Maker Pro
Maker Pro

0.1 ohm Resistors from Scratch Again

  • Thread starter Watson A.Name - \Watt Sun, the Dark Remover\
  • Start date
S

Spehro Pefhany

Jan 1, 1970
0
I wonder if he was really warming it up, or just shunting it with
his thumb sweat?

Thanks,
Rich

He must be sweating lead if his sweat conducts like a 2.3 ohm
resistor.


Best regards,
Spehro Pefhany
 
W

Watson A.Name - \Watt Sun, the Dark Remover\

Jan 1, 1970
0
Chuck Harris said:
Ok, I'll play the straight man.

Seamonkey is the new version of the Mozilla suite. When Firefox was
released, the Mozilla Foundation, (known as MoFo) decided to dump the
much heralded Mozilla Suite (successor to Netscape), and to continue
development only on Firefox and Thunderbird. MoFo froze Mozilla Suite
at version 1.7. Well, the users of Mozilla Suite weren't pleased, and
neither were the folks who did the actual coding on the project, so
they split off from MoFo, and continued development of Mozilla Suite
as Seamonkey.

Seamonkey is vastly superior to the Firefox web browser, and the Thunderbird
mail program. Seamonkey is a web browser, mail program, IRC program,
web authoring program, and debugger all in one. All are top flight, and
all work together seamlessly.

I never call Watsun anything but Watsun.

Ya can't read. But thanx for the info anyway,
 
W

Watson A.Name - \Watt Sun, the Dark Remover\

Jan 1, 1970
0
John Woodgate said:
I read in alt.binaries.schematics.electronic that "Watson A.Name -


Why does the resistance go down when it gets hot?

Maybe I'm assuming that squeezing it with my thumb and finger is warming
it up. It could be that the current thru it is causing it to self-heat,
and when I squeeze it, it's actually cooling it down. I do know that
when I first put it on the DMM, it's lower, and then creeps up and
stabilizes at a higher value. So perhaps that's when it's self-heating.
 
C

Chuck Harris

Jan 1, 1970
0
Watson said:
Ya can't read. But thanx for the info anyway,

I beg your pardon! I most certainly can too read. And I am aware
that you spell your pseudonym Watson, I just always call you
Watsun from your "Dark Remover" theme.

-Chuck
 
W

Watson A.Name - \Watt Sun, the Dark Remover\

Jan 1, 1970
0
Chuck Harris said:
I beg your pardon! I most certainly can too read. And I am aware
that you spell your pseudonym Watson, I just always call you
Watsun from your "Dark Remover" theme.

-Chuck

Suit yourself, Chuck Hairy Ass. ;-)
 
C

Chuck Harris

Jan 1, 1970
0
Watson said:
Suit yourself, Chuck Hairy Ass. ;-)

That takes me right back to Junior high school.

I suppose I could retaliate by spreading your given
name across the newsgroups.

-Chuck
 
D

Don Stauffer

Jan 1, 1970
0
John said:
I read in alt.binaries.schematics.electronic that "Watson A.Name -



Why does the resistance go down when it gets hot?

Virtually all materials have a resistivity which is temperature
dependent. Most are positive relationship (resistance goes up with
temp) but a very few are negative.
 
N

NunYa Bidness

Jan 1, 1970
0
That takes me right back to Junior high school.

I suppose I could retaliate by spreading your given
name across the newsgroups.

Nope. Real e-mail addresses are far more annoying.
 
R

redbelly

Jan 1, 1970
0
Don said:
Virtually all materials have a resistivity which is temperature
dependent. Most are positive relationship (resistance goes up with
temp) but a very few are negative.

Okay ... but we are talking about copper here. What other materials do
isn't relavent.

Mark
 
D

Don Stauffer

Jan 1, 1970
0
redbelly said:
Don Stauffer wrote:




Okay ... but we are talking about copper here. What other materials do
isn't relavent.

Mark
Maybe I should have said, copper, like MOST materials, has a resistivity
which is temperature dependent. Better?
 
R

redbelly

Jan 1, 1970
0
Don said:
Maybe I should have said, copper, like MOST materials, has a resistivity
which is temperature dependent. Better?

Well, it appeared that you were specifically addressing this question:
"Why does the resistance [of the op's COPPER wire] go down when it gets hot?"

Saying that copper's resistance is temperature dependent still does not
answer the question, since the known temperature dependence (for
copper) is in the opposite direction to what the op measured. That is
what the question is all about: why is the measurement contrary to what
is known about copper?

But perhaps I'm mistaken and you weren't trying to address the above
question at all, just interjecting some general knowledge for our
benefit. :)

Mark
 
D

Don Stauffer

Jan 1, 1970
0
redbelly said:
Don Stauffer wrote:

Maybe I should have said, copper, like MOST materials, has a resistivity
which is temperature dependent. Better?


Well, it appeared that you were specifically addressing this question:

"Why does the resistance [of the op's COPPER wire] go down when it gets hot?"


Saying that copper's resistance is temperature dependent still does not
answer the question, since the known temperature dependence (for
copper) is in the opposite direction to what the op measured. That is
what the question is all about: why is the measurement contrary to what
is known about copper?

But perhaps I'm mistaken and you weren't trying to address the above
question at all, just interjecting some general knowledge for our
benefit. :)

Mark
Ah, I had indeed missed that. I didn't pay attention to the sense of
the change. Interesting!
 
D

Don Klipstein

Jan 1, 1970
0
Watson A.Name - \"Watt said:
A tenth of an amp across a tenth of an ohm is a hundredth of a watt.
Even at 150 or 200 mA it shouldn't make much heat. The current limited
PS should be set for well under 1/4 amp (it's good for only 1/2A max).
After reading some of the followups, I think I should be more concerned
about heating from the current changing the resistance. A dozen mW
around a 1/4W resistor might change it a few degrees. BTW I'd rather
let the smoke out of the resistor than a transistor. I'll have to put a
few inches of the wire across a PS and crank the current up and see when
it starts to smoke and glow red. ;-)

A coil on a resistor body will heat up more than a few inches of wire in
open air.
Air cooling is less when the wire is on a surface. In addition,
probably more significant, is wire turns on a resistor body being heated
by heat from adjacent turns of the coiled winding.

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
The real trick is going to be to make a connection to a 3/4 inch long
piece of wire, using solder, and have it come out to within +/- 5% in
resistance value.

That's a more difficult part, given temperature coefficient of
resistance of copper! How to measure cold and when in use? Maybe
make a resistor measuring only a little low ehen cold, and see what it
does when in use, and improve from that!

- Don ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
Watson A.Name - \"Watt said:
IN PART ...

I did, and it's a current sense resistor for use in any circuit, the PS
or battery lead, or output current, etc. I'm becoming intolerant of the
relatively large V drop across the mA ranges of the DMM, usually 0.2VDC
full scale. That's more than 20 percent of the battery voltage of a
single cell when it's getting run down. At 200mA, the tenth ohm has
only 20mV drop across it, which is a lot more tolerable. The current
measurement doesn't have to be all that accurate, just reasonable.


Nope. I think that should be a bit more professional, and should handle
a lot more power. Say, a wire wound resistor of a couple watts.

One thing to keep in mind:

Wirewound power resistors are typically made with nichrome wire, which
has an unusually low temperature coefficient of resistance about half that
of copper and most other metals.

- Don ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
Watson A.Name - \"Watt said:
Well, I decided to take on the challenge. I warmed up the ol' soldering
iron, and grabbed a 22k 1/4W resistor.and cut off maybe 1.3" or 30mm of
the fine wire. I wrapped a few turns around one lead with the
needlenose, and soldered it. I measured off .79" or 20mm, and marked
that point with the felt tip. Then I wrapped a bit more than 2 turns
around the resistor, until the mark was at the other lead. Again I
wrapped the leftover wire around the other lead, and soldered it.

I measured the result with the HP 3478, and I got 0.1013 at first, but
as it cooled off a bit, it went up to 0.106 ohms or thereabouts. When I
warm it up with my thumb and finger, it goes back down to around the
0.101 point. Looks like that's about +6 percent ; not too bad for the
first try. But with the current warming it up a bit, it might be right
on or a little under.

This is opposite of what should be the truth. Copper wire has
resistance increasing with temperature, roughly proportionately with
degrees K.

Looks like you have some measurement error.

- Don ([email protected])
 
J

John Woodgate

Jan 1, 1970
0
I read in alt.binaries.schematics.electronic that Don Klipstein
Wirewound power resistors are typically made with nichrome wire,
which has an unusually low temperature coefficient of resistance about
half that of copper and most other metals.

Well, they MAY be, but there are many alloys with much lower TCRs.
Constantan is one, and Eureka is similar. Manganin has very nearly zero
TCR and is used for high-precision resistors.
 
R

redbelly

Jan 1, 1970
0
Don said:
This is opposite of what should be the truth. Copper wire has
resistance increasing with temperature, roughly proportionately with
degrees K.

Looks like you have some measurement error.

- Don ([email protected])

Perhaps the heating/cooling changes the stress on a contact somewhere
(from thermal expansion), and that might be what is causing the
resistance change. The differences involved, 0.005 ohms, is not very
much.

Mark
 
B

Ban

Jan 1, 1970
0
Don said:
This is opposite of what should be the truth. Copper wire has
resistance increasing with temperature, roughly proportionately with
degrees K.

Looks like you have some measurement error.

- Don ([email protected])

Really, I don't know what this is all about, every bit of the "experiment"
can be predicted by anybody, so WTF is the OP doing?
He should just take some Konstantan wire, I have some at hand. It says 2.5
Ohm/m, has 0.5mm dia. and looks yellowish/greenish. It is solderable, but
should be welded to be useable at higher temperatures, otherwise the total
dissipation would be the same as the resistor.
My 0.1R resistors have to have much more dissipation capability than 1/4W to
be useable.
 
Top