However, since V CEsat is usually quite low, the power dissipation will also be low. In saturation mode, the transistor is dissipating some power, given by This dependence is typically given as a set of curves in the datasheet. V CEsat is also dependent on the collector current and the temperature. This value is given in the transistor datasheet, and is usually between 0.2V for a small transistor to more than 1V for a large one. When a real transistor is saturated its V CE will be at a value of V CEsat. Thus, saturation is defined in relation to external circuit conditions.įinally, note that real transistors cannot be complete short circuits between their collectors and emitters unless they are defective. Note that if, in the example just described, V CC is now increased to, say, 25V, or R L is changed to 1Ω, the transistor will no longer be saturated. So, the equation I C = βI B only holds until the transistor is saturated. In this mode, the transistor collector current is the maximum that the circuit conditions allow, and increasing base current will not cause it to go any higher. In this state, the transistor is said to be in saturation mode. This occurs when V C = 0, meaning that the transistor is a dead short to ground. In other words, the maximum value of I C is also 1A. Since V CC = 10 Volts and R L = 10Ω, the maximum current that can flow through R L is 1A. According to the calculations, this implies that I C = 2000mA, or 2A. The same argument applies if I B = 2mA, and so on. This implies that V C must be 9V then, since V CC is 10V, and the voltage drop across R L is 1V. The voltage across the resistor is I C x R L, or 1V. In this case, I C = 100mA, since β = 100. Note that since no current is flowing in the transistor, it isn’t dissipating any power also in this case, V C is the same as V CC.įor the next part, assume that V CC = 10 Volts, R = 10Ω, and β = 100. In this state, the transistor is in cut-off mode. When BJT’s are used as load switches, they are used in two modes: Cutoff and Saturation. Note that it is not a fixed value for a given transistor, but varies somewhat with the value of the Collector current and temperature, but that will not matter much for the purpose of this article. For the purpose of this article, they mean the same thing. The value of β is given in the datasheet of a given transistor as h FE. So, if β = 100, then the Collector current will be 100X the base current. This says that the Collector current is the β value times the Base current. I C = βI B, where β is the DC current gain, and is maybe 20 to as high as 300, or more. The key to understanding how a transistor can control a large load is this equation: 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. One thing that microcontrollers cannot do is directly control anything other than, maybe, a single LED.
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