Can you show us a picture of what is "face up" and what is "face down" and which side is closest to your temperature measuring device in each case?
Hello Ken,
the arrangement is shown on page 15 in post
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg599876/#msg599876picture:
So face up means that you can read the resistor value and the tolerance from top side.
Face down in this context is when you read the brand (VISHAY), date code (B0940-) and resistor type (Z201T).
The reference temperature sensor (NTC2) is the one lying left of the resistor.
NTC1 is right and NTC_Legs is between the legs.
The NTCs are not calibrated so they have up to 0.3 deg C difference against each other.
For the T.C. measurement I will need only temperature differences and not the absolute value.
When you look at the "naked" Z-Foil resistors (VAR) for "high end audio" applications you will see
that it is impossible that they behave symmetrically.
http://www.vishaypg.com/docs/63140/var.pdfIs this resistor identical to the Z201, you've sent me, 1k also?
Then I'll do the reversal test on my 3458A also.
Hello Frank,
you got the Z201#2 which has the same date code (B0940-) as the Z201#3 which I have
used for the last measurements. Both are 1K resistors.
And thanks again for testing.
If you get different results then we have a problem to find the root cause.
One could be different measurement current. Around 2.5 mA in my case and around 1 mA in your case.
So I have factor 6.25 more self heating of the resistor.
There exist many Pictures of the ceramic /metal foil chip, with attached leads.. That always looks symmetrical, so no reason for asymmetric thermal behavior.
They are symmetrical if you mount them in upright position.
But from front and back side you should have different thermal resistances to the surface.
The resistive element (metal foil) is on the front side then comes the aluminium substate then the wires on the back side.
I guess that the epoxy resin is thicker on the wire side than on the metal foil side.
The problem is to know if vishay does really do consistent printing or if they dont mind printing the values on the "wrong side".
http://www.vishaypg.com/docs/63187/zseries.pdfOr, make a comparison against a poor mans 0.1% resistor like the RT0805BRD071KL from Yageo?
What do you get extra when paying $10 for a resitor, instead of $0,10?
Hello Johnny,
I already made a comparison to a "good" (more in the 1$ range) thin film resistor (RC55Y type) with 15ppm/K spec.
See page 1 of the thread. The question is how much effort is done for long term stability.
Did you consider any SMTs?
Susumu RG and URG are avaliable to 2ppm/C, with URG series being supposedly a higher quality, better aging. Both have some kind of secret inorganic coating. Datasheet: "Unmatched Reliability and Excellent Stability at different environmental conditions".
I think I have already mentioned it in the thread: Definitively no.
The reason is simple: SMTs cannot be measured alone. (At least it makes no sense).
I will in each case get the humidity coefficient and the thermal expansion coefficient of the epoxy material or ceramic substrate where the SMTs are mounted.
It is simply useless to specify T.C. coefficients of SMTs (below 20ppm/K) without exact definition of the mounting method.
With through hole devices I can decouple the resistor from PCB (expansion) by the mounting method.
I think you can reach the 2ppm/K only on ceramic substrates with same thermal expansion than the resistor.
But never on a standard epoxy PCB which shrinks and swells with humidity.
Thermal cycling is sometimes recommended for removing mechanical stress on sensitive components. Do you have any thought on that based on the measurements?
Heating removes humidity from PCB. So the following thermal cycle will get lower hysteresis.
Thats an old trick to let voltage references with plastic package look better in the datasheet.
But it will not help you in real life applications. (except when you heat up the whole device).
With best regards
Andreas