Isolating main power traces on backside of the PCB

Question

I am currently making a build-in PSU for my own audio-effect-rack-thing.

The internal PSU consists of two print-transformers(THT), one for 6VAC and the second one for 9VAC. These are also pretty low power ones (max 1A on the output).

I want to put the PSU circuit on a perfboard, since this is a simple design. I am already familiar, that there needs to be a decent airgap between main-rails on the PCB. Therefore, I can remove all the unconnected dots, so there can't be a spark.

Now my concern is, when I put the psu circuit in the case, that the backside of the PCB (and therefore the mains(230V) power traces) are touching the case internally.

How can I safely prevent this from happening and is there a better way around ?

My thoughts:

• painting the backside with something
• putting a plastic sheet beneath it

Answer 1

First of all, having the PCB properly manufactured would include solder resist all over the back side. Nowadays you can get reasonably good PCBs for a few dollars, if at all possible avoid the perfoboard and use a "real" PCB. If that is not possible, painting or conformal coating is the way to go. Make sure to use proper, non conductive paint.

That said, some sort of plastic sheet is commonly used in commercial products. I am not sure of the exact material, it is quite thin, maybe one fourth of mm, clear and reasonably flexible.

It is also of prime importance to screw the PCB to the case using an appropriate number of mounting holes, and spacers. If the PCB is rigidly connected to the case you can probably skip the plastic, and even the paint, but safety is reduced of course.

For a small, homebrew project a perfo board well fixed to the case is more than enough. If you need to bring this thing around, on stages, where people can possibly touch/spill something on it, please use a real PCB, or paint it with the appropriate material.

Be safe out there! You don't get to touch the mains too many times.

Answer 2

Your enclosure is not big enough.

However, most power supplies in metal enclosure have the enclosure grounded, provide enough distance to the board, and add a insulating material sheet as additional precaution.

Beware that sharp pins or soldering residue may not cut into this sheet, care must be taken when choosing the type of sheet. Looking at strenght, insulation and temperature properties. Eg: Nomex or polyimide.