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UV LED lamp 5W

Purpose

For work with photoresist, a small portable source of ultraviolet-A light was desired. A LED was chosen as a suitable source of good-quality light, based on the projects of Bogdan Raducanu and Yves Lebrac. The latter was chosen as a direct inspiration.

See also Germicidal UV lamp.

Construction

LED

A 5-watt ultraviolet LED was obtained over eBay from somewhere in Hong Kong, with 5-watt power rating, forward voltage of about 6.5 volts, and rated current of 750 milliamps. The LED has four chips, in two pairs in series. Gives fairly homogeneous light intensity, with wavelength of 400-405 nm.

To avoid overheating, the LED is mounted on an aluminium heatsink, cut to size from a larger CPU heatsink and attached using M3 screws and a liberal amount of thermal grease.

Power supply

The original implementation uses a LM317-class chip. It was decided against this approach on the basis of fairly high thermal loss on the linear stabilizer at higher currents and the corresponding cooling/heatsinking requirements.

For maintaining high efficiency, the power circuit is realized as a buck converter, using a LM2576 chip, a 3-ampere step-down regulator, the adjustable version. Due to the requirements of the LED, which desires a current-stabilized source instead of a voltage one, the feedback signal is derived from a current sense resistor instead of from the converter's output.

Current sensing

Method 1

The simplest way would be using a 1.6 Ω resistor on the negative side of the circuit, and the voltage from it fed directly to the feedback pin of the switching chip. The power loss, and corresponding heating, would be almost one watt, and the current through the LED would be difficult to adjust. The resistor value was also impractical and would have to be either made from a resistive wire, or assembled from several resistors of different values.

Method 2

The current sensing is performed with a sense resistor with a small value, to limit power losses and corresponding heating of the assembly. 150 mΩ was chosen on the basis of availability and suitable ballpark-value.

The sense resistor was originally intended to be connected to the ground and the voltage on it directly amplified with a LM386 op-amp and fed to the feedback of the switching chip. However that required a floating ground for the LED, and as the chassis is connected to the ground, it required insulating the connector. This turned out to be impractical, and the failure mode of shorting the negative lead of the LED to the ground, common with the grounded chassis, would send a lot of current through the LED, leading to its failure.

The decision to not use this method was made when an accidental short of such nature fried one of the four LED chips, now visible as a dark spot when the LED is operating and a slightly darker chip when the LED is not powered.

Method 3 (used)

LM358, a dual op-amp, was chosen on the basis of cost and availability for amplifying the current sense resistor signal, and ability to operate with input close to the ground, and ability to drive the output almost to the ground (acting as halfway rail-to-rail). The value of the Rsense was kept on 150 mΩ as a good compromise between sense amplitude and power loss (112 mV and 85 mW at the desired 750 mA).

The sense resistor is connected on the high side of the circuit, attached to a differential amplifier made from the unused half of the LM358. Its output is fed to a non-inverting amplifier with variable gain, adjustable with a trimpot, and the amplified signal is then fed to the feedback input of the switching converter chip.

The amplification determines the output current. With the values of 1 kΩ against the ground and 4.7 kΩ resistor and 10 kΩ trimpot it achieves amplification of about 5..15 times, which gives regulation range of about 0.5..1.4 amps.

Different values of the sense resistor and of the amplification range allow using this circuit for powering other types of LEDs or other constant current requiring devices.

Mechanical assembly

For the device housing, a metal chassis from an old CD-ROM drive was used, based on availability and suitable size.

To allow adjustments of the LED height, the arm with the LED is attached with a set screw to a vertical rail. The vertical tube, on which the LED-arm height can be set, is made from a length of 8 mm aluminium tube, with a M8 thread cut to its end. The tube is attached to a nut cut into a piece of aluminium bar and attached to the unit housing.

The LED arm is made from a hollow rectangular profile, also serving as a heatsink. Original intention of using the arm itself as a heatsink without further modifications turned out to be insufficient, the LED was heating to an excess of 80 °C, after which the thermal test was cancelled (though it took a while to reach this temperature). A piece was then cut off the profile, and replaced with a block of finned aluminium obtained from an old computer CPU heatsink.

To allow easier storage and transport, the LED arm and the vertical tube can be unmounted and stored inside the chassis. For this purpose, the LED is attached via a 3.5mm mono jack connector.

TODO

Images


Top side view

Back panel

LED heatsink

Front panel

LED arm positioning

LED attachment

LED attachment

LED arm back side

Tube mount thread

Tube mount thread, bottom view

Circuit detail

Circuit detail

Circuit detail

Inside view

Switch

Inside circuit detail

Back panel

Back panel

Heat sink assembly

Heat sink assembly

Top view of the assembly

LED powered on

LED powered on

LED powered on

LED powered on

LED powered on, from side

LED powered on, from side

LED powered on, from side

LED powered on, from side

LED powered on, from side

LED chip detail

LED chip detail, visible damage

Inside, with LED assembly stored

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