Discrete Semiconductor Devices and Circuits
|Don’t just sit there! Build something!!|
Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.
You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:
- Carefully measure and record all component values prior to circuit construction, choosing resistor values high enough to make damage to any active components unlikely.
- Draw the schematic diagram for the circuit to be analyzed.
- Carefully build this circuit on a breadboard or other convenient medium.
- Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
- Mathematically analyze the circuit, solving for all voltage and current values.
- Carefully measure all voltages and currents, to verify the accuracy of your analysis.
- If there are any substantial errors (greater than a few percent), carefully check your circuit’s construction against the diagram, then carefully re-calculate the values and re-measure.
When students are first learning about semiconductor devices, and are most likely to damage them by making improper connections in their circuits, I recommend they experiment with large, high-wattage components (1N4001 rectifying diodes, TO-220 or TO-3 case power transistors, etc.), and using dry-cell battery power sources rather than a benchtop power supply. This decreases the likelihood of component damage.
As usual, avoid very high and very low resistor values, to avoid measurement errors caused by meter “loading” (on the high end) and to avoid transistor burnout (on the low end). I recommend resistors between 1 kΩ and 100 kΩ.
One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.
Shown here is an illustration of a large “stud mount” type of SCR, where the body is threaded so as to be fastened to a metal base like a bolt threads into a nut:
With no test instrument other than a simple continuity tester (battery and light bulb connected in series, with two test leads), how could you determine the identities of the three terminals on this SCR?
Hint: The threaded metal base of the SCR constitutes one of the three terminals.
A student of electronics has just recently learned how to build audio amplifier circuits, and this inspires dreams of designing a super-powerful amplifier for a home entertainment system. One day, this student comes across a donation of electronic components from a local business, and in this donation are several industrial SCRs, rated at 20 amps each.
“Wow,” says the student, “these components look like really big transistors, but they’re rated for a lot of current. I could build a huge amplifier with these!”
The student approaches you for advice, because you’ve just recently learned how SCRs function in your electronics class. What do you tell the student, concerning the use of SCRs as audio amplification devices? How do you explain to this excited student that these devices will not work in an amplifier circuit?
One way that SCRs may be triggered into their “on” state is by a transient voltage applied between the anode and cathode terminals. Normally, this method of triggering is considered a flaw of the device, as it opens the possibility of unwanted triggering resulting from disturbances in the power supply voltage.
Explain why a high [dv/dt] present on the power supply rail is able to trigger an SCR, with reference to the SCR’s equivalent circuit. Also suggest what means might be employed to prevent false triggering from power supply transients.
The unijunction transistor, or UJT, is an interesting device, exhibiting hysteresis just like SCRs and TRIACs. Its schematic symbol is as follows:
One equivalent circuit diagram for the UJT uses a pair of transistors and a pair of resistors:
When the two base terminals of a UJT are connected across a source of DC voltage, the two base resistances (RB1 and RB2) form a voltage divider, splitting the applied voltage into lesser portions:
How much voltage, and of what polarity, must be applied to the emitter terminal of the UJT to turn it on? Write an equation solving for the magnitude of this triggering voltage (symbolized as VP), given RB1, RB2, and VBB.
Predict how the operation of this UJT latch circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):
- Capacitor C1 fails open:
- Capacitor C1 fails shorted:
- Resistor R1 fails open:
- Solder bridge (short) past resistor R1:
- Resistor R2 fails open:
- Solder bridge (short) past resistor R2:
For each of these conditions, explain why the resulting effects will occur.
Find one or two silicon-controlled rectifiers and bring them with you to class for discussion. Identify as much information as you can about your SCRs prior to discussion:
- Terminal identification (which terminal is gate, anode, and cathode)
- Continuous voltage rating
- Continuous current rating
- Continuous power rating
- Whether or not it is a ßensitive gate” device
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