Electronics > Metrology

Teardown: Standard Resistors

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zlymex:
I've been fascinated by standard resistors of their performance, cannot stop thinking of what's on earth inside.

Before teardown, let's look at some of the important factors of a standard resistor, such as specified by esi SR104:


 - Stability
Specified as ppm/year, It is a long time factor that most important for a standard resistor.
Every Chinese and Japanese resistor has a CLASS, they classify them according to annual drift.

 - Temperature Coefficient, alpha23
This is the slop of the temperature chart at 23 deg C, in ppm/C. Because the temperature chart is curved for most resistors, alpha varies with temperature. That's why it has a subscript specifying at what temperature the alpha is measured. Also, for standard resistors that often used in lab environment, they don't specify or test in large temperature range. Normally 18 to 28 degree C is suffice.

 - Temperature Coefficient, beta
The unit of beta is ppm/C2(per sq deg C), not ppm/C as above spec. The temperature chart can be roughly represented by a quadratic curve, this is especially true for standard resistors where the temperature range is limited. This beta is the coefficient(constant) of the second order equation. The larger the beta, the more the curvature. Most of the metals and resistors have negative beta(parabolic shaped). If a WW made of manganin, the beta is usually -0.6ppm/C2, If made of evanohm the beta usually around -0.028ppm/C2. Foil resistors often have even smaller beta.

 - Power Coefficient
Represent in ppm/Watt. This parameter depends on how heat dissipated and also related to tempco.

 - Adjustment to nominal value, ppm
Its the deviation from nominal value at the time of being made, or may call it initial error, initial accuracy.

One thing also worth noting is that when people measure a standard resistor by a DMM, the standard resistor is superior in most of the cases even the DMM is a 8.5 digits. Most of the imperfection/noise/instability in the test results are probably from the DMM rather than the standard resistor.

ADVANTEST 10k, find no spec, 1 hermetically sealed WW resistor inside.



Chinese BZ3, 0.001 Ohm, class 0.01%, manganin.




Remarks: This is one of the nine BZ3 resistor set, 0.001 Ohm, max current 33A, everything is heavy from binding post, thick manganin stripe. There is a small compensation resistor in parallel with the main.


esi SR104, 10k, stability 0.5ppm/a. Oil filled, hermetically sealed evanohm WW in a stainless steel container.





What can I say, the best 10k resistor ever made. Pity that no further teardown.

zlymex:
esi SR1, 100 ohm, 50ppm/year


Normal wire on mica, the favorite way of esi.


Fluke 742A, 10k







Stability 4ppm/year, tempco is very small. Four 39992 Ohm hermetically sealed evanohm WW resistors in parallel, a 32 ohm adjust resistor is in series with one before parallel. Fluke is probably the largest hermetic WW manufacturer.


Fluke 742A, 1 Ohm



Stability specified as 8ppm/year. Twenty 20.01 Ohm hermetically sealed evanohm WW resistors in parallel, parallel another small adjust resistor of about 2k.
The inside photo was by Leicamr of 38hot.net

zlymex:
IET SRL 1 Ohm
Same specification as Fluke 742A 1 Ohm but different internal construction.



The small resistor on the top is probably the compensation resistor.

IET SRX, 1Ok
I bought this one from IET 11.5 years ago, specified as 10ppm/year, 1ppm/C. One hermetically sealed WW resistor with some compensation/adjustment





MI 9331, 100 Ohm, Specified as 5ppm/year, 0.1ppm/C




Transmille RS3000


http://www.transmille.com/references-and-standards/precision-references-and-standards/3000rs

Although this is not an actual teardown but I know what's inside.
When this UK standard was first released in 2009, I saw something interesting on their website that must be mistakenly put by someone the manuscript of the resistors they used in the standard. A few days later it's gone but I saved it.



Another interesting thing about the script is that they described the tempco of  VSC232Z as 0.05ppm/C, apparently mislead by Visahy's datasheet. Later, they modified this to 0.5ppm/C >:D

zlymex:
Wekomm RS9010A, 10k




More photo and info see Dave's blog #834 and xdevs.com
Specified as 1ppm/year, 0.3ppm/C, one hermetically sealed Vishay foil resistor


I must say this is not up to German standard as the actual tempco of -0.47ppm/C is larger than specified. Wekomm may have misinterpret the datasheet from Vishay like most of us did, and they didn't throw away or compensate ones with large tempco.
Also they are contradictory on those max voltage and max current specs. How can you apply 10mA max current on a 10k without exceed the 20V max voltage. Well, they may say they are on 'which is smaller' basis, but why 10mA max for 10k in the first place?
Another thing is that the resistor is off by as much as 7.72ppm, but the spec says it should be within 2ppm.

Inside VHA518-7, there are seven foil resistor elements.



Yokogawa, 1k, class 50ppm, not hermetically sealed






Lastly, let's teardown some hermetically sealed WW and see how they are made.






Apart from usual bobbin/teflon insulation etc, the special thing about the hermetic resistor is the seal. They have those pre-made glass/ceramic disk with a small tube in the middle and thin layer of metal ring outside to be later soldered to bass tube case.
Although brass has different temp expansion co-efficient with glass, it's flexible and will expand and extract with the hard glass as temperature vary. If the casing is thick, they cut the edge part to make thinner. The inner metal on the hand must be Kovar which expand the same with glass, otherwise, the not-so-easy-compressed 'rod' will soon destroy the seal or even break the glass especially in high temperature when soldering the leads. But, Kovar is notorious for its large thermal EMF that should not be used in precision WW. They solved this dilemma by first implant a Kovar tube(instead of Kovar wire) that bond with glass well, and pass the copper lead through the hole and fill the gap by soldering. Solder is soft that act as cushion if temperature varies and still keep the seal.

The end of the teardown. Sorry for taking too long and split, I had difficulty in uploading images that often stopped in the middle.

Vgkid:
Looking forward to the rest. I have a 1k SR1 resistor(I tore it down too).

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