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Exposing PC Boards In A Modified Microwave Oven

I decided to build the UV exposure box for photo-sensitive PC boards, as published in the November 2007 SILICON CHIP. The first thought I had was to use the timer module in an old microwave oven, instead of building the timer design used in the project. Then I thought: "Why not use the whole oven?"

By Graeme Rixon

WARNING: MICROWAVE OVENS CAN BE LETHAL!

A microwave oven is a very dangerous device. Even service personnel who are very familiar with them will tell you they are not their favourite equipment to work on. A microwave oven operates at a potential of several thousand volts and contact with any part of the high voltage circuitry can be (and indeed has been) fatal. Even a microwave oven which has been turned off for some time may have enough energy stored to give a potentially lethal shock. NEVER operate a microwave oven without the cover on and secured and without the door interlocks operating.
This article is intended for those who are experienced with mains device wiring in general and microwave ovens in particular. Follow exactly the steps outlined in this article regarding making the microwave oven safe before, and as you modify it.

Why not, indeed? I wouldn’t have the lid of the original design to clamp the transparency down onto the PC board but this could be overcome by making a platter and using a sheet of glass to apply enough weight to ensure intimate contact.

When activated, the platter-motor of the oven, slowly rotates the PC board pattern within the UV light, providing a very even exposure of your pattern. I can obtain very even and consistent results with transparency film and about 30-40 seconds exposure time (although I tend to pre-heat the tubes for five minutes first). There is a significant difference in both light output and colour temperature between a just-turned-on tube and one which has been on for a few minutes.

One big advantage of using an old microwave oven: just as when the oven was an oven, when the time is up, the oven beeps loudly at you, to tell you that your board has finished cooking exposing!

So I bought a secondhand microwave oven for $5 from my local refuse recycle centre, in a "working but no glass turntable" state. If your council has kerbside cleanups, you may well find one that’s about $5 cheaper.

By far the majority of microwave ovens are discarded because a 30c high-voltage diode has blown. But we don’t even need that because we aren’t interested in the cooking side of things.

The only things you need to check, when looking for an oven to convert, is that (a) the timer and its display works and (b) that the platter motor turns. It does not matter if the oven has a glass platter or not – I made my own from a sheet of MDF and an old speaker.

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