Passive Components and Soldering Supplies

Through-hole or SMD, and how to read the sizes

Passive parts come in two mounting styles. Through-hole resistors and capacitors have wire leads that go through the board, so they are easy to hand-solder and to use on breadboards. Surface-mount parts are tiny and need a fine tip, flux and steady hands, but they save board space. SMD sizes use codes: 0805 is about 2.0 x 1.25 mm, 0603 is 1.6 x 0.8 mm, and 1206 is 3.2 x 1.6 mm. Leaded solder melts at about 183 C while lead-free alloys need roughly 217 C or more, so set the iron to suit.

Ratings that keep a part from failing

Resistors need the right power rating as well as the right value. A 0.25 W resistor suits signal circuits, while power circuits call for wire-wound types, such as a 20 W part, which can get very hot. Pick a rating at least double the power the part will dissipate. Capacitors have a voltage rating that must exceed the circuit voltage with margin, and electrolytic types are polarized: the stripe marks the negative lead on aluminum parts but the positive lead on tantalum ones. Check tolerance too: 1 percent parts suit measurement circuits and 5 percent parts suit general use.

Potentiometers and trimmers add two more choices. Taper decides the response: linear suits general adjustment, logarithmic suits audio volume. Mounting style decides how you set them: panel pots have a shaft for a knob, while trimmers are adjusted with a screwdriver and then left alone. Check the resistance value and rated power as well.

Questions about Passive Components and Soldering Supplies

What does resistor tolerance mean?

It is how far the actual resistance may differ from the stated value. A 5 percent 1 kilohm resistor can measure anywhere from 950 to 1,050 ohms. Use 1 percent parts for precision work such as voltage dividers and measurement circuits.

Can I use a higher-wattage resistor than the original?

Yes, provided the resistance value matches and the part physically fits the board, unless the original is a fusible resistor meant to burn out like a fuse. Otherwise a higher power rating only adds a safety margin and often runs cooler. Never go lower, because the resistor can overheat and fail.

Why does a capacitor have a stripe or a plus sign?

Electrolytic capacitors are polarized. On aluminum electrolytics the stripe marks the negative lead, while on tantalum capacitors a stripe or plus sign marks the positive lead. Fitting either backwards can make it overheat or burst. Ceramic capacitors have no polarity and can go in either way round.