RMS  -  True RMS  -  Voltage and current ..  - Page 1
Forum section: electronics / projects

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 26, 2010, 10:35:39 am
content: Well, I an trying to find for days few explanations , especially about measurements of DC amperes.The most " Dirty " DC sources look to be the pulsing power supply ( computer PSU ) .I call them " Dirty " , because of the large bandwidth in Hz or KHz , that they use ..That are forcing us to use RMS or True RMS tools.And in order to judge if one tool is good for the job , by reading the specs .I must be aware of " Some how " , of the expected for measurement bandwidth.Is there any " Rule of thump " that it predicts the use of the bandwidth VS device type ?Example :An Uninterruptible power supply ( on battery) It will produce an output of 220V at 50/60Hz ,even if its not an perfect AC , the attenuation in the frequency are "predicted - calculated" to stay at 50/60Hz , if not , the powered device will be on trouble .. as it is designed to operate at 50/60Hz.The bad made " Uninterruptible power supply" will use more bandwidth like 10Hz to 100Hz ,and we are getting in the calculation even the first harmonic at 83Hz ..For this measurement we need anRMStool.But which one can be called as an RMS tool ? the one that detects 0-100Hz or 0-400Hz ...Or what is any way ? , the limit of the measured bandwidth called as RMS , so to actually need and use one True RMS tool !!At 2010 , should all tools be as True RMS ? , and any as " just " RMS to be parked on the museums ?And if some how, we define the limits of the AC RMS & True RMSWhat about the defined limits of DC RMS & True RMS ?

author: tecman
author_rank: Frequent Contributor
timestamp: May 26, 2010, 02:22:20 pm
content: RMS vs True RMS is an interesting question. Nearly every meter today reads RMS (on AC). Not a true RMS, but being scaled by 0.707 of the peak. In the bad old days, most true RMS instruments were thermally based. The voltage/current heated a resistor, and the meter would measure the temperature rise. Since the way to measure true RMS is to calculate the value in real time (analog multiply works as well), the technology did not exist in the past.Your question is a valid one. The bandwidth of the instrument (how fast it can calculate the value) will determine the accuracy. Since Mr. Fourier taught us that anything other than a pure sine wave is a series of sine waves of increasing frequency, the bandwidth of the instrument will determine what harmonics get measured and what harmonics don't get measured. Most meters today usually give accuracy verses frequency. As a rule of thumb, you should have good accuracy at 10 times the frequency of interest. So for 50-60 Hz, go for a minimum of 1 KHz of accurate reading capability.Paul

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 26, 2010, 03:09:21 pm
content: I do find this first explanation, as accurate, by my logicSo I will get a bit deeper in it .... the modern multimeters uses processors so to calculate the True RMS.But if we go to the Current Clamps "Hall effect principle" .How they " Understand " - capture - the bandwidth ..Its well known , that if the clamp was like a coil , it should had a very specific range.I am looking for one AC/DC clamp probe, and most documents about them are unclear ..In the clamp probes , the Amperes gets converted to Millivolts , and the digital multimeter will read those and it will display them as Amperes ..From the technical document of one manufacturer ...... The clamp can do True RMS measurements if the connected multimeter are an True RMS one !!The above , its like saying that the clamps do not have limits , and all the weight falls over the quality of the digital multimeter ..Usually on the clamps on multimeters , we do not bother with their inner connection as clamp and multimeter ... thats why most of times we get poor products ..But in the case of the clamp as probe .. and since I do own an respectable True RMS multimeter ,I finally need to know the truth about the damn "Hall effect principle".... and to be able to tell two things like :1) Are all the "Hall effect principle" clamps, the same item (identical construction) ? ...2) If yes and everything that matters are just the digital multimeter its self , why they just,do not say so !!Damn marketing !!

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 26, 2010, 07:12:13 pm
content: Ahh and some good news, got my CHAUVIN ARNOUX PAC12 clamp, yes it is made in France.End I even got it from Paris - France .. for 60 EUR shipping included. (special agreement)http://cgi.ebay.fr/ws/eBayISAPI.dll?ViewItem&item=370374988821In specs looks ever better than the Fluke i1000. (accuracy)But even so, I will not stop poking the theory behind this technology.