Procedure – PASCO EM-8656 AC_DC ELECTRONICS LABORATORY User Manual

Page 90

AC/DC Electronics Laboratory

012-05892A

A transistor circuit can serve as a ‘digitial’ electric switch. In a mechanical electric switch, a small
amount of power is required to ‘switch on’ an electrical device (e.g., a motor) that can deliver a
large amount of power. In a ‘digital’ transistor circuit, a small amount of power supplied to the
base is used to “switch on” a much larger amount of power from the collector.

Here is some general information. A transistor is a three-terminal device. Voltage at a transistor
terminal relative to ground is indicated by a single subscript. For example, V

is the collector

voltage. Voltage between two terminals is indicated by a double subscript: V

is the base-to-

emitter voltage drop, for instance. If the same letter is repeated, it means a power-supply voltage:
V

is the positive power-supply voltage associated with the collector.

A typical npn transistor follows these “rules”:

➀

The collector must be more positive than the emitter.

➁

The base-to-emitter and base-to-collector circuits behave like diodes. The base-emitter diode is
normally conducting if the base is more positive than the emitter by 0.6 to 0.8 Volts (the
typical forward “turn on” voltage for a diode). The base-collector diode is reverse-biased. (See
previous experiments for information about diodes.)

➂

The transistor has maximum values of I

, I

, and V

and other limits such as power dissipa-

tion (I

) and temperature.

➃

If rules 1 – 3 are obeyed, the current gain (or amplification) is the ratio of the collector current,
I

, to the base current, I

. A small current flowing into the base controls a much larger current

flowing into the collector. The ratio, called “beta”, is typically around 100.

PROCEDURE

PART I: Computer Setup

➀

Connect the Science Workshop interface to the computer, turn on the interface, and turn on the
computer.

➁

Connect the Voltage Sensor to Analog Channel A.

➂

Connect the Power Amplifier to Analog Channel B. Plug the power cord into the back of the
Power Amplifier and connect the power cord to an appropriate electrical receptacle.

➃

In the Physics Folder of the Science Workshop Experiment Library, open the document:

Macintosh: “P54 Transistor Lab 1” / Windows: “P54_TRN1.SWS”