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| SEC. 3-1 BASIC IDEAS A diode is a nonlinear device. The knee voltage, approximately 0.7 V for a silicon diode, is where the forward curve turns upward. The bulk resistance is the ohmic resistance of the p and n regions. Diodes have a maximum forward current and a power rating. SEC. 3-2 THE IDEAL DIODE This is the first approximation of a diode. The equivalent circuit is a switch that closes when forward biased and opens when reverse biased. SEC. 3-3 THE SECOND APPROXIMATION In this approximation, we visualize a silicon diode as a switch in series with a knee voltage of 0.7 V. If the Thevenin voltage facing the diode is greater than 0.7 V, the switch closes. SEC. 3-4 THE THIRD APPROXIMATION We seldom use this approximation because bulk resistance is usually small enough to ignore. In this approximation, we visualize the diode as a switch in series with a knee voltage and a bulk resistance. SEC. 3-5 TROUBLESHOOTING When you suspect that a diode is the trouble, remove it from the circuit and use an ohmmeter to measure its resistance in each direction. You should get a high resistance one way and a low resistance the other way, at least 1000:1 ratio. Remember to use a high enough resistance range when testing a diode, to avoid possible diode damage. A DMM will display 0.5–0.7 V when a diode is forward biased and an overrange indication when it is reverse biased. SEC. 3-6 UP-DOWN CIRCUIT ANALYSIS No calculation is required in this type of circuit analysis. All you are after is up, down, or no change. When you know beforehand how a dependent variableshould respond to an increase in an independent variable, you will be more successful at troubleshooting, analysis, and design. SEC. 3-7 READING A DATA SHEET Data sheets are useful to a circuit designer and may be useful to a repair technician for selecting a substitute device, which is sometimes required. Diode data sheets from different manufacturers contain similar information, but different symbols are used to indicate different operating conditions. Diode data sheets may list the following: breakdown voltage (VR, VRRM, VRWM, PIV, PRV, BV), maximum forward current (IF(max), IF(av), I0), forward voltage drop (VF(max), VF), and maximum reverse current IR(max), IRRM). SEC. 3-8 HOW TO CALCULATE BULK RESISTANCE You need two points in the forward region of the third approximation. One point can be 0.7 V with zero current. The second point comes from the data sheet at a largeforward current where both a voltage and a current are given. SEC. 3-9 DC RESISTANCE OF A DIODE The dc resistance equals the diode voltage divided by the diode current at some operating point. This resistance is what an ohmmeter will measure. DC resistance has limited application, aside from telling you that it is small in the forward direction and large in the reverse direction. SEC. 3-10 LOAD LINES The current and voltage in a diode circuit have to satisfy both the diode curve and Ohm’s law for the load resistor. These are two separate requirements that graphically translate to the intersection of the diode curve and the load line. SEC. 3-11 SURFACE-MOUNT DIODES Surface-mount diodes are often found on modern electronics circuits boards. These diodes are small, efficient, and typically found either as an SM (surface mount) or an SOT (small outline transistor) case style. |