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Posted in: Developing Perspectives (December 2010)

What are the “Best” Components for Your Project?

By Bryan Bergeron

Two major determinants in defining what constitutes best are cost and application. You’re obviously not going to spend $100 on a precision op-amp that’s part of an LED blinker circuit. Similarly, it would be unwise to use an electrolytic capacitor rated at 6 VDC in at 12.6 VDC circuit.

If you’re just starting out and want to have a supply of components around so that you don’t have to wait two weeks for parts to arrive when you get the urge to build something, the application area — say, RF or audio — readily defines the types of components you should consider. No need to splurge on a high-powered RF MOSFET when a low-powered, audio MOSFET will do.

In addition to addressing specific needs, there are several systematic approaches to building up a supply of components. One is to find a book or website that explains specific classes of components. The Art of Electronics by Horowitz and Hill comes to mind for a book on components. It’s a bit dated, but still a goldmine of information.

Another approach is to rely on product data from a major online supplier such as Mouser or Digi-Key. Online or print catalogs from these and similar parts suppliers are great because they highlight the key parameters for classes of components. For example, let’s consider electrolytic capacitors. If you log in to Digi-Key and search for ‘electrolytic capacitor,’ you’ll get a table of selection criteria that you can use to further define your search. It turns out that the selection criteria are usually the most important for a given component class.

Returning to our search for electrolytic capacitors, the table produced by Digi-Key includes capacitance, voltage rating, tolerance, lifetime at temperature, operating temp range, special features, ripple current, equivalent series resistance, impedance, and a variety of physical parameters. If your application is an output filter for a DC power supply, then you’ll be primarily concerned with voltage rating, capacitance value, and ripple current. However, if you’re using the capacitor as part of an RC timing element, then you’re going to pay particular attention to the tolerance rating. If you don’t know what the individual parameters mean, it’s an easy enough exercise to look up, say, equivalent series resistance on Wikipedia.

Now, let’s perform a search for ‘diode,’ sub-category ‘bridge rectifier.’ The criteria now include peak reverse voltage, DC forward current, speed of recovery, and reverse recovery time. If you’re building a power supply, you’re probably going to be concerned primarily with peak reverse voltage and DC forward current. Again, if you’re unfamiliar with a term, it’s a great time to Google it.

Before I place the order for a component, I make a practice of reading the official data sheet. The data sheet which is published by the component manufacturer usually contains too much information to be used as a screening tool, but is a great sanity check. Once you’ve narrowed your selection down to two or three components, official data sheets can help you make your final decision. Data sheets are especially helpful when they contain example circuits that show the practical application of components. They’re also great at identifying general application areas, such as RF, audio, or power.

Another approach to component selection worth noting is to explore existing circuits. That is, to tear down whatever you can get your hands on. Want to know what kind of electrolytic capacitors go into a good AC-DC power supply? Take one apart. Given the output voltage range of the supply, how conservatively are the output capacitors rated? Unless you’re dealing with a super cheap supply, you’ll find the capacitor max voltage rating of about twice the maximum expected output voltage. After a few teardowns, you’ll not only get a feel for the specific components you need to have on hand, but also practical information on component layout and overall circuit design. NV

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