resistor measurement power level in Watt versus current in Amp ? - Page 1
Forum section: electronics / general

author: Overspeed
author_rank: Frequent Contributor
timestamp: October 08, 2024, 09:07:51 am
content: HelloDoes someone know the rule / formula / specs regarding the power in watt for resistor measurement as the current value is linked to the output voltage in 4 wires setupBUTDoes the power in Watt have an effect and what is the factor base on the maximum allowed resistor power as more he power is high more the thermal effect will be highI have designed and built a DIY SMD 4 wires jig but from 0402 to 1206 , injected power will the same . Problem is similar with 1 or 2 Watt resistorRegardsOS

author: laichh
author_rank: Regular Contributor
timestamp: October 08, 2024, 10:53:56 am
content: Per NIST Technical Note 1298 "NIST Measurement Service for DC Standard Resistors", section 2.2, the recommended power dissipation should be between 10 mW and 100 mW.https://nvlpubs.nist.gov/nistpubs/Legacy/TN/nbstechnicalnote1298.pdf

author: Overspeed
author_rank: Frequent Contributor
timestamp: October 08, 2024, 01:07:50 pm
content: HelloThanksI agree on the NIST reference but if I apply 10 mW ( page 11 ) to a small SMD resistorThe setup is to measure 2 branches and make comparison between two resistors . one calibrated one to measure the calibrated resistor is quite BIG ( 1 W) the 0603 is small ( 1/10 Watt ) so even in short duration self heating will not be the samePurpose is to sort SMD by true resistance valueRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2024, 07:03:39 pm
content: The NIST article is about standard resistors. These are quite large, often oil cooled - more like the size of a 100 W type as a normal part.For PT100 / PT1000 the recommendet power is usually 1 mW, though this one may be a bit more sensitive to heating than just a normal resistor.For more normal resistors nominal power results in a temperature rise of some 50 K for precision resistors and maybe 120 K for more normal resistors.So I would aim for some 1% of the nominal power if resonable. The heating is mainly an issue for a small range maybe 100 ohm to 10 K.With small power parts one may want to check the range on the DMM. Often one has a somewhat limited choice anyway. Not so many meters offer more than 1 mA and usually ore 1:10 steps.

author: Overspeed
author_rank: Frequent Contributor
timestamp: October 10, 2024, 07:46:18 am
content: Quote from: Kleinstein on October 08, 2024, 07:03:39 pmHelloThanksthat sound realistic , my purpose is to match / built resistor for strain gage bridge as precision resistor in 120 and 350 Ohms are very costly and not available in small SMD .RegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: October 10, 2024, 10:03:19 am
content: Because of the heating effect I would not go for a very small form factor anyway. As it is likely about matching and not the absolute value I would really consider a resistor array, like ORN or NOMCT (the NiCr version, not the TaN variant).For a the ref. arm of the bridge one would not need to have exactly the typical 350 ohms. A slightly different resistor value like 500, 330 or 250 ohm should be OK too. So one could also get away with more normal resistor arrays.Precise 350 ohm resistor sets are available in the LT5401.

author: Overspeed
author_rank: Frequent Contributor
timestamp: October 11, 2024, 09:29:45 am
content: HelloThanks for the LT5401. an interesting IC with impressive performances in a MSOP footprintI agree on absolute value in the bridge I focus more on the branch matching and up to now I use 1206 sizeRegardsOS

author: Overspeed
author_rank: Frequent Contributor
timestamp: October 18, 2024, 06:56:54 pm
content: HelloAn interesting white paper on small size resistor measurement to calibrate conductive probe atomic force microscopyThey use voltage source and measure the current with a current amplifier , all measurement are done following strict lb requirements , interesting readingRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: October 19, 2024, 08:19:12 am
content: A fixed voltage and measuring the current is a good way for large resistors. It can also make it a little easier to prevent excessive voltage for sensitive samples. However there is still the possible transient response at the TIA to measure the current - so not all is as easy as it looks at first view.