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 Post subject: Question on Resistor Precision
PostPosted: Jan Mon 09, 2017 5:14 pm 
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Location: DFW Texas
If I need say a 100 ohm 1% resistor for a project and I cherry pick my stock of 20% resistors and find one that's 99.5 ohms and use that, is it any different than ordering a real 1% resistor?
Or are 1% resistors really different from 20%? Considering stock from the same manufacturer.

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 Post subject: Re: Question on Resistor Precision
PostPosted: Jan Mon 09, 2017 6:04 pm 
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Location: Santa Rosa, CA
Why do you need a 1% resistor? Your approach is sound, but what about temperature drift and aging?

Make sure what ever you pick is stable.

Rich


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 Post subject: Re: Question on Resistor Precision
PostPosted: Jan Mon 09, 2017 6:13 pm 
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Depending on the technology and age of your inventory, the cherry- picking may have happened at the factory. At least with carbon comps, the 10% parts were the ones left after picking the 5%, and the 20% were the ones left after.......you get the picture.

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 Post subject: Re: Question on Resistor Precision
PostPosted: Jan Tue 10, 2017 12:27 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
Quote:
If I need say a 100 ohm 1% resistor for a project and I cherry pick my stock of 20% resistors and find one that's 99.5 ohms and use that, is it any different than ordering a real 1% resistor?
Or are 1% resistors really different from 20%? Considering stock from the same manufacturer.


If a manufacturer tells you a resistor is 1%, that means it will stay within 1% of its marked value over some temperature range given on the data sheet. This is usually in a specification called the temperature coefficient, or tempco of the part.

A 20% resistor is only expected to stay within 20% over its temperature range. Consequently, even if its resistance measures perfectly dead-on at room temperature, there's no guarantee it will still be there when the equipment gets up to operating temperature. So no, they're not really the same thing.

Nowadays one would be hard pressed to find a newly manufactured 20% resistor available anywhere. 10% is the new 20%. If one needs a 1% resistor, they are readily available and inexpensive.

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 Post subject: Re: Question on Resistor Precision
PostPosted: Jan Tue 10, 2017 2:21 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
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Location: Peekskill, NY
This is indeed how carbon resistors were graded in the past. If you checked the one percent
resistors, the distribution cut off hard at the limits. The five percent ones, had a hole
in the distribution exactly where the one percents were selected out. And they cut off hard
at the limits.

Ten percents? Same, with a hole in the middle where the ones and fives were taken away.

Twenty percents had gaussian tails with a big hole where the rest had been removed.

Put 'em all together, and you get a full gaussian distribution which is what the manufacturing
produces. They were all made on the same line. Not sure if this is the same these days.

Metal film resistors, different story.


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 Post subject: Re: Question on Resistor Precision
PostPosted: Jan Tue 10, 2017 2:44 pm 
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Joined: Aug Mon 17, 2009 4:11 pm
Posts: 2800
Location: DFW Texas
Chris108 wrote:
Quote:


If a manufacturer tells you a resistor is 1%, that means it will stay within 1% of its marked value over some temperature range given on the data sheet. This is usually in a specification called the temperature coefficient, or tempco of the part.

A 20% resistor is only expected to stay within 20% over its temperature range. Consequently, even if its resistance measures perfectly dead-on at room temperature, there's no guarantee it will still be there when the equipment gets up to operating temperature. So no, they're not really the same thing.

Nowadays one would be hard pressed to find a newly manufactured 20% resistor available anywhere. 10% is the new 20%. If one needs a 1% resistor, they are readily available and inexpensive.


This is exactly what I wanted to know.

I don't have a project in mind, I just was looking to understand resistors more.

Thanks all!

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 Post subject: Re: Question on Resistor Precision
PostPosted: Jan Tue 10, 2017 2:45 pm 
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Joined: Aug Mon 17, 2009 4:11 pm
Posts: 2800
Location: DFW Texas
jim rozen wrote:
This is indeed how carbon resistors were graded in the past. If you checked the one percent
resistors, the distribution cut off hard at the limits. The five percent ones, had a hole
in the distribution exactly where the one percents were selected out. And they cut off hard
at the limits.

Ten percents? Same, with a hole in the middle where the ones and fives were taken away.

Twenty percents had gaussian tails with a big hole where the rest had been removed.

Put 'em all together, and you get a full gaussian distribution which is what the manufacturing
produces. They were all made on the same line. Not sure if this is the same these days.

Metal film resistors, different story.


Can you expand on how metal film resistors are different?

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 Post subject: Re: Question on Resistor Precision
PostPosted: Jan Tue 10, 2017 3:11 pm 
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Location: Annapolis, MD
My understanding is that some types of modern resistors are automatically laser-trimmed to the right value, and that no money is saved by buying loose tolerance parts.

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 Post subject: Re: Question on Resistor Precision
PostPosted: Jan Tue 10, 2017 6:05 pm 
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Location: Perrysburg, OH, U.S.A.
From the data sheet for the Ohmite Little Devil carbon composition resistors:
Quote:
Carbon composition resistors are manufactured by extruding a blend of carbon and organic binders inside a phenolic outer
body. The extrusion is cut to length, leads inserted, cured, and marked to form a finished resistor. The carbon and binder
mixture is adjusted to produce different resistance values. The resistors are sorted for 5%, 10%, and 20% tolerance values.

Carbon composition resistors are able to withstand larger short-term pulses and higher voltages than film resistors and are virtually impervious to ESD events (Electro-static discharge). Carbon composition resistors are also sensitive to moisture and, therefore, storage recommendations should be adhered to. Generally, any moisture absorbed during storage will be "baked out" during the soldering operation. If the product is stored properly the resistance shift during the soldering operation will be minimal, less than 2% or 3%.

Carbon composition resistors are highly hygroscopic and changes in resistance value can occur if too much moisture is
absorbed. For this reason, it is recommended not to use water or water-soluble solvents to clean these components. Alcohol
or hydrocarbon solvents are recommended for rinsing.


Just for grins I went to the Mouser website and did a comparison of resistor types. For 100K, 1/4W resistors, axial lead, quantity of 1, the prices are:

Ohmite Carbon Comp, 5%, P/N OD104JE $0.70
Yageo Carbon Film, 5%, P/N CFR-25JR-52-100K $0.10
Yageo Metal Film, 1%, P/N MFR-25FRF52-100K $0.10

So, you can see why most of the smaller wattage parts I order are 1% metal film.

John

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 Post subject: Re: Question on Resistor Precision
PostPosted: Jan Wed 11, 2017 3:45 am 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 385
Location: Peekskill, NY
The metal film resistors I work with are one percent. They've got three bands and a multipler,
and are I think precision manufactured so there's no selection process once manufactured.


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