In electronics, polarity determines how a component should be inserted into a circuit. A non-polarized component can be connected in any direction and will still function properly. A polarized component can only be inserted in one direction. Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity. When it comes to through-hole LED’s a polarized component usually has two pins.
If a polarized component is inserted incorrectly it may not work at all, or it may smoke, spark or fail. It’s a little bit harder to determine the polarity with Surface Mount LEDs (SMT LEDs). Surface mounted device LEDs (SMD LEDs) are electronics components without the connecting wires used with through-hole devices. Although initially intended for automatic manufacturing their use has expanded into hobby electronics. Smaller components lead to more compact circuitry and smaller devices. In fact, some components are available in SMD versions only.
Continuity Tests using a Multimeter
Often the easiest way to determine polarity of an SMT LED is by using a simple multimeter. This is achieved by carrying out a simple test: we set the selector to ‘Diode’ or ‘Continuity Test’ mode and place a probe on each end of the LED. Usually, the multimeter will supply enough current into the LED which will just barely light it up. However, there’s no guarantee your meter won’t over-do the current, so we recommend doing just a quick touch, or putting a resistor in line. The black (common) lead on the multimeter indicates the negative (cathode) lead, and the red indicates the positive or anode side. It will only emit light when the negative probe is on the cathode end and the positive probe is on the anode end. When the voltage across the p-n junction of the LED is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reversed biased, very little current flows, and no light is emitted. Of course, the best method to determine the polarity of your LED, the pinout, along with all the other specs is to locate the manufacturer’s datasheet.
Visual Inspection
The majority of LEDs have the cathode end marked, however it has been noticed that even within the same range of LEDs from the same manufacturer, there are differences. Red LEDs seem to be the exception, having a mark on the anode end, not the cathode one. Most often if you cannot see any of these markings, a small notch or dot will indicate the negative side of an LED. SMT LEDs typically have a dot or small green line indicating their cathode. Lighthouse LEDs provide a selection of SMD images in their Polarity Guide for 0402, 0603, 0805, 1206 and most all SMD LEDs.
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Article Source: http://www.limetrace.co.uk/how-to-determine-the-polarity-of-an-smd-led
