switching. One of the simplest approaches to controlling large loads that operate on DC current is the saturated switch. However, this approach means that some liberties have been taken with technical rigors.įREE GUIDE: Introduction to Microcontrollers A few simple math calculations are required to determine typical component values, and these will be presented in easily accessible formats. Let’s look at several ways to switch heavier loads on the low side from a typical microcontroller output. That’s because the output drive of most microcontrollers can directly source or sink only about 10mA. The above circuit was implemented in Proteus.One thing that microcontrollers cannot do is directly control anything other than, maybe, a single LED. The current is diverted through the RED LED D2 instead of damaging the entire circuit. 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. Transistor Q2 also stops conducting as its collector voltage also dropped to 0V and it too, is not biased anymore. Q1 stops conducting as its base voltage is 0V now and it is no longer biased. Now the switch is closed i.e The circuit is ‘shorted’, the output voltage drops to 0V as there exists no potential difference. As long as the green light is on, there is no short circuit. During this time Red LED D2 remains off as there is no flow of current through it. The current flows through the green LED D1 and it turns ON. When the power supply is ON, transistors Q1 and Q2 get biased and start conducting. The circuit is shorted when the switch is closed.Ĭurrent chooses the path that offers the least resistance. A switch is used to replicate the shorting of the circuit. LEDs are connected in order to identify the flow of current and the presence of a short circuit. The circuit is powered by a 5V DC supply. The circuit discussed here uses two simple transistors to protect the circuitry from short circuit. A short circuit protection circuit can be built using relays, transistors and even basic diodes. There are many ways to override the short circuit. ![]() ![]() The short circuit protection circuit discussed below plays the same role of a fuse in the household electric circuits. Hence, using circuits that protect the circuit from short circuiting are important. A short circuit can cause significant damage to the components such as ICs, rendering them useless. We might make wrong connections or connect the wrong terminals i.e., anything can go wrong. Similarly, while working with smaller circuits, we are always at the risk of a short circuit. A fuse is used to stop the surplus current. A surplus of current then flows through the appliances and can damage all the other household appliances if not stopped. 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. Short circuits are common while working with electricity. ![]()
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