Build-It Part II

In the first part of this construction series, all of the automotive and electrical/electronic components for the Distributorless Ignition System (DIS) were identified. Figure 1 is the schematic for the simple electronic control circuit that controls the speed of the motor. The 500 ohm pot varies the base current of the the 2N3055 power transistor. Beta or current gain of this series of power transistors varies from a minimum of 20 to a maximum of over 150. The RPM indication on the scale for the potentiometer given on the unit is only a reference as the beta of the power transistor will determine the amount of rotation to achieve a specific RPM. Be sure to mount the transistor on the specified (or better) heat sink.

Many good activities can be obtained from this simple circuit for advanced students such as;

1. The transistor is manufactured in a TO-3 package. Identify the transistor leads, base, emitter, and collector.

2. What is meant by "current gain"?

3. What are the maximum and minimum values of transistor collector current for the stated value of beta variation?

4. What is the amount of power dissipated by the transistor if the motor turning at 500 RPM has a current draw of one amp and a voltage drop of 2 volts?

5.(Difficult) Why was an emitter-follower circuit configuration chosen as opposed to a common emitter configuration?

6. Measure the motor's resistance with an ohmmeter, then measure the voltage and current when the motor is operating. Explain the difference.

Figure 1

Partial Vehicle System Diagram

Figure 2

Circuits # 424, 423, 430, and 453 on the wiring diagram in figure 2 do not need to be connected on the trainer. They are normally connected to the Engine Control Module (ECM) to allow control of spark advance. The DIS module will operate in "base timing mode" without a control signal from the ECM. These lines can be connected to the Electronic Fuel Injection trainer to give a complete operational engine control system. On the original ATech trainer, they were connected to a terminal block for ease of connection to the EFI and to allow easy access for measurement.