I'm putting together a circuit board to take in 12V at less than 1/2 amp from a wall wart. The unit will have a power jack to suit.
In this design a fuse will be very large or non-resettable or expensive. I already have a diode so reverse polarity won't be a problem but what exactly are the rules regarding fuses? Do I need one?
I'm in the UK.
I
exactly
The wall wart should come CE approved/marked - which means it meets all the necessary regulations for it's intended market place. That includes safety and EMC standards. In which case the output should be intrinsically safe (eg you should be able to touch the output safely or short the 12V output to 0V without the wall wart catching fire or melting or presenting a shock hazard). So you shouldn't need a fuse to protect the wall wart.
See Internal Fuses..
However... If the wall wart can be detached from your unit then you may need to consider what happens if the user connects the wrong type of wall wart to your product. Would your product catch fire? Other issues that might be a factor....Will the wall wart have a functioning earth? (eg will the 0V output be connected to the earth pin or will the wall wart be double insulated and have no connection between 0V and earth?) If your product has any power outlets (eg 5V on a USB port or PS2 keyboard port) then it may make a difference. You might need a fuse on the input or elsewhere for some of these reasons - but I'm too rusty on the rules for all this.
In the past when I was involved with product safety certification (8 years ago now) we would hire someone at the BSI for an hour and take along a prototype along for an informal meeting. (In Hemel Hempstead I think it was). We'd take along a prototype and go over the design to see if they can spot any issues we need to address before we get too far down the design path. You might be able to get this initial meeting free if you explain you haven't done this before and want to know what services the BSI can offer.
When it comes to selecting a wall wart don't just take the manufacturers word for it when they say it's CE approved! Ask them to provide evidence of compliance (eg copies of test reports). Batches of wall warts shipped to you should also come with a certificate of conformance. Some of the relevant EN standards are mentioned on this makers site:
Colin
fuses are meant more for fire-protection than for protecting the equiment itself... if size is an issue, there are very small pico fuses avail
It is a good practice to use a fuse in any equipment. The wall wart supply most likely has an internal fuse. Putting an external one that is a bit smaller than the one in the wall wart, will save you extra inconvenience if there is a short. Having an extra fuse in line adds to the safety factor as well.
--
Greetings,
JANA ============
Iexactly
There should already be a fuse in your UK13-A plug, but I have often noticed it was sometimes omitted (naughty!) from a monolithic 13-A plug format PSU (you may have called it something else). The fuse at the *correct* rating in a plug top is there just to protect the consumer unit (fuse board) and then the *local mains supply*. When that blows due to any number of failure reasons, the resultant cut-off of maintained supply just to the appliance is pure spin-off. Many consumer units now have resettable trips, a bit in action like RCDs. Anti-Surge (T) fuses on the AC side to handle both inrush and normal current to a AC to DC PSU (allow 20% above peak inrush) and Quick-Blow (F) protection fuses (allow 10% above max traffic) after the o/p of a DC PSU are recommended.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.