Isolation distance for Mains PCB?

[Llamarama]

Hello everyone, apologies if this is a restricted topic but i genuinely can't find anything about it in any of my books.

I'm making a switch mode power supply that will be run from the mains, input 240V AC output adjustable from about 1V to 24V. This will all be persormed on a single sided fibreglass PCB.

Naturally safety is the main concern, I have read a few pages of people oohing and ahhing over anything from 5mm to 8mm. I have decided to play it safe and try for the largest I can get away with, which in my case is some 15-20mm. The only point of concern is the optocoupler for feedback. I can only find ones in the narrow dip style packaging. Is there any way the distance between HV and LV sides can be increaded?

Sorry for seeming a little apprehensive, this is my first mains project that involvs a PCB, I have worked with mains voltages and higher many times, just never with a PCB.

 

[canadaelk]

Application note AN978 by Motorola (now ON Semi) is a good start. E

 

[dr pepper]

There are guidelins, but it depends on what part of the world you are in, the smallest I've seen on existing gear is about 4mm, 15mm sounds plenty, unless your running it in a high humidity environment.

 

[tunedwolf]

You will also find in common practise, that the PCB is slotted underneath the opto coupler to increase the isolation barrier

 

[House0Fwax]

You could always make your own opto coupler, that way you choose the 'gap'.

 

[Llamarama]

Hello everone, problem solved in an odd way. I did end up using a standard opto-isolator, however I cut the PCB to leave a nice 12mm isolation gap, coubled with some well isolated traces. I also added plenty of quick blow fuses and an earth leakage circuit breaker. I don't mind being slightly paranoid with the mains

 

[Misterbenn]

Putting a slot in the PCB is a very common practice for HV isolation.