Question about power and frequency relationship - Page 1
Forum section: electronics / beginners

author: skysurf76
author_rank: Regular Contributor
timestamp: March 06, 2013, 10:44:11 pm
content: I'm an EE major and working on a lab report for lab we did where we ran a sine wave at different frequencies through a 1W resistor and measured the temp. My data points are...500Hz 30.7C2kHz 31.9C5kHz 32.1CAccording to the root mean square equation frequency should have no effect on power delivered. I realize that the difference in theoretical and actual values isn't enormous, but with results like that I can't help but ask the pro's if something else is going on here beyond the basic stuff we are studying. I'm really curious so I had to ask.Thanks in advance.UPDATE: To answer some of the questions from the replies, I don't know what type of resistor it was. It looked like the 3rd one from the left in this picture...http://www.bcae1.com/images/jpegs/IMG_5660b.jpgAdditionally we were using a fluke thermocouple module attached to a fluke 87V DMM, and we had the thermocouple in a clamp stand along with the resistor with some thermal compound on the end of the thermocouple probe. The resistor wasn't shielded from drafts at all. Also, we used a second fluke 87V to measure AC rms which gave the same reading for each of the frequencies. AC rms for each frequency was 2.48 volts. The sine wave came from an HP function generator. We also allowed a couple minutes between each frequency change for the temperature to stabilize. The temperature wasn't bouncing around at all and was very stable after a couple minutes.I was thinking maybe something like some capacitance or inductance was coming into play.

author: IanB
author_rank: Super Contributor
timestamp: March 06, 2013, 11:27:20 pm
content: I think your data is incomplete. Without an accurate measure of the RMS voltage across the resistor you do not have verification that the voltage was the same in each case.Strictly speaking you should also be measuring temperature rise above ambient, so you need to record two temperatures for each test: the ambient temperature and the resistor temperature and look at the difference between the two.Also consider air currents (did you have the resistor shielded from drafts?). A small flow of air across the resistor can cause a very measurable change in temperature.How did you measure the resistor temperature? Did you measure the exactly the same point on the surface in each test in exactly the same way? (A thermocouple glued to the resistor would be a good repeatable setup.)Did you make sure to let the resistor reach a stable, steady temperature before each measurement? Time is a variable that can affect measurements.

author: MikeK
author_rank: Super Contributor
timestamp: March 06, 2013, 11:33:18 pm
content: Is this a wirewound resistor?

author: IanB
author_rank: Super Contributor
timestamp: March 06, 2013, 11:35:51 pm
content: Quote from: MikeK on March 06, 2013, 11:33:18 pmThat's a probability, but I doubt low frequencies down in the 5 kHz range would be affected by any resulting inductance.

author: uprightsquire
author_rank: Contributor
timestamp: March 06, 2013, 11:39:18 pm
content: 31.9 and 32.1 could easily be within the error of your temperature measurement. Was the temp stable? If you didnt let it equilibrate, the warming could just be due to passing some current through the resistor, and not related to the freq.I would also have repeated the tests.

author: amspire
author_rank: Super Contributor
timestamp: March 06, 2013, 11:39:40 pm
content: Quote from: skysurf76 on March 06, 2013, 10:44:11 pmThe heat in the resistor will be identical within at least 0.01% at those frequencies so something else is going on. What are you using to measure the sinewave voltage? If it is a multimeter without a True RMS AC option, then I would bet that the answer is that you are applying more AC voltage at 5KHz and your multimeter is wrong. Look at the specifications for the meter you are using. Is it only specified up to 1KHz? Perhaps you have a DC offset in the sinewave that is not showing up on the AC range of your multimeter. Perhaps whatever you are using to measure temperature is influenced by capacitive or inductive coupling to the AC in the resistor.

author: IanB
author_rank: Super Contributor
timestamp: March 06, 2013, 11:41:43 pm
content: Quote from: amspire on March 06, 2013, 11:39:40 pmPerhaps my "shoot from the hip" idea of attaching a thermocouple to the resistor would not be such a good idea then...?

author: robrenz
author_rank: Super Contributor
timestamp: March 06, 2013, 11:46:01 pm
content: With values that close I would go back down thru the 2kHz and 500Hz and make sure they repeat. If not your ambient is wandering and or you did not give enough time for the temps to stabilize at each frequency in the first place.

author: amspire
author_rank: Super Contributor
timestamp: March 07, 2013, 12:41:42 am
content: Quote from: IanB on March 06, 2013, 11:41:43 pmI think thermocouples would probably work OK. The point is that the one thing you can absolutely bank on is that at a constant AC voltage, you will get an identical heat. So all that has been established is that something in the test setup is wrong. You are not to get frequency dependent resistors at a few kHz and P=V*I is absolutely accurate. If P is not constant and R is constant, then V is not constant. Even if it is a lousy resistor with a big temperature coefficient, it would still end up at the same temperature for each frequency as long as the ambient temperature is identical. If the resistor is mounted on a panel, then if the panel is gradually heating, the ambient temperature for the resistor is increasing with each test so the resistor temp will increase for each test.There are only two possibilities -1. Measurements of P (power which is indicated by temperature) is flawed in some way - ambient temp changes, bad temp sensor, not allowing resistor temp to fully stabilize, etc2. V is not constant at the different frequencies.Experiments like this should always be repeated at least three times and if the test procedure is bad, you will often get different results on each test.

author: AlfBaz
author_rank: Super Contributor
timestamp: March 07, 2013, 01:09:23 am
content: Wouldn't the need for precision measurements, voltages etc be mitigated somewhat if the temperature generated due to power dissipation be increased?It seems to me that ambient temperature would be the dominant force here

author: skysurf76
author_rank: Regular Contributor
timestamp: March 07, 2013, 02:03:48 am
content: Added an update in the original post with information that was mentioned in the replies.

author: skysurf76
author_rank: Regular Contributor
timestamp: March 07, 2013, 02:09:19 am
content: Quote from: amspire on March 07, 2013, 12:41:42 amThats kind of what I was looking for. I just wanted to verify that my understanding was correct. The reason I asked is that it seems like every time you learn about something in electronics it just uncovers 2 or 3 more subtleties that lie just below the surface of what you just learned, and I don't want to be one of those guys just standing scratching his head when something doesn't behave like it should. I was just curious if something like someone stated about inductance coming into play because perhaps it was a wire wound resistor was a factor.

author: uprightsquire
author_rank: Contributor
timestamp: March 07, 2013, 02:18:15 am
content: The fluke 80bk temp probe has a accuracy of 2.2oC.https://www.distrelec.com/ishop/Datasheets/07608722.pdfThis would mean that 30.7, 31.9 and 32.1oC are the same temperature.

author: amspire
author_rank: Super Contributor
timestamp: March 07, 2013, 03:08:15 am
content: Quote from: skysurf76 on March 06, 2013, 10:44:11 pmDid you check the DC out of the function generator rather then AC? Make sure the Fluke is not on autorange when you do it. You would want a range that can do at least 3.99 volts DC. The total power is the DC power + AC power.

author: IanB
author_rank: Super Contributor
timestamp: March 07, 2013, 04:23:21 am
content: Quote from: uprightsquire on March 07, 2013, 02:18:15 amIt doesn't mean that at all. If that were true it would be impossible to get a reading of 31.9°C on the display, all you would see is some kind of fuzz around 31 degrees with no way to read a number off it.If you use the same meter with the same probe and take two successive measurements that are 1 degree apart, you can be highly confident that you have measured a temperature difference of 1 degree.

author: IanB
author_rank: Super Contributor
timestamp: March 07, 2013, 04:31:11 am
content: Quote from: skysurf76 on March 07, 2013, 02:09:19 amI think the biggest issue is you needed to measure the temperature rise of the resistor above ambient temperature, and not the absolute temperature of the resistor. If a load of people pile into an empty lab the temperature of the room is going to go up at least a degree or two. Especially if those people start running electrical equipment to conduct experiments.Good multimeters can display the ambient temperature measured by their internal reference temperature sensor and it always pays to record that as well as the thermocouple reading in this kind of experiment. I don't recall if the Fluke 87V can do that. It's not the best DMM out there in spite of its fabled status. It's actually a bit of an expensive dinosaur

author: gotwood
author_rank: Contributor
timestamp: March 07, 2013, 04:46:01 pm
content: saying those temperatures are the same is wrongthe accuracy of the temperature probe gives the variance to the ACTUAL temperaturewhat you need to know is the precision of the instrument to be able to say those temperature variances are essentially equal

author: alanb
author_rank: Frequent Contributor
timestamp: March 07, 2013, 05:06:17 pm
content: What you need to measure is the rate at which the temprature increase at each frequency rather than the actual temp. I would put some thermal isulation round the resistor and the thermocouple and then plot the temperature against time for each frequency, allowing the set up to cool down between each run. The slope of the line will give an indication of the rate of power dissipation in the resistor.