INSULATION 3775948 - Caterpillar

Testing Diodes
When a diode is functioning properly in a circuit, the diode acts as a large voltage drop in one direction, and as a very small voltage drop in the other. Unfortunately, testing diodes is not always this simple. In fact, there are four possible ways in which you can test diodes:
Take the diode out of the circuit. Sometimes this is not possible.
If the diode is in a series circuit, the diode can be tested with the circuit power off.
If the diode is in a series circuit, the diode can be tested with the power on. For a typical silicon diode, the forward biased voltage drop should be approximately 0.6 volts.
If the diode is in a parallel circuit, the diode must be tested with an analog meter, not with a digital meter.TRANSISTORS
A diode is only one type of semiconductor. By combining several kinds of semiconductor material, you can create transistors. Like diodes, transistors control current flow. Transistors can perform practically all the functions that were once performed by vacuum tubes, but in much less space and without creating as much heat. Transistors are used in many applications, including radios, electronic control modules and other solid state switches.Transistor Types
There are many kinds of transistors. Transistors can be divided into two major groups: bipolar and unipolar (also called Field Effect Transistors, or FETs). While there are several differences between the two types, the most important difference is this:
Bipolar transistors vary current to control voltage.
FET transistors vary voltage to control current.Bipolar transistors are more common in Caterpillar electrical circuits. Transistor Construction
Illustration 11 g01072621
Like diodes, transistors contain a combination of N type and P type material. However, transistors contain three materials instead of two. The three materials are arranged so that N type and P type materials alternate (either as an NPN or a PNP group). This means that diodes have two leads while transistors have three. Illustration 11 is a symbolic representation of transistor construction.Emitter, Base, and Collector
In Illustration 11, the material on the left is called the emitter. The material that is sandwiched in the middle is the base. The material on the right is the collector. The symbols on the top of Illustration 11 are the schematic symbols for a transistor. The arrow indicates current flow direction this is conventional theory, and is always on the emitter. The arrow points in a different direction depending on whether the transistor is PNP or NPN.FETs also have three sections. These sections are referred to as the gate, the source, and the drain .
The gate approximates the function of the base.
The source is similar to the emitter.
The drain is similar to the collector.Basic Function
A transistor works by using the base to control the current flow between the emitter and the collector. When the transistor is turned ON, current can flow in the direction of the arrow only. When the transistor is OFF, current cannot flow in either direction. Base Paths
It is important to realize that the base leg of a bipolar transistor controls