CHAPTER 10 ■ LEDSIf there isn’t a problem with the test setup, it could be that the LED is damaged. A diode voltage value
below 400 mV (0.4 V) indicates the power is passing almost directly from the anode to the cathode without
passing through the die. This is called a short circuit. It’s unlikely that an LED would kick the bucket this way.
An open or partially open circuit is more common.
If all goes well, the multimeter will display a red LED’s voltage value as being somewhere around
1.6 volts (see Figure 10-17). A lot of meters display this rating in millivolts (mV). For example, 1632 mV is the
same as 1.632 V.
■ Tip Take a close look at your LED as the multimeter tests it. The LED may be lit up dimly. (You may need
to cup your hands around the LED to block the room’s lighting.) This is a good trick for proving that the LED is
working and also to give you a sense of the LED’s efficiency. Poorer-quality LEDs won’t light up at all or will
have just a glimmer. High-efficiency LEDs will be quite noticeable.
If you haven’t already, try reversing the LED leads to see what the meter displays when an LED is
hooked up backwards. It should be “open” or “0L.” If the LED works in both directions, it’s likely a bicolor.
Try a bunch of different kinds of LEDs. You may notice that the physical size of the LED doesn’t affect
the voltage rating displayed. However, the color does!
Colors nearer to red (including infrared) generally have a lower voltage value and colors nearer to
blue have a higher voltage value. In fact, many older or cheaper meters’ diode modes usually can’t go high
enough to test the 2.5 volts required by true green, blue, and white LEDs. That’s why I had you start by testing
a red LED.
Figure 10-16. Short circuit or zero volts indicate a test error or damaged LED
Figure 10-17. Multimeter displaying normal LED value