Problem with power resistors - Page 1
Forum section: electronics / beginners

author: Fantasma25
author_rank: Regular Contributor
timestamp: November 27, 2013, 04:08:23 pm
content: I'm making an h bridge for my power electronics course, but when I try to measure the power in the load it does not match. I am using a 25W 27 ohm resistor which at 10V (measured with the multimeter) should produce 370mA, but instead I am getting about 500 mA. Is there something I am not considering? The output waveform is a sqare wave with 50% duty cycle with no DC component. All my measurments are RMS. Please help!

author: AndersAnd
author_rank: Frequent Contributor
timestamp: November 27, 2013, 04:21:10 pm
content: What kin of resistor do you use? Do you have a datasheet for it?A lot of power resistors are wirewound which means they also act as inductors too.Fine for DC, but not for square waves which contains very high frequency components.https://en.wikipedia.org/wiki/Resistor#Wirewoundhttp://www.resistorguide.com/wirewound-resistor

author: robrenz
author_rank: Super Contributor
timestamp: November 27, 2013, 04:29:43 pm
content: Also that resistor may be 10% tolerance or worse. 10% low value would give you .411A at 10V. Add the instrument RMS measurement errors and you are close to what you are reading.

author: AndersAnd
author_rank: Frequent Contributor
timestamp: November 27, 2013, 04:37:09 pm
content: Quote from: Fantasma25 on November 27, 2013, 04:08:23 pmWhat test instrument do you use?A multimeter, is it a "True RMS" and at what frequencies etc.?A scope with a current probe?If you have access to a current probe for an oscilloscope you should use this, not a multimeter.What does the square wave look like?A drawing of the schematic with BOM, signals and test instruments would help.

author: PA4TIM
author_rank: Super Contributor
timestamp: November 27, 2013, 05:10:12 pm
content: Most TRMS meters are limmited in frequency and crest factor and on top of that only measure AC coupled. For a squarewave the bandwidth must be around ten times the frequency of the signal.This are two squarwaves with the same amplitude. One pure AC with a crest factior of 1, the other has the same amplitude but a DC offset. An AC coupled true RMS meter will measure both as 500 mV. But the one with the DC offset is of 700 mV. You can measure it in DC, then in AC and then calculate RMS from that. (as long as bandwidth and crestfactor are within range of the meter) = squarroot(Vdc2+Vac2)EDIIT: I had not seen you wrote there is no DC offset so that leaves bandwidth of the used meter/scope

author: Fantasma25
author_rank: Regular Contributor
timestamp: November 27, 2013, 05:18:59 pm
content: Here, I uploaded a picture. The oscilloscope is measuring the voltage across the resistor, and the multimeter is measuring the current. I measured the resistance and it is exactly 27 ohms, yet when I calculate it with the voltage and current measured, it says that the value should be about 15.3 ohms

author: wraper
author_rank: Unknown
timestamp: November 27, 2013, 05:27:06 pm
content: Quote from: Fantasma25 on November 27, 2013, 05:18:59 pmYour multimeter is not true RMS therefore it shows wrong reading.

author: c4757p
author_rank: Super Contributor
timestamp: November 27, 2013, 05:28:40 pm
content: The Fluke 23 does not measure true RMS; it is average responding. For a 660mA display, that's an average value of 420mA - also a square wave RMS current of 420 mA. Much closer to 370mA.

author: fcb
author_rank: Super Contributor
timestamp: November 27, 2013, 05:54:36 pm
content: Look up "crest factor".

author: Fantasma25
author_rank: Regular Contributor
timestamp: November 27, 2013, 06:02:26 pm
content: Quote from: c4757p on November 27, 2013, 05:28:40 pmYou are right. I am now using a voltage and current isolator to make my measurments and they are closer to what it should be. But I am still 7 ohms away from what I should be getting. Is that correct? Channel 1 is the voltage measured divided by 3, and channel 2 is measuring the current divided by 10.

author: Jon86
author_rank: Frequent Contributor
timestamp: November 27, 2013, 06:04:16 pm
content: If I was you, I'd probably try and use the scope to calculate the current.

author: robrenz
author_rank: Super Contributor
timestamp: November 27, 2013, 06:16:48 pm
content: Are you taking the TC and operating temperature of the resistor into account?

author: IanB
author_rank: Super Contributor
timestamp: November 27, 2013, 06:21:11 pm
content: Why don't you apply 10 V DC across the resistor and measure the DC current to make sure you really do get 370 mA in a baseline reference test?

author: Fantasma25
author_rank: Regular Contributor
timestamp: November 27, 2013, 07:27:04 pm
content: Quote from: IanB on November 27, 2013, 06:21:11 pmWell, I just feel stupid. Turns out that the scope was not calibrated. I'm getting much better results with a good scope. Thank you all!