Resistor Value in Journeyman Exam

LarryFine

Master Electrician Electric Contractor Richmond VA
I wonder whether they intentionally manufacture resistors to such close tolerances, or they make them, measure them, and mark them according to the tolerance for which they qualify.
 

GoldDigger

Moderator
Staff member
I wonder whether they intentionally manufacture resistors to such close tolerances, or they make them, measure them, and mark them according to the tolerance for which they qualify.
1% resistors are definitely manufactured differently. Among other things a different construction can be necessary to get a stability appropriate to the tolerance.
I would not be surprised if 5% and 10% resistors came off the same production line.
20% resistors might come off a different production line.
 

PaulMmn

Senior Member
1% resistors are definitely manufactured differently. Among other things a different construction can be necessary to get a stability appropriate to the tolerance.
I would not be surprised if 5% and 10% resistors came off the same production line.
20% resistors might come off a different production line.
I suspect that if they're mixing carbon slurry for 1K resistors, and aiming for at least 20%, if they hit 1K on the nose, some of the 20% resistors are going to be a lot closer to 1K than the color bands admit to!
 

LarryFine

Master Electrician Electric Contractor Richmond VA
I suspect that if they're mixing carbon slurry for 1K resistors, and aiming for at least 20%, if they hit 1K on the nose, some of the 20% resistors are going to be a lot closer to 1K than the color bands admit to!
The important thing is that they're not outside the marked tolerance.

I was wondering if they make a batch with a target resistance, then actually measure each one, and simply mark the ones that are within 1% as 1%, the ones between 1% and 5% as 5%, those between 5% and 10% as 10%, etc.

That would virtually guarantee that, for example, a 10% resistor would never fall within 5% of the target value.
 

PaulMmn

Senior Member
The important thing is that they're not outside the marked tolerance.

I was wondering if they make a batch with a target resistance, then actually measure each one, and simply mark the ones that are within 1% as 1%, the ones between 1% and 5% as 5%, those between 5% and 10% as 10%, etc.

That would virtually guarantee that, for example, a 10% resistor would never fall within 5% of the target value.
Nowadays with automated testing, they could mark each one individually. I suspect that in days of yore they'd check a representative sample, then mark the whole batch.

I think I was looking for pairs of matched resistors for a project, and buying 1% was expensive. IIRC, I heard that if you get a strip of resistors from Parts-R-Us that all of the resistors in a row will pretty much match ohm for ohm (unless you get the place where one batch replaces another).
 

gar

Senior Member
190521-0743 EDT

I have never been involved in resistor manufacture. So in part my comments are guesses.

In the carbon composition days a resistor was a mixture of carbon and filler molded into a block.

In the 1920s this was a somewhat round bar with a wire wrapped around the bar near each end, then painted. The resistance would be whatever you got falling within some distribution curve. These would be sorted into tolerance bands. Pricing would be based on the probability of building within a band, and demand for a given tolerance. After some experience in what the distribution curve was the price would be set. Price would need to be somewhat of a constant. This means that the resulting distribution curve for 5% units is different than 10% units. Its also means that if demand for 5% units is not as high as expected that some 5% items will be marked as 10%. Probably the poorer 5% would go into the 10% mix. So distribution curves might not be very consistent.

By 1940 carbon comp resistors had the wires molded into the carbon block, and an outer coating molded around that assembly.

Wire wound resistors were available with tighter tolerances. Here number of turns could be adjusted to adjust resistance.

Then came film resistors. These are a ceramic core with a film surface of a resistive material, carbon or metal. A spiral grove is cut around the resistor to determine resistance. Resistance can be moderately accurately adjusted by the length of the spiral. This can be adjusted during manufacture, and so one is less dependent upon the variabilities of the resistive mix. Sorting still may be done. 1% metal film resistors are close to the price of 5% carbon comp.

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