Файл: Embedded Controller Hardware Design (Ken Arnold, 2001).pdf

8EMBEDDED CONTROLLER

Hardware Design

equation V = I * R, as shown in Figure 1-5. This is known as Ohm’s law.

their performance in the real world and are subject to other limitations, such as operating temperature, power limits, etc. Current flows only through a complete circuit, and in most cases (for a positive power supply) current flows from the power source through the circuitry and returns to the power supply through the common “ground” connection. Current flowing through any resistance results in the dissipation of power as heat. The power dissi­ pated is P = I2R = V*I = V2/R. Note that voltage is sometimes denoted by the variable V and sometimes by E, for electromotive force.

All practical components have some resistance. Real batteries have an internal resistance, for example, which provides an upper limit to the current the battery can supply to an external circuit. Real wires have resistance as well, so the actual performance of a circuit will deviate somewhat from the ideal. These effects are obvious in some cases, but not in others. In an automobile starting circuit, it’s not surprising that the battery, supplying 12 volts to a starter with internal resistance on the order of 0.01 to 0.1 ohms, will result in currents of hundreds of amperes in order to start the engine. On the other hand, while consulting with a prominent notebook computer manufacturer, I uncovered a design error resulting in an internal current of hundreds of amperes flowing in the circuit for a few nanoseconds. Obviously, this wreaked havoc on the operation of the computer, and generated a great deal of electro­ magnetic noise!

One of the things you will learn in this book is how to avoid those kinds of mistakes. It’s also important to remember that power is dissipated in any resistance present in the circuit. The power is proportional to the voltage times

9CHAPTER ONE

Review of Electronics Fundamentals

the current across the resistance, which is dissipating the power. In the last two examples, the amount of power dissipated instantaneously is quite high while the current is flowing. When the current pulse is only a few nanoseconds long, however, it may not be

+

(Purists please note:

This book does not use electron current flow.

All electrical current flow will be “positive” or “conventional” current flow, meaning current

always flows from the most positive terminal to the most negative terminal of a component. The use of positive current flow follows the intuitive direction of the arrows inherent in the component drawings for diodes, transistors, etc.)

Transistors

The flow analogy can also be used to model how a transistor operates in a logic circuit. The transistor is an amplifier. It uses a small amount of energy to control a larger energy source, just as a valve controls a high-pressure water source.

There are two kinds of transistors: bipolar and field-effect transistors (FETs). We will look at bipolar transistors first; these amplify current. A small amount

11CHAPTER ONE

Review of Electronics Fundamentals

Transistor Switch ON

Transistors can be configured to function as switches. As can be seen in Figure 1-10, an NPN transistor operating as a current controlled switch can be used to build a simple inverter. It changes a logic one on its input to a logic zero at its output, and vice versa. In this case, logic one is represented as a positive voltage, and a logic zero is represented by zero volts. The logic one input (positive input voltage) is supplied through a resistor from the power supply voltage to the transistor base terminal, resulting in a small base control current into the base.

Figure 1-10: The transistor inverter; input = 1 and transistor ON. The transistor

ON configuration is at left and the equivalent circuit is at right.

The transistor is used because it has gain allowing a larger output current to flow as controlled by a weaker input. When the transistor is turned on as much as it can be, the collector emitter circuit looks almost like a short circuit, effectively connecting the output to ground or zero volts. This gives a logic zero on the collector output. When the transistor collector is shorted to ground, current flows from the supply through the resistor and into the transistor collector to ground. The transistor is said to sink the resistor current into ground. If there is an external load, such as another inverter or gate, connected to the collector output, the transistor can also sink current from the load. This is also referred to as pulling down the output voltage. The current sinking capacity of the transistor limits the number of devices this inverter can drive.