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
 
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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