Tempereture effect on resistance - Page 1
Forum section: electronics / projects

author: Asim
author_rank: Regular Contributor
timestamp: December 29, 2014, 05:49:31 am
content: Hi all,If I have a 0.1 ohms resistor rated at 2W , this means it can handle 4.47 A.My question is at which current will the resistance drift due to tempereture given that the resistor is not mounted to a heatsink, is it at 4.47A , more,less?Thanks

author: Monkeh
author_rank: Super Contributor
timestamp: December 29, 2014, 05:53:54 am
content: The resistance will drift due to temperature at any current. Including none at all.

author: Asim
author_rank: Regular Contributor
timestamp: December 29, 2014, 08:07:11 am
content: Ok, but is their a graph that shows how much the resistance drift with respect to current/ tempereture ?What i want to know is how much the drift if i have a 0.1 ohm resistor ( 2W) drawing 2A.

author: mzzj
author_rank: Super Contributor
timestamp: December 29, 2014, 08:27:12 am
content: Quote from: Asim on December 29, 2014, 08:07:11 amYou have to calculate and/or guess based on the temperature coefficient.If this is typical ceramic power resistor it will run probably pretty hot.If your resistor temperature coefficient is 100ppm/c and your resistor runs at +200 degrees temperature rise the change is going to be 20000ppm or 2%

author: Asim
author_rank: Regular Contributor
timestamp: December 29, 2014, 09:46:32 am
content: Quote from: mzzj on December 29, 2014, 08:27:12 amRESISTOR, 2512, 0R100, 1% 2WResistance:0.100ohmResistance Tolerance:Â¬Â± 1%Power Rating:2WVoltage Rating:3VTemperature Coefficient:Â¬Â± 100ppm/Â¬_CResistor Element Material:Thick FilmResistor Case Style:2512Resistor Mounting:SMDSVHC:No SVHC (18-Jun-2012)External Depth:2.7mmExternal Length / Height:3.7mmExternal Width:2.45mmNo. of Pins:2This is the resistor i am planning to use as a shunt resistor

author: Christopher
author_rank: Frequent Contributor
timestamp: December 29, 2014, 10:02:40 am
content: Seems very likely that resistor will get a bit toasty...First work out the temp rise, then use ppm/degree rating in datasheet as explained above

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2014, 10:12:12 am
content: Warning, tempco is only true for small changes around room temperature.Typically, metals increase in resistivity according to a power law, such as T^2 or T^3. Not only is the tempco fairly large, it increases with temperature. A 100 or 200K difference makes a huge difference.Resistors are generally made of special alloys or compounds which exhibit a low tempco near room temperature, but the behavior at other temperatures (very low or very high) can be anything.I would treat the 100ppm/K figure as a 'typical' or lower bound, and a rough guess from the "pure metal" case as an upper bound (i.e., it doesn't get much worse than that).If the manufacturer provides a graph of resistance or tempco versus temperature, over a much wider range of temperatures than 25.000C, you can determine this with a much better level of certainty.Then again, if you need that kind of numerical and statistical certainty, you probably shouldn't be playing around with 1%, 100ppm resistors. Lower drift resistors are more expensive, but that's a small cost when you need it.You can also connect many resistors in parallel, or use parts with much higher power ratings, to reduce the temperature increase, thus reducing the error due to self-heating. The expected error will then be much closer to the expected 100ppm/K change.Tim

author: MrAl
author_rank: Super Contributor
timestamp: December 29, 2014, 10:46:43 am
content: Quote from: Asim on December 29, 2014, 05:49:31 amHi,It is a good general practice to use power resistors at one half power not to their full power. They get far too hot at full power. When they are in an enclosure even more special mounting techniques may be required.But all that is for the normal power resistor used in the normal way. For sense resistors the limit is even more extreme. You dont want the resistance to change too much or you start to get false readings.Resistors are not all the same however and they can vary by quite a bit. This means the best bet is to run it at the required power level and measure the resistance to see exactly what it changes to. But even then half power is a good limit.Calculating resistance change with temperature is a little tricky, but even more tricky is calculating the actual temperature rise for a given input power. Better off measuring the change in resistance with the resistor mounted exactly as it will be in the end application, which might include an enclosure, and it should have free air flow at the least.For short runs at home with low voltages power resistors can be immersed in distilled water to keep them cool for a while.This is interesting but of course you have to be careful and not let the water get too hot either.

author: Asim
author_rank: Regular Contributor
timestamp: December 29, 2014, 11:34:04 am
content: Quote from: MrAl on December 29, 2014, 10:46:43 amI will be using it at half power, i guess i should experiment with it. if it failed me i will parallel 2 together which means i will be running them at 1/4 of their rated power

author: rx8pilot
author_rank: Super Contributor
timestamp: December 29, 2014, 05:15:03 pm
content: Does your circuit have the sensitivity to use a smaller resistor? I use .005 and .01 Ohm sense resistors for many of my circuits to measure 10A and 5A. They don't get hot enough to need a heat sink - the temp rise is modest. Save a lot of hassle with lower resistance.

author: MrAl
author_rank: Super Contributor
timestamp: December 30, 2014, 09:11:45 am
content: Hi,Yes good point, the smaller resistance you can get away with the better as there is less heat loss.I have used current sense resistors from as high as 10 ohms down to 0.010 ohms. The 10 ohms was for a low current like 100ma and so 1/10 watt. The voltage drop has to be acceptable too, and for 10 ohms 0.1 amps it was of course 1 volt but that was not a problem for the type of circuit being used.On the other hand, to sense up to 15 amps i used 0.010 ohms because i could not afford to loose too much voltage.What works nice with a microcontroller is an 8 ohm resistor for sensing low current. That's because the current is then V/8 which means the current calculation is a division by a power of 2 which means a couple shift instructions rather than an actual division. Of course that is when the drop from an 8 ohm resistor is acceptable.

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 09:20:32 am
content: I was doing some calculations and i decided to order ina213 ( 50 gain version) and i will use 0.02 ohm as a shunt. This way i will still achieve the results I need 1v=1A

author: rx8pilot
author_rank: Super Contributor
timestamp: December 30, 2014, 07:51:13 pm
content: If you are using a microcontroller for this project, an INA226 may be interesting. It has 16bit A/D on board and spits out V and I values over i2c. It also has on-board averaging and an alarm output.