Diode switching circuits

Chapter 3.10 Diode switching circuits

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 132CHAPTER 3. DIODES AND RECTIFIERSelectromechanical relay, because the relay will possess a natural “time delay” upon coil de-energization, and an unwanted delay of even a fraction of a second may wreak havoc in somecircuits.Unfortunately, one cannot eliminate the high-voltage transient of inductive kickback andmaintain fast de-magnetization of the coil: Faraday’s Law will not be violated. However, if slowde-magnetization is unacceptable, a compromise may be struck between transient voltage andtime by allowing the coil’s voltage to rise to some higher level (but not so high as without acommutating diode in place). The schematic in Figure 141,3.52 shows how this can be done.offoffon(a)(b)(e)V(d)(c)Figure 3.52:(a) Commutating diode with series resistor. (b) Voltage waveform. (c) Level withno diode. (d) Level with diode, no resistor. (e) Compromise level with diode and resistor.A resistor placed in series with the commutating diode allows the coil’s induced voltage torise to a level greater than the diode’s forward voltage drop, thus hastening the process of de-magnetization. This, of course, will place the switch contacts under greater stress, and so theresistor must be sized to limit that transient voltage at an acceptable maximum level.3.10Diode switching circuitsDiodes can perform switching and digital logic operations. Forward and reverse bias switch adiode between the low and high impedance states, respectively. Thus, it serves as a switch.3.10.1LogicDiodes can perform digital logic functions: AND, and OR. Diode logic was used in early digitalcomputers. It only finds limited application today. Sometimes it is convenient to fashion asingle logic gate from a few diodes.An AND gate is shown in Figure 142,3.53. Logic gates have inputs and an output (Y) which isa function of the inputs. The inputs to the gate are high (logic 1), say 10 V, or low, 0 V (logic0). In the figure, the logic levels are generated by switches. If a switch is up, the input iseffectively high (1). If the switch is down, it connects a diode cathode to ground, which is low(0). The output depends on the combination of inputs at A and B. The inputs and output arecustomarily recorded in a “truth table” at (c) to describe the logic of a gate. At (a) all inputsare high (1). This is recorded in the last line of the truth table at (c). The output, Y, is high(1) due to the V+ on the top of the resistor. It is unaffected by open switches. At (b) switch Apulls the cathode of the connected diode low, pulling output Y low (0.7 V). This is recorded in