0.1 ohm Resistors from Scratch Again

RST Engineering (jw) wrote: > The OP stated that the wire length would be about three quarters of an inch. > It would be a real trick to get a solenoid out of that size wire. With #42 > wire (the best guess so far) the #22 wire lead of a quarter watt resistor is > near enough an infinite heat sink to make the approximation reasonably > correct. >

I agree, but I thought the limits of the ratings should be mentioned.

One of my early consulting jobs was to try and figure out what a large Swedish company did wrong when it converted a 240V, 50Hz instrument to

120V, 60Hz. What they did wrong was think that they could use the original wiring, and connectors, unchanged at the lower voltage, and higher current because the wire table said the wire was okay with the higher current, and connector table said the connectors were okay at the higher current, but... the tables in both cases expected the wire to be infinitely long, and all by itself in 25C still air. Sadly, the wire was only 2 feet long, and was in a bundle, and as a result, both the wire, and the connector burned up!

And some young engineer in Sweden got his bottom paddled when his boss read my report.

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.

-Chuck

I don't recall if the OP posted the ultimate use of the device, but I rather suspect it is an emitter resistor in a suck and blow (totem pole) power output stage. Sometimes called "ballast resistors", in which case +/- 20% should be adequate.

Jim

Wats> I'm up to my tricks of winding 0.1 ohm resistors from some fine copper

< snip >

One question to ask is, just how accurate is the .0025" reading? If you're off by just .0001" or .0002", that could account for the resistance descrepancy.

More questions to ask yourself:

Did you check that the calipers read .0000" when fully closed? Did you press the calipers against the wire with enough force to crimp the wire down to less than it's original diameter?

Mark

They needed a consultant to tell them, that -- despite what the young engineer had perhaps "read into" the data sheets for the cable and connectors -- if the things were burning up, you just go and get a bigger cable and connectors and call it good?

Just curious :-)

The employer probably regretted it when they had to pay him for 2 days while he stood in line for his state-provided health care to fix his butt again...

---Joel KOlstad

A heady bunch, Swedish engineers. They were insisting that there was something wrong with the US power. The president of the US division of the company couldn't get the Swedish engineers to fix the problem, so he brought in an independent engineer (me) to evaluate the system. He dropped a brand new unit into my lap, and asked me to find any problems that might be there. I went over the entire design with a fine tooth comb, and found a handful of "rookie" mistakes.

It was a job that never should have had to be hired out, but because of some european chauvanism, it was necessary. The president of the US division got the redesign he wanted, and the Swedish company learned that not all US engineers are stupid.

-Chuck

an inch.

With #42

resistor is

reasonably

original

current

and

but...

all by

was in

piece

resistance

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.

I put it across the PS after cranking the current control down to min. I then cranked up the current, watching the mA meter. I got to 200mA, and felt nothing. I then cranked it up to 300 ma and felt nothing. So I decided to go for it, and cranked it up to 500 mA, and I may be just imagining it, but I think it's getting perceptably warm, but it's only

50mW. Maybe later, just for the halibut, I'll put it on the HP 66312 PS and crank it up to 2A, and see if it gets hot or burns out.

Maybe a couple coats of paint over the wire, or heat shrink, to keep it from being damaged. So far, seems I'm not doing too bad. Who started this ugly sans serif font? Now everything I type is also that font. Sucks! I can't find any way to change the font back to Courier. Grrrrr!

rather

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.

power

20%

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.

BYW, I've used the wire table, calculator and a couple pins to do some current sensing. Say 24 AWG is 25.7 milliohms per foot. I measure off a bit less than 4 ft., say 3 ft. 11 in., and poke the pins thru the insulation and measure the V drop across the pins. Just remember to multiply the result by ten to get the actual current.

long

resistance

Watson A.Name - "Watt Sun, the Dark Remover" wrote: > Well, I decided to take on the challenge. I warmed up the ol' soldering

a proper little Cu thermister!

I am undoubtedly the one that set the thread to Unicode 8. Your browser is the one that decided to show Unicode, fixed space, as whatever you are seeing. I am seeing a nice little fixed space Courier typewriter font.

You can probably change your settings to make things appear any way you want them to appear. (but since you are using Outhouse Express, I don't know that for a fact)

(OBTW, you do know that a "serif" is the little horizontal lines that make a capital 'I" look like a piece of I-beam, right? In a sans-serif font, the "I" looks like "|".)

I am trying out a new version of Mozilla, Seamonkey and it has changed a few of my settings. I have set it back to not change the font of the replied to postings. We'll see if it does.

-Chuck

This is passing strange. In your first post in this thread you said: "I'm up to my tricks of winding 0.1 ohm resistors from some fine copper wire and a resistor to wind it on. But I'm having trouble with the wire, in more ways than one."

Last time I checked, copper had a positive temperature coefficient, but you are measuring a decrease in resistance with an increase in temperature. Did you switch to a different wire alloy?

I read in alt.binaries.schematics.electronic that "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote (in ) about '0.1 ohm Resistors from Scratch Again', on Thu, 22 Sep 2005:

Why does the resistance go down when it gets hot?

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in alt.binaries.schematics.electronic that Chuck Harris wrote (in ) about '0.1 ohm Resistors from Scratch Again', on Thu, 22 Sep 2005:

What's a Seamon key? Or is that what you are calling Watson?

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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.

-Chuck

You'll have to set your pink tutu aside for that role.

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

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

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

Scratch

Thunderbird

and

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

but

When

the

the

right

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.

selection.

Try blowing some hot air from a hair dryer or hot air gun on it and see what happens.

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

program,

flight,

Suit yourself, Chuck Hairy Ass. ;-)