"Crest Factor" - is a measure of how well the True RMS meter will measure the non-sinusoidal waveform. - maybe compare this spec with various meters ?
From: Rich Nute [mailto:[email protected]] Sent: Monday, February 17, 2003 2:52 PM To: [email protected] Cc: [email protected] Subject: Re: Measuring Power Supply Output Current Hi Rick: > On the surface it seemed like a simple exercise of putting a DC current > meter in series with each of the outputs. Given that the current demand > for each of these outputs is dynamic, corresponding with the processing > activity, does it make sense to measure this output current with a True > RMS meter? Since you said you wanted to "baseline output power," then you would want to use a true-rms meter. Power is a measure of the electric energy that could be converted to thermal energy. Consequently, you must measure any ac component with a true-rms meter. A true-rms meter will measure both dc and ac. However, this may not give you what you want if the dynamic periods of current change are longer than the period of the meter measurement. However, if you get a constant current reading, then you are getting an accurate measure. > If this is the case, I would assume that the True RMS meter takes the > measurement based on some type of a time weighted average or sample > time. Do any of you have a feel for how this is calculated? There are basically two kinds of true-rms meters. The first kind essentially converts the energy to thermal energy, and measures the temperature. The second kind does an analog-to-digital conversion, and then calculates, point-by-point, the rms value of the waveform. If you have access to an electronic instrument museum, you can use any analog meter that has an iron-vane meter movement. RMS = Root of the Mean of the Squares of each increment of the waveform. > How do manufacturers of these PC supplies address the maximum output > current ratings for each voltage. Does this rating take into account > PEAK demands for current (or over current)? Most power supplies are rated according to the maximum continuous load. The ratings for multiple-output supplies is when all outputs are loaded to their maximum continuous load. (Typically, if any one output is loaded and all others are not loaded, then that one output can be quite a bit higher than its rating. But, this is taken as a fault condition in the load.) Peak ratings are specific to the power supply and its application. Essentially, the peak rating (and its repetition) are determined by the output storage capacitor, both discharge time and charge time. Best regards, Rich This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
