To fuse equipment or not to?

F

Fred

Guest
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.
 
"Fred" <Fred@nospam.com> wrote in message
news:4242a5ae$0$8743$db0fefd9@news.zen.co.uk...
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.
Click to expand...

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

Leon
--
Leon Heller, G1HSM
http://www.geocities.com/leon_heller
 
"Leon Heller" <leon_heller@hotmail.com> wrote in
news:4242adad$0$10947$cc9e4d1f@news-text.dial.pipex.com:

"Fred" <Fred@nospam.com> wrote in message
news:4242a5ae$0$8743$db0fefd9@news.zen.co.uk...
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
Click to expand...
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 <Fred@nospam.com> wrote (in
<4242a5ae$0$8743$db0fefd9@news.zen.co.uk>) about 'To fuse equipment or
not to?', on Thu, 24 Mar 2005:
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.
Click to expand...
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.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
I read in sci.electronics.design that Ken Moffett <KMoffet@mn.rr.com>
wrote (in <Xns96234145271BAqwerty@24.94.170.87>) about 'To fuse
equipment or not to?', on Thu, 24 Mar 2005:
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.
Click to expand...
Mains transformers have the primary wound with specially-insulated wire,
which goes quietly open-circuit if overloaded.

Not even in the little SWMP types.
Click to expand...
I don't know what protection is in those, but I'm sure there is some,
however well-hidden.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
"Ken Moffett" <KMoffet@mn.rr.com> wrote in message
news:Xns96234145271BAqwerty@24.94.170.87...
"Leon Heller" <leon_heller@hotmail.com> wrote in
news:4242adad$0$10947$cc9e4d1f@news-text.dial.pipex.com:

"Fred" <Fred@nospam.com> wrote in message
news:4242a5ae$0$8743$db0fefd9@news.zen.co.uk...
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

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.
Click to expand...

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

Leon
 
On Thu, 24 Mar 2005 13:24:01 +0000, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that Fred <Fred@nospam.com> wrote (in
4242a5ae$0$8743$db0fefd9@news.zen.co.uk>) about 'To fuse equipment or
not to?', on Thu, 24 Mar 2005:
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.

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.
Click to expand...
Around here some transformers (like for warts or doorbells) are "class
2" impedance limited. If you short them, they just get sort of hot.
Regulation is ghastly, of course.

Some others have tiny fuses inside the windings, too.

I'd vote that the OP go with no fuse, if a fuse complicates things.

John
 
"Leon Heller" <leon_heller@hotmail.com> wrote in message
news:4242e4a0$0$2763$cc9e4d1f@news-text.dial.pipex.com...
"Ken Moffett" <KMoffet@mn.rr.com> wrote in message
news:Xns96234145271BAqwerty@24.94.170.87...
"Leon Heller" <leon_heller@hotmail.com> wrote in
news:4242adad$0$10947$cc9e4d1f@news-text.dial.pipex.com:

"Fred" <Fred@nospam.com> wrote in message
news:4242a5ae$0$8743$db0fefd9@news.zen.co.uk...
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

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 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!.
Click to expand...
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
<jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote (in
<0dq54198d00h6l4h1m0ogjvf6hbbpfk664@4ax.com>) about 'To fuse equipment
or not to?', on Thu, 24 Mar 2005:

Around here some transformers (like for warts or doorbells) are "class
2" impedance limited. If you short them, they just get sort of hot.
Regulation is ghastly, of course.
Click to expand...
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.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
On Thu, 24 Mar 2005 17:30:36 +0000, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that John Larkin
jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote (in
0dq54198d00h6l4h1m0ogjvf6hbbpfk664@4ax.com>) about 'To fuse equipment
or not to?', on Thu, 24 Mar 2005:

Around here some transformers (like for warts or doorbells) are "class
2" impedance limited. If you short them, they just get sort of hot.
Regulation is ghastly, of course.

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

John
 
Fred wrote:
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.
Click to expand...
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

--
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
..
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Wanted 13" LCD for Mitac 6133 Samsung HT13X13-201
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
 
Fred wrote:

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.
Click to expand...
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
 
mike wrote:

Fred wrote:
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 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.
Click to expand...
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
 
Ken Moffett wrote:

"Leon Heller" <leon_heller@hotmail.com> wrote in
news:4242adad$0$10947$cc9e4d1f@news-text.dial.pipex.com:

"Fred" <Fred@nospam.com> wrote in message
news:4242a5ae$0$8743$db0fefd9@news.zen.co.uk...
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

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.
Click to expand...
There may be a thermal fuse buried inside the windings.

Graham
 
bigcat@meeow.co.uk wrote:

Fred wrote:
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.

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
Click to expand...
4 mm !

What kind of joke is that ?

Graham
 
Pooh Bear wrote:
bigcat@meeow.co.uk wrote:


Fred wrote:

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.

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


4 mm !

What kind of joke is that ?

Graham
Click to expand...
I hear 4mm NiChrome wire works well for that.... (Not!)
 
bigcat@meeow.co.uk wrote:

Pooh Bear wrote:

4 mm !

What kind of joke is that ?

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.
Click to expand...
Ahhh - you meant the length !


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.
Click to expand...
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
 
I read in sci.electronics.design that Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote (in
<42444ECC.FBAF0ABB@hotmail.com>) about 'To fuse equipment or not to?',
on Fri, 25 Mar 2005:

I'd not advise using bare fuse wire though. It works by melting i.e.
hot ! You could have a potential fire hazard.
Click to expand...
Not enough energy involved, unless you use one of those Celluloid
printed boards. (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
On Fri, 25 Mar 2005 17:47:56 +0000, Pooh Bear wrote:
I'd not advise using bare fuse wire though. It works by melting i.e. hot !
You could have a potential fire hazard.
Click to expand...
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
 
I read in sci.electronics.design that Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote (in
<42448C19.C1FC919F@hotmail.com>) about 'To fuse equipment or not to?',
on Fri, 25 Mar 2005:

Interesting case regarding fusing. We just had an email about one of
our amplifiers that had failed, where the PSU pcb was destroyed by some
internal fault elsewhere. The track got so hot in one place that the
board was charred rotten and the track wasn't there anymore. FR4
material so no fire of course.
Click to expand...
That could indicated a failure to conform to IEC/EN 60065.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
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