Capacitors are components that store electricity and release it on demand. Their primary role is to accept electrical current in a slow trickle, then release that accumulation in one burst. For comparison, picture leaving a glass under your dripping tap until it's full, then pouring it out. That's how a camera flash can power such a strong light with a relatively small battery. Choosing the right capacitor for your project means understanding how to compare them.
Read the capacitance of each capacitor, measured in farads, from its printed side. If they're stated in different units -- one in microfarads, for example, and one in nanofarads -- convert them to the same unit for comparison purposes (see Tips). Use the capacitance required by your circuit.
Check that the voltage printed on the capacitor is suitable for your circuit. Higher-voltage capacitors might increase your cost unnecessarily, while lower-voltage capacitors might burn out and endanger the rest of your components.
Compare the physical dimensions of your capacitors, if space is at a premium. A tiny tantalum capacitor might be best for a circuit board. In other uses, a larger electrolytic or ceramic capacitor might serve your purpose.
Examine the capacitor for a letter code signifying its tolerance rating. That's how much it might vary from its stated rating, which can be a crucial factor in exacting applications. Some are accurate within one picofarad, while others vary by as much as plus or minus 20 percent. Capacitors with low tolerances are manufactured to higher standards, and cost correspondingly more.
Review your capacitor choice depending on the number you need and your budget for the circuit. If all other factors are equal, price is a perfectly valid reason to choose one capacitor over another.
Write down the order and color of the bands on your capacitor. If one of the bands is extra-wide, it's actually two bands of the same color and should be recorded that way.
Look up the values for each colored band on a color-code chart, or enter them into an online color-code calculator. For tantalum capacitors, the dot in the middle is equal to the third band. If you're using a calculator, it will furnish you with a written set of values for the capacitor. If you're using a chart, write each value on a piece of paper.
Read the first two digits. For example, if your colors were yellow and violet, your basic number would be 47. The third digit will be a multiplier. If it was red, for instance, you'd multiply your base number by 100 and get 4700. That's the rating in picofarads. Convert it if necessary into the same unit as your other capacitors (see Tips).
Compare the fourth and fifth bands, if they're present. The fourth indicates tolerance, and the fifth is voltage. For example, a black band fourth band indicates a tolerance of plus or minus 20 percent, while a brown fifth band signifies a 100-volt rating.
- A farad is the amount of current required to raise the capacitor's stored capacity by one volt. Capacitors follow the metric or SI convention of measuring in hundredths, thousandths and ten thousandths, yielding microfarads, nanofarads and picofarads respectively. Converting between them for comparison purposes is a straighforward matter of putting the decimal point in the right place. For example:
- 4,700 picofarads = 47 nanofarads = 0.047 microfarads = 0.00047 farads
- Handle capacitors with care. They'll retain a charge for an extended period, and even a relatively small capacitor can hold enough current to cause serious injury or death.
- Some capacitors are polarized, with positive and negative terminals. They'll be marked with either a plus sign and a minus sign, or arrows pointing in the direction of current flow. If you wire one in reverse, your capacitor will fail and might damage the circuit.
- Capacitor color chart
- Capacitor calculator (optional)
- Paper and pen or pencil
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