Diodes
Diodes are very important in electronic circuits. Everyone working in electronics must be familiar with them. Your study of diodes will enable you to predict when they will be on and when they will be off. You will be able to read their characteristic curves and identify their symbols and their terminals. This chapter also introduces several important types of diodes and some of the many applications for them.Types of Diodes
Rectifier diodes
A rectifier diode is designed specifically for circuits that need to convert alternating current to direct current. The most common rectifier diodes are identified by the model numbers 1N4001 through 1N4007. These diodes can pass currents of up to 1 A, and they have peak inverse voltage (PIV) ratings that range from 50 to 1,000 V.Here is a list of the peak inverse voltages for each of these common diodes. When choosing one of these diodes for your circuit, pick one that has a PIV that's at least double the voltage you expect it to be exposed to. For most battery-power circuits, the 50 V PIV of the 1N4001 is more than sufficient.
| Model Number | Diode Type | Peak Inverse Voltage | Current |
|---|---|---|---|
| 1N4001 | Rectifier | 50 V | 1 A |
| 1N4002 | Rectifier | 100 V | 1 A |
| 1N4003 | Rectifier | 200 V | 1 A |
| 1N4004 | Rectifier | 400 V | 1 A |
| 1N4005 | Rectifier | 600 V | 1 A |
| 1N4006 | Rectifier | 800 V | 1 A |
| 1N4007 | Rectifier | 1,000 V | 1 A |
Signal diodes
A signal diode is designed for much smaller current loads than a rectifier diode and can typically handle about 100 mA or 200 mA of current.The most commonly used signal diode is the 1N4148. This diode has a close brother called 1N914 that can be used in its place if you can't find a 1N4148. This diode has a forward-voltage drop of 0.7 and a peak inverse voltage of 100 V, and can carry a maximum of 200 mA of current.
Here are a few other interesting points to ponder about signal diodes:
- They're noticeably smaller than rectifier diodes and are often made of glass. You have to look at them closely to see it, but the cathode end of a signal diode is marked by a small black band.
- They're better than rectifier diodes when dealing with high-frequency signals, so they're often used in circuits that process audio or radio frequency signals. Because of its ability to respond quickly at high frequencies, signal diodes are sometimes called high-speed diodes. They're also sometimes called switching diodes because digital circuits often use them as high-speed switches.
- Some signal diodes are made of germanium rather than silicon. (Germanium the crystal, not to be confused with geranium the flower.) Germanium diodes have a much smaller forward-voltage drop than silicon diodes — as low as 0.15 V. This makes them useful for radio applications, which often deal with very weak signals.
Zener diodes
In a normal diode, the peak inverse voltage is usually pretty high — 50, 100, even 1,000 V. If the reverse voltage across the diode exceeds this number, current floods across the diode in the reverse direction in an avalanche, which usually results in the diode's demise.Normal diodes aren't designed to withstand a reverse avalanche of current. Zener diodes are. They're specially designed to withstand current that flows when the peak inverse voltage is reached or exceeded.
And more than that, Zener diodes are designed so that as the reverse voltage applied to them exceeds the threshold voltage, current flows more and more in a way that holds the voltage drop across the diode at a fixed level. In other words, Zener diodes can be used to regulate the voltage across a circuit.
In a Zener diode, the peak inverse voltage is called the Zener voltage. This voltage can be quite low — in the range of a few volts — or it can be hundreds of volts.
Zener diodes are often used in circuits where a predictable voltage is required. For example, suppose you have a circuit that will be damaged if you feed it with more than 5 V. In that case, you could place a 5 V Zener diode across the circuit, effectively limiting the circuit to 5 V. If more than 5 V is applied to the circuit, the Zener diode conducts the excess voltage away from the sensitive circuit.Zener diodes have their own variation of the standard diode schematic symbol.
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