Electronics Questions and Answers
1. A diode has a certain characteristic when operating. Explain this characteristic.
The main operating characteristic of a diode is that it allows current in one direction and blocks current in the opposite direction.
2. What must the conditions be for a LED to emit light?
An LED emits light when the diode is forward biased allowing current to flow.
3. Transistors have two main functions, what are they?
Amplification and switching.
4. A bipolar junction transistor has three semiconductor sections, what are they?
Emitter (E), base (B), and collector (C).
5. Give a detailed explanation on how a bipolar junction transistor amplifies current.
The BJT is a current amplifier in that a small current flow from the base to emitter results in a larger flow from collector to emitter.
6. What are the names of the three leads attached to a junction field-effect transistor?
Gate, source, and drain.
7. What are the similarities between a thyristor and a mechanical switches operation?
Similar to a mechanical switch, thyristors have only two states: on (conducting) and off (non-conducting).
8. List some of the similarities and differences between an SCR and a diode?
Silicon controlled rectifiers (SCRs) are similar to diodes except for a third terminal, or gate, which controls, or turns on the SCR.
9. The control of an SCR is different when operated from an AC source than a DC source, explain the difference.
When operating from a DC source, once the SCR is turned on it stays on.
With an AC source, the SCR will automatically switch to off when the sine wave goes through the zero volts.
10. Explain how the operation concerning an SCR which is unidirectional and a Triac which is bi-directional is different.
An SCR can only control the power delivered to a load from 0 to 50%. The triac can deliver power to the load from 0 to 100%.
11. A single wave and half wave rectifier change AC to DC, what is the difference between the two?
During the positive half cycle of the AC input wave, the anode side of the diode is positive.
12. What is the difference in the output when a single phase half-rectifier is replaced by a full-wave rectifier?
The diode is forward biased, allowing it to conduct a current to the load. Because the diode acts as a closed switch during this time, the positive half cycle of the AC wave form is developed across the load. During the negative half cycle of the AC input, the anode side of the diode is negative. The diode is now reversed biased; as a result, no current can flow through it. The diode acts as an open switch during this time, so no voltage is produced across the load.
13. Transistors can be a switching device or an amplifying device, how do the operations compare?
When a transistor is used as a switch, it has only two operating states, on and off.
14. A MOSFET has certain operating characteristics that are utilized for providing long time-delay periods for electronic timers, explain them.
The high input impedance, the low current into the gate, are the characteristics of the MOSFET that are utilized to provide long time delay periods for electronic timers.
15. To provide a varying amount of power to a three phase, reduced voltage starter there is a certain SCR control utilized to accomplish this, explain.
Phase angle control.
16. For switching AC power loads, there are certain characteristics of a triac that make it a perfect electronic switch, define this characteristic.
The operating characteristic of being bi-directional makes the triac an ideal component for switching AC power loads.
17. Diacs can be utilized to control power in a triac lamp dimmer circuit, explain how this is accomplished.
The diac is bi-directional and when the control voltage charges to the break over voltage the diac triggers the triac into conduction for the remainder of the half cycle.
18. An electronic motor drive has what primary function?
The primary function of an electronic motor drive is to control speed, torque, direction, and resulting horsepower of a motor.
19. An electronic frequency drive has three major sections, list them and state the main function of each.
Rectifier section: The full-wave three phase diode rectifier converts the 60 Hz power from a standard utility supply to either fixed or adjustable DC voltage.
Inverter section: Electronic switches, switch the rectified DC on and off, and produce a current or voltage waveform at the desired new frequency.
Control section: An electronic circuit receives feedback information from the driven motor and adjusts the output voltage or frequency to the selected values.
