Transistors - Briggs & Stratton 86262GS Familiarization & Troubleshooting Manual

Silicone-Controlled Rectifiers
The silicone-controlled rectifier is often referred to as
simply an "SCR" (Figure 2.41).
The SCR permits current to flow in one direction when a
voltage is applied at its "gate." Applying a small (DC) voltage
to the SCR gate is said to "turn on" the SCR; it then permits
only direct current (DC) to flow.
Figure 2.41 — Silicone-Controlled Rectifier
Reverse current alternations are blocked by the diode action
of the SCR.When the current again reverses itself, a small
(DC) voltage must pulse the gate once more before current
can flow.Thus, the SCR acts much like a hydraulic check
valve that must be opened to permit fluid flow but blocks
the flow of fluid in the opposite direction.
The Zener Diode
The zener diode (Figure 2.42) is often used to provide
protection against over voltage that might damage some
components in a circuit, or to protect transistors against
excessive current flow.
For example:
A zener diode rated at 10VAC acts much like any other
diode up to 10VAC. However, if the applied voltage should
exceed 10VAC, the diode begins to pass current to maintain
the 10VAC level.Thus, if 15VAC is applied to the 10VAC
zener diode, it shunts five volts to ground while permitting
10VAC to pass.
Portable Generator Familiarization & Troubleshooting Guide
Section 2 • Generator Components & Systems
Figure 2.42 — The Zener Diode

Transistors

The construction of a transistor is similar to that of a diode.
A diode is made up of two layers of metal. However, a
transistor consists of three metallic layers and is called a
"triode."
As illustrated in Figure 2.43, a "triode" (or transistor)
consists of a wafer "N-Type" material sandwiched between
two thicker sections of "P-Type" material.
Figure 2.43 — Construction Of A Transistor
34