![]() 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. This article teaches you how to get around some of their major limitations. Microcontrollers are great for implementing the smarts behind a given product. ![]() How to Switch Large Loads with a Microcontroller Using Transistors
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