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  • Stu WØSTU

Technician Figure T-1

The 2022-2026 Technician License question pool contains five questions referencing figure T-1 below. Let's get a solid understanding of the simple circuit of this figure.

Circuit diagram T-1 from Technician Exam questions.

Four of these questions simply ask you to identify one of the electronic components in the circuit diagram. The fifth asks you to identify the function of component #2. Let’s examine this circuit, identifying the components’ symbols and functions along the way.

The circuit of Figure T1 is a simple lamp dimmer. It adjusts the brightness of a lamp by altering the amount of current that is allowed to flow through the lamp from a battery. Let’s trace the flow of current through the circuit. (We will trace the flow of conventional current, or the positive charge, for convenience; the flow of negatively charged electrons is the reverse of this examination.)

From the battery (#4) current flows to the lamp (#3). When current flows through the lamp the lamp’s filament glows. The brightness of the lamp’s glow depends upon how much current is allowed to flow through it. Just like water flowing through a pipe, the amount of electric current may be increased or decreased by a “valve” further downstream that may be opened wider or closed tighter to regulate the current flow.

The valve is the transistor (#2). A transistor regulates the current flowing through it from top to bottom, as oriented in Figure T1. The current flows through the transistor and out to the common ground connection (#5) where it is able to return to the opposite terminal of the battery (at the battery’s ground connection symbol).

The size of the transistor valve’s “opening,” or the amount of current allowed to pass through the transistor, is controlled by the amount of a much smaller current flowing into the transistor from the connection on its left, as oriented in Figure T1. If the controlling current is relatively large, the amount of battery current allowed to flow through the transistor top-to-bottom and, thus, through the lamp, is large. The lamp glows brightly! If the transistor’s controlling current is made to be relatively small, the amount of battery current allowed to flow is also small, so the lamp dims. By varying the small transistor control current (left side connection) the larger lamp current and resulting brightness is varied.

A resistor (#1) is included in the line providing the transistor’s control current. A resistor opposes the flow of electrical current in a circuit. In this case it’s receiving current from an undefined source to the left and reducing the control current to be consistent with the transistor’s acceptable range. We assume that the undefined source is one that can provide variable current in order that the lamp brightness may be controlled.

To summarize, the current flow through this circuit may be depicted as follows:

Conventional current flow (positive charges) in the circuit of Figure T-1
Conventional current flow (positive charges) in the circuit of Figure T-1

It should be noted that the transistor’s control current may be set to a level that allows no current to pass, thereby turning off the lamp altogether. This way the transistor may be used as a switch.

Further, imagine if you rapidly changed the transistor control current higher and lower. The much larger current allowed to flow through the transistor would mimic that small variable control input current – that is, the control input signal (variations in small current) would be amplified into much larger current signal variations through the transistor. In this way the transistor may be used as a signal amplifier. And yes, the lamp would flash brighter and dimmer in response to this amplified “signal.”

For Figure T1, “What are components 1, 2, 3, and 4?” the correct answers are:

  • 1 – Resistor (T6C02)

  • 2 – Transistor (T6C03)

  • 3 – Lamp (T6C04)

  • 4 – Battery (T6C05)

And the function of component #2 is to control the flow of current. (T6D10)

-- Stu WØSTU


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