PCB Design for beginners. (safety first!)

[czdt8m]

PCB safety, especially when HV is involved;
Looking at the teardowns by Dave (and others), once problems are pointed out (creepage distances, opto-coupling) I can understand why they are a problem. Designing something from a blank page as a beginner is differrent.

This is why;

As my first project in electronics in many years, I decided to go for Yet Another Nixie Clock. (YANC), in this case, GPS controlled.
Mainly because it has a good mix of a microcontroller (Parallax Propeller in this case), power supplies (5V, 3.3V, +140V), shift registers with I2C, GPS signal (RS232), ...
In time, once working, I would like to include network capability and NTS. (being a software engineer during the day job)

So the ambition is there, the EE knowledge, not so much.

At this moment, I am at the stage that I have a breadboard with the uC and GPS working.
Power supply and high voltage stuff is next.

At some point I would like to design a single PCB for the whole thing. Maybe even create a kit for some of my friends who have shown interest.

So I am looking for do's and dont's, best practises, anything that can help is welcome. And Google did not help much.

Might even be worth an EEvBlog video. 

[poorchava]

First thing: make sure that anything which can be connected to live or neutral is separated from anything anyone can touch. Separated means pcb clearance, board cutouts, inductive and capacitive components with proper volage rating, optocoupling. The case should preferably be grounded it it's conductive. I tend to just reserve some space on the pcb for mains stuff, that will be isolated from everything else in every possible way. I avoid using wallwarts, as they are often a total crap. Not only are they often dangerous but also introduce shitload of interfereneces into your circuit. For mains-related stuff use only parts quality of which you are sure. This is no place for cheap chinese crap.

Make sure you are careful with nixie HV generator. It's not as dangerous as mains, but can give you a really nasty zap. you also need to make sure that there is no way that the 140V rail can make it to the other parts of the circuitry (eg. during power-up phase). 140V is absolutely lethal to any CMOS circuitry.

Since you are not a spoiled American citizen with access to Digikey, Mouser and Allied with $1 shipping, you need to first check what you can actually buy from Farnell, RS, TME, Elfa and other places which won't charge more for shipping that the whole order is worth.

That's for starters...

[czdt8m]

Thanks!

I am still not sure if I want to power the clock from a 12V wall wart, or include a linear power supply on the Pcb.
Since it is a learning project, might as well go the whole way.

Even more important the need for a proper Pcb design.

[Ton]

Hi

Here is a link to a application note discussing safety in power supply design according to EN60950,
This is the base standard for most of the electrical safety standards.

www.ti.com/lit/ml/slup227/slup227.pdf

Key words for you would be creepage, pollution degree 2, equipment class l, ll, or lll, reinforced insulation.
If you get your head around those therms and the associated distances, your PCB layout will most probably be a decent safe one .

Br ton

[GreyWoolfe]

Spoiled American???!!!  Harumph, I resemble that remark! 

Tom, NW0LF