# [volt-nuts] Review: Vishay Z foil resistors as Secondary OhmStandards

Andreas Jahn Andreas_-_Jahn at t-online.de
Sat Nov 19 18:29:15 UTC 2011

```>A goal which could not be achieved, is the usage of the VHP202Z
>resistors as secondary Ohm standards, as the TCR of roughly -0.5ppm/K is
>unexpectedly too high.'

>Best regards - Frank

Hello Frank,

yes: reading between the lines of a datasheet is very difficult
because they are made from salesmen which want to sell their stuff
and have to be "better" than competition.

For understanding the datasheet values you have to understand how they are
specified.
In most cases the "box method" is used for specifying values over
temperature.
In the box method the value is measured  at 3 temperatures: (to save
cost/measurement time against a continuous curve).
At 25 degrees, at minimum operating temperature (e.g. -55 degrees) and at
maximum temperature (e.g. 125 degrees).
Maximum and Minimum of the 3 values give now the "spread" of the
measurement.
So for TC calculation (RMax-RMin) / RNom / (TMax-TMin) is calculated.
For the Datasheet the TC is even halfed by writing +/- TC = +/- (RMax-RMin)
/ 2 / RNom / (TMax-TMin).

The only thing that you can say from TC specification is that the difference
between minimum and maximum resistance is 2 * (+/-)TC (including "spread") *
(TMax-TMin) over the temperature range.
There is no assumption to make over the actual slope at a specific
temperature.
Although typical you will measure double TC for the "slope" as in the +/-
specification for TC due to the box method.

And: the only thing you can expect from the same "Lot" is that the resistors
behave mostly equal.
So for a voltage divider you can expect that the division factor will be
nearly constant over temperature.

By the way: from where did you order the parts: from England or from Teltow?

With best regards

Andreas

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