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Electronics => Projects, Designs, and Technical Stuff => Topic started by: homebrew on January 03, 2014, 08:42:23 am

Title: Mains voltage on perfboard?
Post by: homebrew on January 03, 2014, 08:42:23 am
Hi,

for a simple little fixed power supply +-15V @100mA I would like to use a small print transformer. However, being a prototype, I don't want to invest in a PCB.
Is it possible (from a safety point of view) to use a perfboard (dot PCB)?

To get the required clearance I would remove two rows of dots around each 230V trace / four rows between primary and secondary side.

Beside the mains terminal and the transformer there wouldn't be any other components on the mains side (primary fuse holder is mounted in the case).

The whole thing will be mounted inside a fully closed and mains earthed aluminium case. The secondary side would be floating and accessible to the outside.

Is that safe to construct this way?

Best,
Peter
Title: Re: Mains voltage on perfboard?
Post by: AndyC_772 on January 03, 2014, 08:56:46 am
If you're worried - and it doesn't sound like you really need to be - you could use a transformer with wires or solder tags instead of PCB pins, and keep the mains wiring off the board entirely.
Title: Re: Mains voltage on perfboard?
Post by: Lurch on January 03, 2014, 09:26:44 am
I wouldn't run mains on perfboard, as in across any of the tracks. To make it slightly less rickety you could use solid wire between the tx pins and the cables/input pins rather than running power across tracks.
Title: Re: Mains voltage on perfboard?
Post by: calexanian on January 03, 2014, 09:29:46 am
I wouldn't run mains on perfboard, as in across any of the tracks. To make it slightly less rickety you could use solid wire between the tx pins and the cables/input pins rather than running power across tracks.

Agreed. I would isolate the pads the transformer is mounted to and run wires above the board to them. If for no other reason than you will not have a long energized track on the board with no benefit if the mains is only going to the primary of the transformer.
Title: Re: Mains voltage on perfboard?
Post by: homebrew on January 03, 2014, 10:26:11 am
Thanks for your thoughts and comments!

I really meant a dotted board not a strip board. I would have made the trace out of bare copper wire on the copper-side of the board soldered down any 5 points (underneath and inaccessible even with open enclosure)

I did google that question and there is a general opinion that one just shouldn't do it. But why is that?
It is just the same FR4 as any other 'normal' PCB. There are just a bunch of extra holes in it ...

Thanks a lot,
Peter
Title: Re: Mains voltage on perfboard?
Post by: nctnico on January 03, 2014, 10:44:31 am
You'd need to remove the pads between the track carrying mains. This is easy with a soldering iron.
Title: Re: Mains voltage on perfboard?
Post by: Psi on January 03, 2014, 03:21:54 pm
You'd need to remove the pads between the track carrying mains.

+1
Title: Re: Mains voltage on perfboard?
Post by: Andreas on January 03, 2014, 04:25:57 pm

To get the required clearance I would remove two rows of dots around each 230V trace / four rows between primary and secondary side.


The requirements are a (copper free) clearance of 3.2 mm (3 mm + tolerance) betwenn L + N. And (depending on device and environment category) 5.5-8.2 mm between primary and secondary side. So with 4 rows = > 10mm you are on the safe side.

Do not forget to isolate primary from case accordingly (isolation foil in the mains area of the pcb since the standard 5mm standoffs barely give a reliable isolation) and supply additional fuses (best in AC plug) which are dimensioned to blow before the mains wiring within device gets overheated in case of a unintended short cirquit.

On the other side: it's usually cheaper to use a wall wart transformer like this:
http://www.pollin.de/shop/dt/MzU4OTQ2OTk-/Stromversorgung/Netzgeraete/Steckernetzgeraete/Steckernetzteil_FRIWO_FW6299_12_V_0_5_A.html (http://www.pollin.de/shop/dt/MzU4OTQ2OTk-/Stromversorgung/Netzgeraete/Steckernetzgeraete/Steckernetzteil_FRIWO_FW6299_12_V_0_5_A.html)

Ok in your case you would need around 15V AC to get reliable 15V DC out with standard voltage regulators.
Most wall warts are either 12V or 24V AC (christmas tree outdoor).

with best regards

Andreas