by Harry Lythall - SM0VPO

I have been making PCBs for quite a number of years but one om my material suppliers has recently changed the Photoresist spray ingredients and it can no-longer be used with "high-spectrum visible light. This means I am now forced to have a UV-light box. I therefore built mine and the moment it is a "lashup" fixed to the workbench with masking tape. So please excuse me if I do not include a photograph.

There is no PCB since there are no components to mount on one. the object was to create a source of Ultr Violet light as fast as possible. The UV tubes I bought from a lighting shop for use with handheld battery lamps using 4-watt tubes. the tubes are white in colour and deliver light from about 390nm to 450nm which is ideal for making PCBs.

The transistor is almost any NPN power transistor and heatsink that can deliver 15 watts comfortably. The resistor is selected for the highest value that will cause the tubes to strike when cold with 12-volts applied to the circuit. Use TIP31, 2N3055, almost anything. just do not forget the heatsink.

The transformer T1 is a 4cm Dia. ferrite "Pot Core" wound with 50 + 50 turns of about 22 SWG enamelled wire for the primary and 750 turns of about 38 SWG enamelled wire. Wind the secondary in two layers separated with a layer of waxed paper. If you get any arcing inside the transformer then it does not work for very long!

Finally, keep your fingers away from the tube terminals, they sting a bit! There is not enough power to do you any injury but you will certainly know about it if you touch them. Also be very carefull if you use the "black light" UV tubes sold for forged banknote detectors. I believe they work Ok ut I have not tried them in the past 10 years. UV light from "black light" tubes can be dangerous to the eyes. If you become blinded then it's your own fault, don't look at me!

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