In other words, the maximum value of IC is also 1A. Since VCC = 10 Volts and RL = 10Ω, the maximum current that can flow through RL is 1A. According to the calculations, this implies that IC = 2000mA, or 2A. The same argument applies if IB = 2mA, and so on. This implies that VC must be 9V then, since VCC is 10V, and the voltage drop across RL is 1V. The voltage across the resistor is IC x RL, or 1V. Note that since no current is flowing in the transistor, it isn’t dissipating any power also in this case, VC is the same as VCC.įor the next part, assume that VCC = 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.ĭownload your PDF cheat sheet: 15 Steps to Develop Your New Electronic Hardware Product 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 hFE. 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. IC = βIB, where β is the DC current gain, and is maybe 20 to as high as 300, or more. In other words, for the NPN transistor the collector voltage is generally higher than the emitter voltage.įollowing the same convention, VBE is the voltage between the Base and the Emitter. With the voltages, VCE is the voltage between the Collector and the Emitter, and is generally a positive value for NPN transistors. The same arguments apply for IC and IE, with IE shown leaving the transistor. Starting with the current, IB 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.
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