Nuts & Volts - September 2004

Just For Starters

help achieve a critical threshold point. Oscillators, pulse generators, and RF energy generation are the areas where the tunnel diode is most applicable.

Zener Diode

Voltage regulation is an important function in modern electronics and the zener diode is designed to do just that. It achieves this in the reverse bias mode. You can see in the diagram (Figure 2) that the zener is wired across a power supply output with the anode lead connected to ground and the cathode lead connected to the supply output.

Like regular diodes, zener devices resist the flow of voltage when reverse biased. The zener, however, is designed to resist the flow of voltage only to a given breakdown point. Anything over the breakdown point will be passed to ground. Acting as a reference source, the zener diode is capable of providing very accurate, constant current loads. Popular cutoff points are 5, 9, and 12 volts to accommodate most power supply requirements.

Figure 3. Diode types

diode. The varactor acts as a voltage sensitive capacitor in series with a resistance. Its use is basically confined to microwave equipment.

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SCRs

Diode Types

An SCR (Silicone Controlled Rectifier) is like a diode with an extra lead. The device will not pass current until a pulse is applied to the third lead, which is called a gate. Many switching functions that were formerly controlled

Figure 4. SCR operation

Varactor

One of the most unusual devices is the varactor or variable-reactance

As you can see in the illustration, diodes come in a wide variety of shapes and sizes. The size of a diode will most likely relate to its power handling capacity. Leads are identified as anode and cathode and — on most diodes — polarity is identified by a line on the cathode side, as shown in the diode type illustration (Figure 3).

Your local supplier should have most types in stock. Low level glass diodes, such as the 1N914, are inexpensive (about 10 for $1.00) and widely available. Harder to find diodes can be ordered from supplier catalogs or from the advertisers