Determining Polarity of Electronic Components
There are a few different ways that components are marked to show polarity.
To determine polarity in CAPACITORS:
Electrolytic capacitors are often marked with a stripe. That stripe indicates the NEGATIVE lead.
If it's an axial leaded capacitor (leads come out of opposite ends of the capacitor), the stripe may be accompanied by an arrow that points to the negative lead.
Sometimes, you can look to the length of the leads as an indication of polarity. The positive lead is usually longer but be careful if you're reusing old or 2nd hand capacitors - the leads may have been trimmed.
Tantalum capacitors are often marked with a + sign. In addition, there are other capacitors that are not polarized such as ceramic, polyester, film, polystyrene and paper.
Always use caution when trying to figure out the positive and negative leads of a capacitor. If you make a mistake with an electrolytic capacitor and hook it up backwards, the capacitor may still work if the voltage across the capacitors is low enough. If the voltage across the capacitors is not low enough, you may blow the cap (or worse) by not correctly figuring out the polarity before installation.
To determine polarity in DIODES:
On plastic case diodes, there is a white or silver band stamped on one end of the diode that indicates the polarity of the diode. Glass diodes might have a black band. On either one, the positive current flows from the terminal furthest from the band to the terminal closest to the band (and is blocked in the opposite direction). On a schematic drawing, the band is represented by the 'T'. The banded side is the cathode side.
If you have a diode in a TO-220 package, the two outer leads are marked either '+' or '-'.
Finally, on stud diodes, the threaded end is the cathode and the solder terminal is the anode.
To determine polarity in LEDS (light emitting diodes):
The easiest way to determine polarity in an LED is by looking at the lead length. The longer lead is the anode; the shorter is the cathode.
Another method you can use is to look for a flat spot on the edge of the LED. The flat spot indicates the cathode side of the lamp.