Using a Power Supply to Test LEDs

Because LEDs are becoming so pervasive in the lighting industry, one requires an efficient means of verifying correct operation. A regulated DC power supply with an adjustable current limit (constant current) is indispensable for this verification. Since LED strings are available in various voltage and current ratings, a regulated power supply with an adjustable output is required.

When testing a single LED lamp, the operator must determine the appropriate voltage and current rating for the diode. The color of the LED determines the required voltage level to illuminate the diode. Red, green, yellow, and orange LEDs typically have a forward voltage requirement of less than two volts, but blue and white LEDs normally require three or more volts.

Therefore the voltage output of the power supply may need to be adjusted to obtain illumination. The current limit or constant current value should be set initially to 20 mA or less to prevent damage to the LED.

LED strings typically contain a current limiting resistor that allows the string to be powered by a readily available standard voltage level (normally 12 or 24 volts). In this case you would need to adjust the voltage level of the power supply to a higher level than required for testing a single LED.

The procedure described is valid for either single LED lamps or strings of multiple LEDs. The power supply should be set initially to a low output voltage level of one or two volts and the constant-current feature should be activated. The output of the programmable power supply should now be shorted out using a jumper wire or shorting bar  and the current limit should be slowly raised to the desired value.

Once the current level is set to a value close to 20 mA, the shorting wire must be removed. After the current limit has been set, the voltage should be slowly raised until the LED illuminates. If the LED or LED string does not illuminate and no current is shown on the power supply meter, the polarity of the LEDs should be verified.

At this point the voltage may be increased slowly while watching for an increase in the current reading of the power supply. If the current value shown on the power supply is at or near the 20 mA setting and the LEDs are dimly lit, the current limit value may be slowly increased while looking for a corresponding increase in the brightness.



George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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