DIY Pico fuses

N

Norm Dresner

Guest
Yeah, I forgot to order them. And now I need one. Actually, I needed one
this morning when I plugged the metal DB37 connector into the DCC5-P
Counter/Timer board and a slight misalignment shorted the PC's +5V to the
surrounding shell to ground. It actually vaporized about .035" of a trace
that's about .050" wide in what looks like 1 oz circuitry. Rather than just
patch it, I wanted to solder in a picofuse to prevent randon parts of the
circuit from vaporizing the next time I'm too lazy to walk around the other
side of the rack and turn the computer off. [1] But, as I said above, I
completely forgot to order them SO ... I'm going to do the next best thing
and bridge the gap with a single strand of very thin wire. Now this time
I'm going to do it by the seat of my pants but I know that in ages past I've
seen charts for the current carrying capacity of single strands of wire but
nothing I've found via Google tells me at what current a solid conductor of
a particular size of wire will vaporize, but that's exactly how fast-blow
fuses are made so someone's got to have the information somewhere. Links
and references welcome.

NOTE 1: Putting the +5v output of the corner pin of a male connector isn't
the greatest feat of engineering. Putting it in the middle of the row is a
lot safer. And, for the user, using plastic shell DB-connectors instead of
metal shell ones would be smarter too.

Norm
 
On Tue, 24 Aug 2004 18:34:56 GMT, the renowned "Norm Dresner"
<ndrez@att.net> wrote:

Yeah, I forgot to order them. And now I need one. Actually, I needed one
this morning when I plugged the metal DB37 connector into the DCC5-P
Counter/Timer board and a slight misalignment shorted the PC's +5V to the
surrounding shell to ground. It actually vaporized about .035" of a trace
that's about .050" wide in what looks like 1 oz circuitry. Rather than just
patch it, I wanted to solder in a picofuse to prevent randon parts of the
circuit from vaporizing the next time I'm too lazy to walk around the other
side of the rack and turn the computer off. [1] But, as I said above, I
completely forgot to order them SO ... I'm going to do the next best thing
and bridge the gap with a single strand of very thin wire. Now this time
I'm going to do it by the seat of my pants but I know that in ages past I've
seen charts for the current carrying capacity of single strands of wire but
nothing I've found via Google tells me at what current a solid conductor of
a particular size of wire will vaporize, but that's exactly how fast-blow
fuses are made so someone's got to have the information somewhere. Links
and references welcome.
Click to expand...
http://www.speff.com and click on "Fusing current for various wires"

You can probably scavenge a proper one off an old PC motherboard.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Hi Norm,

Just a hint: Don't know the amps you want but you can locally buy these
tiny fuses of 1/4" length or so that are meant to go into small power
plugs. They are for Christmas lights, small lamps where the cord isn't
rated as high as a typical mains circuits etc. I believe they start at
2A and I have seen them at a hardware store.

Regards, Joerg

http://www.analogconsultants.com
 
Thanks to Spehro Pefhany for the link to the fusing current chart.

The situation is that the PC board I was using has a pin on the DB-37
connected to the PC's +5v directly. It also happens to be the corner pin on
a male connector surrounded by a metal shell. When I inserted the metal
shelled female connector slightly off center it created a short between pin
20 and the shell which is connected to ground.
The circuit I power from that pin draws about, maybe, 250 ma peak but
more like 75-125ma normally.
The short I caused vaporized a short section of pc board trace; I'm
guessing it was a few amps.
What I wanted was to jump the gap with something that would provide
current limiting and a known place for a failure instead of possibly
vaporizing something inside of a mutlilayered board where it would be
impossible to find. For this application, something around 1/2 A would have
probably been ideal.
It just occurred to me that I have some fast blow 3AG fuses around that
rating that I could have cracked open. Doh!

But I *will* order some pico fuses ASAP.

Norm


"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:ycNWc.11165$3z7.2515@newssvr27.news.prodigy.com...
Hi Norm,

Just a hint: Don't know the amps you want but you can locally buy these
tiny fuses of 1/4" length or so that are meant to go into small power
plugs. They are for Christmas lights, small lamps where the cord isn't
rated as high as a typical mains circuits etc. I believe they start at
2A and I have seen them at a hardware store.

Regards, Joerg

http://www.analogconsultants.com
Click to expand...
 
On Tue, 24 Aug 2004 18:34:56 GMT, Norm Dresner wrote:

I googled that stuff some time ago and got a table of ampacities
with fusing currents.

--
Best Regards,
Mike
 
Hi Norm,

BTW, tiny light bulbs, such as those scavenged from a dead watch or a
Christmas light chain can also be used as fuses. But they won't stand
1/2 amp unless you parallel some. These bulbs often have leads.

Regards, Joerg

http://www.analogconsultants.com
 
On Tue, 24 Aug 2004 18:34:56 GMT, "Norm Dresner" <ndrez@att.net>
wrote:

Yeah, I forgot to order them. And now I need one. Actually, I needed one
this morning when I plugged the metal DB37 connector into the DCC5-P
Counter/Timer board and a slight misalignment shorted the PC's +5V to the
surrounding shell to ground. It actually vaporized about .035" of a trace
that's about .050" wide in what looks like 1 oz circuitry. Rather than just
patch it, I wanted to solder in a picofuse to prevent randon parts of the
circuit from vaporizing the next time I'm too lazy to walk around the other
side of the rack and turn the computer off. [1] But, as I said above, I
completely forgot to order them SO ... I'm going to do the next best thing
and bridge the gap with a single strand of very thin wire. Now this time
I'm going to do it by the seat of my pants but I know that in ages past I've
seen charts for the current carrying capacity of single strands of wire but
nothing I've found via Google tells me at what current a solid conductor of
a particular size of wire will vaporize, but that's exactly how fast-blow
fuses are made so someone's got to have the information somewhere. Links
and references welcome.
Click to expand...
---
http://www.ultracad.com/fusing.pdf

http://www-d0.fnal.gov/hardware/cal/lvps_info/engineering/wirefusing.pdf

--
John Fields
 
Norm Dresner wrote:

Thanks to Spehro Pefhany for the link to the fusing current chart.

The situation is that the PC board I was using has a pin on the DB-37
connected to the PC's +5v directly. It also happens to be the corner pin
on
a male connector surrounded by a metal shell. When I inserted the metal
shelled female connector slightly off center it created a short between
pin 20 and the shell which is connected to ground.
Click to expand...
What you really need to do is redesign the unit such that you don't have
a power supply supplying a naked male pin. Not only is that bad practice,
but band-aiding a fuse on a board that's already damaged by the poor design
is like bad squared.

Have Fun!
Rich
 
Hi Rich,

What you really need to do is redesign the unit such that you don't have
a power supply supplying a naked male pin. Not only is that bad practice,
but band-aiding a fuse on a board that's already damaged by the poor design
is like bad squared.


Would it be ok if duct tape is wrapped around it afterwards? Just
Click to expand...
kidding, but this one is not a joke: Once I came across a strip chart
recorder which could be reset if it hung. A it hung a lot. The reset
consisted of a large push button that shorted VCC to ground. Five amps
or so, bzzzt.

Regards, Joerg

http://www.analogconsultants.com
 
"Rich Grise" <null@example.net> wrote in message
news:taPWc.190$Cc.185@trnddc07...
Norm Dresner wrote:

Thanks to Spehro Pefhany for the link to the fusing current chart.

The situation is that the PC board I was using has a pin on the
DB-37
connected to the PC's +5v directly. It also happens to be the corner
pin
on
a male connector surrounded by a metal shell. When I inserted the metal
shelled female connector slightly off center it created a short between
pin 20 and the shell which is connected to ground.

What you really need to do is redesign the unit such that you don't have
a power supply supplying a naked male pin. Not only is that bad practice,
but band-aiding a fuse on a board that's already damaged by the poor
design
is like bad squared.
Click to expand...
Alas, the board isn't my design. It's a DCCx-P [x=5,10,15]counter-timer
from from ICS/Advent that I've used in several different embedded systems
over the last decade or so. But it *is* a bad design. All I want to do is
protect the single board that I have in my rackmount MS-DOS computer from
subsequent damage if I can.

Norm
 
John Fields wrote:
On Tue, 24 Aug 2004 18:34:56 GMT, "Norm Dresner" <ndrez@att.net
wrote:


Yeah, I forgot to order them. And now I need one. Actually, I needed one
this morning when I plugged the metal DB37 connector into the DCC5-P
Counter/Timer board and a slight misalignment shorted the PC's +5V to the
surrounding shell to ground. It actually vaporized about .035" of a trace
that's about .050" wide in what looks like 1 oz circuitry. Rather than just
patch it, I wanted to solder in a picofuse to prevent randon parts of the
circuit from vaporizing the next time I'm too lazy to walk around the other
side of the rack and turn the computer off. [1] But, as I said above, I
completely forgot to order them SO ... I'm going to do the next best thing
and bridge the gap with a single strand of very thin wire. Now this time
I'm going to do it by the seat of my pants but I know that in ages past I've
seen charts for the current carrying capacity of single strands of wire but
nothing I've found via Google tells me at what current a solid conductor of
a particular size of wire will vaporize, but that's exactly how fast-blow
fuses are made so someone's got to have the information somewhere. Links
and references welcome.


---
http://www.ultracad.com/fusing.pdf

http://www-d0.fnal.gov/hardware/cal/lvps_info/engineering/wirefusing.pdf
Click to expand...
Copper is the WRONG stuff to be making fuses. You want something that
melts at a relatively low temperature without a large region where it
just softens or glows red. Last thing you want is a piece of red-hot
wire flinging itself off the board and landing who-knows-where.
Remember, Murphy has a summer home in your system.
Patch the trace for now and fix it when you can get a proper fuse.
mike

--
Return address is VALID.
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
Compaq Aero floppy,ram,battery.
MINT HP-41CV, 2-METER AMPS, 200CH SCANNER
http://www.geocities.com/SiliconValley/Monitor/4710/
 
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