CHAPTER 10 ■ LEDS
Interpreting LED Test Results
If the multimeter display doesn’t change values when the LED is connected, it’s likely the LED has been put
in backwards (see Figure 10-15). If that’s not the problem, double check that all the meter leads are firmly
connected to the meter terminals and that the probes are making good contact with the LED leads.
If there isn’t a problem with the test setup, it could be that the LED is damaged. It’s unlikely that a fresh
red LED is going to be broken. However, electrically abused LEDs are likely to have their bond wire melted or
some other form of disconnection. In those cases, the LED’s internal circuitry is permanently broken open
and can’t pass power through.
There’s also a problem if the meter displays a voltage rating around zero (see Figure 10-16). It is very
likely that the LED’s anode and cathode leads are accidentally touching each other or that the multimeter
test probe’s tips are accidentally touching each other.
Figure 10-15. Open circuit indicating a test error or damaged LED
Figure 10-14. LED connected to hook adapter probes for testing