To fuse equipment or not to?

I don't understand that. Seems like it violates conservation of energy or something important like that.

John

I called the psychic hotline to fill in the gaps. They said that the actual safety of the device is independent of location. The ONLY reason it might matter that you're in the UK is that you need third party agency approvals for the device. The cost of a fuse is inconsequential unless you're in high volume commercial production of a very low priced item.

While it's fun to let people vote on the internet, the only vote that counts is that of the safety test engineer that will approve your product. Read the regulations and/or call the testing agency. You'll save yourself a LOT of grief. And if you expect to sell one outside the UK, you might wanna be reading many other safety standards. You might find that the presence or absence of the fuse is the least of your problems. Back in the day, standards for different countries required mutually exclusive wire colors. This is probably fixed by now.

Wall Warts dramatically reduce, but do not eliminate your need to understand the safety rules.

Don't get me started on product liability laws. Say you specify a wart and it all passes the test. Next year, some bean counter substitutes a different wart. Same specs and certifications on the wart, but it's 8 cents cheaper. Nobody else even knows it happened, much less tested it. Creates new fire hazard...somebody's gonna get sued... Or maybe someone substitutes a car battery for the wart. If the cord ain't permanently attached, you're at risk.

So, even if your safety test engineer says you don't need a fuse, your attorney might disagree.

Are we having fun yet? mike

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.

The wall wart will have one, presumably. That's all you need.

Leon

"Leon Heller" wrote in news:4242adad$0$10947$ snipped-for-privacy@news-text.dial.pipex.com:

I've torn apart many,many wall warts to build into various projects. There are no fuses. Generally there is a transformer, a capacitor and 2 or 4 diodes. Not even in the little SWMP types. In the bigger linear and SWMP types, yes, but they are no longer on the wall.

I read in sci.electronics.design that Fred wrote (in ) about 'To fuse equipment or not to?', on Thu, 24 Mar 2005:

If you short-circuit the wall-wart output, the transformer primary will go open-circuit and the wall-wart will be useless except as a short-range ballistic missile.

To prevent this, which generally creates negative credit with the affected user, include in your product a non-resettable fuse. It doesn't matter that it's non-resettable, because whatever caused it to operate has to be fixed by a technician anyway.

I read in sci.electronics.design that Ken Moffett wrote (in ) about 'To fuse equipment or not to?', on Thu, 24 Mar 2005:

Mains transformers have the primary wound with specially-insulated wire, which goes quietly open-circuit if overloaded.

I don't know what protection is in those, but I'm sure there is some, however well-hidden.

I meant the wall wart plug. That should always have a fuse.

Leon

A diode will make a faster fuse, than any but the most expensive units!. The 'wall wart', is meant to meet the safety requirements, and many units use no other fuse/protection. Generally it should be fine to go without an extra fuse.

Best Wishes

I read in sci.electronics.design that John Larkin wrote (in ) about 'To fuse equipment or not to?', on Thu, 24 Mar 2005:

It doesn't have to be. In a normal transformer, with just two windings, you allocate half the winding area to the primary and half to the secondary, so that the resistive loss is equal (well, nearly) in both.

For an impedance-limited design, you have to allocate much less area to the primary so that its resistance is high enough to limit the short-circuit current to a safe value. You then let the secondary take up all the remaining winding area, so that its resistive loss is much smaller than in a normal transformer.

You get approximately the same regulation as for a normal transformer, but of course if it's a very small transformer, using laminations with conventional geometry, it won't be all that good anyway. These small transformers really need lams with a BIG window in proportion to their size, because the insulation on the wire, and the bobbin if there is one, take up more winding area in proportion, at the expense of the copper, than they do in a larger transformer.

amp

expensive. I

exactly

Fuse wire is incredibly cheap. Get some, use it, ass covered. You have to realise a user could plug it into another wart by mistake, maybe one the wrong voltage. I cant think of any reason a wire fuse link need be large or expensive. 4mm is probably enough to fuse 12v non-inductive when youre using thin .5 - 1A wire.

The one gotcha is that such low rated fuse wire doesnt last forever, but it should have no problem satisfying guarantees, assuming its not being exposed to corrosives.

NT

The rules say it mustn't overheat in any fault condition. If you supply the PSU ( and specify that it must be used with the supplied PSU ) , it's simplest to deal with the issue there. If you don't supply the PSU - it's best to fuse it.

Graham

Nothing special about the UK. The UK is part of the European Union. All electronic products sold in the EU have to comply with the 'low voltage directive'. This basically means compliance with EN60065 or EN60950 typically. These are derived from the equivalent IEC standards which are applied widely worldwide e.g. in Australia, China, Taiwan, Korea etc etc...

Since these standards only apply to 'mains operated' equipment effectively, anything with 12V DC input isn't covered by them. The power supply is though !

You might care to consider the EMC issues too !

Graham

There may be a thermal fuse buried inside the windings.

Graham

What kind of joke is that ?

Graham

have

one

be

Not a joke at all. I've fused many a transformer by soldering fuse wire from one TF tag to the next, often the length of unsoldered wire is only 4 or 5mm. Its fine for low voltage non inductive uses.

The OP needs to realise though that fuse wire rating depends on length as well as cross section area, when you use very short fuses you need thinner wire to achieve the same current rating.

NT

I hear 4mm NiChrome wire works well for that.... (Not!)

Real fuse wire melts way before it gets hot enough to burn anything. It melts even cooler than solder. I found this out the hard way - I lost a lug in a solder pot. (think Wood's metal spoon) :-)

Cheers! Rich

Ahhh - you meant the length !

True enough.

I'd not advise using bare fuse wire though. It works by melting i.e. hot ! You could have a potential fire hazard.

Graham