wirewound resistors and 4.5 digit meters - Page 1
Forum section: electronics / general

author: 001
author_rank: Super Contributor
timestamp: December 12, 2018, 08:17:11 am
content: Hi!I`m gonna check my 4.5 dmms with set of 0.25%100ppm wirewound resistorsIs it good idea?Can I use this type resistors for hamon divider?

author: Echo88
author_rank: Frequent Contributor
timestamp: December 12, 2018, 10:20:04 am
content: If the resistor accuracy is better than the DMM measurement range-accuracy: yes, good idea.Yes you can build a Hamon Divider with those resistors, but will get dissapointing results (if you have high expectations) due to the 100ppm tempco of the resistors, which the hamon divider cant magically compensate.

author: 001
author_rank: Super Contributor
timestamp: December 12, 2018, 10:24:14 am
content: Quote from: Echo88 on December 12, 2018, 10:20:04 amWhy? All parts have same tempco, isnt it?

author: Echo88
author_rank: Frequent Contributor
timestamp: December 12, 2018, 10:29:28 am
content: Yeah, if one resistor has +100ppm/K tempco and another one in the other hamon-divider-branch has -100ppm/K tempco, then the results will vary with temperature. Only if you select the resistors for equal tempco you can build a good hamon-divider or you just buy better resistors with less tempco.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 12, 2018, 02:46:03 pm
content: You might actually do better with good metal film resistors that you can buy cheap in bulk, and then select them. IMHO, the advantages of wire wounds aren't what they used to be, unless you pay quite a lot, and then foils might make more sense.

author: 001
author_rank: Super Contributor
timestamp: December 12, 2018, 02:50:10 pm
content: Quote from: Conrad Hoffman on December 12, 2018, 02:46:03 pmCan You post some links where I can get it for resonable price online? Thanx

author: Kleinstein
author_rank: Super Contributor
timestamp: December 12, 2018, 06:05:57 pm
content: It might be possible to use 2 versions of the Hamon divider with some resistors swapped. This way one would combine different resistors.Besides environmental temperature changes (which may not be that bad for the test of the DMM), there is also self heating of the resistors. In the Hamon divider the 3 resistors in parallel get much lower power. So one needs a low TC. Just having good TC matching is not enough, unless one has very good thermal coupling.Wire wound resistors could have a slight advantage from higher power rating and thus less self heating compared to usually physical smaller thin film resistors. Wire would resistors with a TC of 100 ppm/K are more like power resistors, not precision resistors. So there can be other effects like hysteresis, stress effects. On the other hand chances are good that resistors from the same batch have a similar TC.Thin film resistors with some 15-25 ppm/K can be relatively cheap (e.g. some 20-100 cents) so one could even use 2 resistors each to get bettermatching / averaging and less self heating.The TC at some 25 C can be much better than the data-sheet value given for a large temperature range. This especially true for relatively good resistors where the square law part makes up a large part of the TC.With a 4.5 digit DMM one might have to look at the loading due to the usual 10 M input resistance.

author: GregDunn
author_rank: Frequent Contributor
timestamp: December 12, 2018, 06:38:57 pm
content: Here's a nice nomograph which will answer the question about the DMM's accuracy in % :https://static.elektroda.pl/attach/AN82_6405927.pngTypically you're going to need much greater accuracy than the meter (general rule suggests 4x greater) in a standard in order to calibrate a meter. So if the meter is 4.5 digits with a good accurate A/D, that's between 0.01% and 0.005% precision and you'll need resistors of around 0.001% accuracy to be sure they're good enough to use for meter calibration. 0.25% resistors aren't even close, unless you hand-select them using a meter considerably more accurate than the one you're using for measurements (say, a 3458A).I bought the PRC set (0.01%-0.02%) for about $70 and they're really not good enough for my 4.5 digit DMM, but then I only am using them for a spot check.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: December 12, 2018, 07:22:56 pm
content: Quote from: GregDunn on December 12, 2018, 06:38:57 pmIn general, the absolute value of the resistors is not that important, as long as they are reasonably close to nominal (0.1% is fine), and you know to a high degree of accuracy what the actual value is-- (thus, they need to be measured by a Cal-Lab, or someone with an in-cal high-spec meter or bridge). The TCR should be zero or as close to zero as you can manage, and the technology should be something that has excellent time drift specs (precision WW, hermetic metal foil, tantalum nitride).There is nothing wrong with building a statistical array of tantalum nitride (Ta2N) resistors-- selected for a zero TCR, and burned in at 125C for 90 days-- this will artificially age the resistors and the result will be exceptionally stable. Further trim of TCR can be done with a platinum thermistor (with appropriate shunt resistor), and further trim of the absolute value can also be done with an appropriate trim resistor in series or parallel with one of the resistors in the array. When you get all done, have a friend with a calibrated 8.5-digit meter measure them.

author: 001
author_rank: Super Contributor
timestamp: December 13, 2018, 05:48:39 am
content: Quote from: GregDunn on December 12, 2018, 06:38:57 pmcool infowhere I can read full text?thanx

author: BNElecEng
author_rank: Regular Contributor
timestamp: December 13, 2018, 07:39:31 am
content: Full text linkhttps://www.analog.com/media/en/technical-documentation/application-notes/an82f.pdf