Mains voltage on perfboard? - Page 1
Forum section: electronics / projects

author: homebrew
author_rank: Frequent Contributor
timestamp: January 02, 2014, 09:42:23 pm
content: 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

author: AndyC_772
author_rank: Super Contributor
timestamp: January 02, 2014, 09:56:46 pm
content: 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.

author: Unknown
author_rank: Unknown
timestamp: January 02, 2014, 10:26:44 pm
content: 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.

author: calexanian
author_rank: Super Contributor
timestamp: January 02, 2014, 10:29:46 pm
content: Quote from: Lurch on January 02, 2014, 10:26:44 pmAgreed. 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.

author: homebrew
author_rank: Frequent Contributor
timestamp: January 02, 2014, 11:26:11 pm
content: 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

author: nctnico
author_rank: Super Contributor
timestamp: January 02, 2014, 11:44:31 pm
content: You'd need to remove the pads between the track carrying mains. This is easy with a soldering iron.

author: Psi
author_rank: Super Contributor
timestamp: January 03, 2014, 04:21:54 am
content: Quote from: nctnico on January 02, 2014, 11:44:31 pm+1

author: Andreas
author_rank: Super Contributor
timestamp: January 03, 2014, 05:25:57 am
content: Quote from: homebrew on January 02, 2014, 09:42:23 pmThe 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.htmlOk 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 regardsAndreas