The purpose of this project is to build an ultraviolet exposure box for exposing UV-sensitive films, such as those used for PCB photoresists.

UV Box

Finished the UV exposure box. The working area is 9x12 inches with 99 LEDs in a triangular grid. The light pattern is not as consistent as I was hoping, but I haven't tried aligning any of the LEDs yet. Haven't tried exposing anything with it yet either. Note that neither the top cover nor the pcb tray (bottom and front pieces) are shown; the box is intended to be mostly light-resistant when closed so the board can "simmer" therein after exposure.

Update: I aligned the LEDs somewhat, and have been using it. Works great, with an exposure time around 5 minutes.

UV Power Supply

I asked on sci.electronics.design about the design of a power supply that could feed 20 mA through 100 LEDs. The best suggestion was to use LM317's in current limiting mode with 62 ohm resistors. The supply in the photo is designed to work with a 25.2 VAC transformer, producing 38 VDC when unloaded. The photos show one bank of nine LEDS and ten other "dummy" loads (470 ohm resistors) to simulate 11 banks of 9 leds, or 99 leds. Results:

• Power supply drops to 35.5 VDC with 0.22 amps load
• Vf across the LED string is 29.4 V, or 3.27 V per LED.
• That leaves 1.25 V across the 62 ohm resistors and 4.85 V across the LM317's.

The board itself is really a board - it's 1/4 inch maple plywood with a piece of 10 mil etched clad taped to it. Note the 10K resistor on the back to bleed the charge out of the capacitor. Without it, as soon as the cap's voltage dropped below the LED's Vf, it nearly stopped discharging, leaving about 30v across it. The resistor also helps shut the LEDs off more forcefully