Electrical – LED lamp loses intensity very fast


I have designed a UV lamp using UV LEDs (VAOL-5EUV8T4-ND), and it loses intensity very fast. I attach a figure with a picture of the lamp and the circuit. I am using a LM317 to generate a constant current for the LEDs. With the resistor values shown in the figure, the LM317 should generate a constant current of 0.160 A, which is split among the eight columns of LEDs. The LEDs data sheet reports a forward current of 20 mA, which what the circuit provides. The voltage drop across one LED is 3.2 V (measured with voltmeter). A voltmeter reader of the drop of potential across R2 and R3 gives 1.273 V as expected. I use the resistor R1 to reduce the voltage drop across the LM317. Not shown in the circuit, I have also placed two fans to avoid heating and all the circuitry is very cold as a result. The capacitor is a 0.1 microF tantalum capacitor. The 6.8 Ohm and 1 Ohm resistors are rated at 1 W and the 27 Ohm resistor is rated at 3 W. The AC/DC power supply provides 48 V. The lamp consists of eight columns with 12 LED each.

I have measured the UV intensity with the UV sensor ML8511 connected to the Arduino. The inset shows the intensity decreasing over time. I use the internal voltage reference of the Arduino to get a precise measurement of the UV light intensity. The lamp is in a temperature-controlled room in a shelf covered with black curtains.

The intensity drop is even visible by eye, so I don't think that the light intensity measurement is the issue, but rather something in the lamp. The issue is permanent, meaning that if I switch off the lamp and switch it on after some time, the UV intensity is equal to the last intensity measured. Do you have any idea of what could be the problem? Thank you very much.

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Best Answer

This is probably to be expected, assuming these are 20mA LEDs.

Without a separate current sharing resistor in each string, each string will conduct at a slightly different voltage, due to natural variations between devices.

The one that starts conducting earliest will take most of the current, heat up, reduce its conduction voltage, and take more current, until it is operating at about 8x its rated power, and ultimately destroyed.

Repeat for each string in turn.

See ANY description of how to drive multiple strings of LEDs for the solution.

I would keep the regulator, and split the 27 ohm resistor : short out the current one, and put a separate (27*8 = 216) or 220 ohm resistor in series with each string.

This must be a duplicate of dozens of questions here.