I wasn't going to weigh in on this one, but this last posting captured what I had been thinking. I think the emphasis on true rms readings on a dc supply is mistaken. I think you need to know two things. The maximum power your boards draw because that is what what you were told to determine, and, based on the other postings, that the dc rails don't droop when your boards draw their maximum currents. To that end you make two measurements. Rail potential as a function of time, and current draw as a function of time. If the power supply is adequate, the potential drop at maximum current should be a small fraction of a Volt, so that you can ignore the Volts x Amps integration over time, and just look at current draw over time.
> From: [email protected] > Reply-To: [email protected] > Date: Mon, 17 Feb 2003 15:18:26 -0800 > To: [email protected] > Subject: Re: Measuring Power Supply Output Current > > > > If the power supply voltage is "stiff", then all you need measure is the > average current. The average current times the voltage will give you the > correct average power. > > If the supply voltage is not stiff, then the process is more complex. You > will need to measure the voltage and the current, multiply them > point-by-point, and then average. > > Measuring RMS current in this case is WRONG. RMS measurements are used when > voltage and current vary in direct proportion to each others (as in a > resistor). That is, when the current goes up across the load, the voltage > goes up in the same proportion. In a power supply, the output voltage will > most likely DECREASE a bit as the load current INCREASES. While you can > still make an RMS current measurement even in this case, it will NOT tell > you the average power consumption. > > Don Borowski > Schweitzer Engineering Labs > Pullman, WA > > > > > > <[email protected]>@majordomo.ieee.org on 02/17/2003 12:19:27 PM > > Please respond to <[email protected]> > > Sent by: [email protected] > > > To: <[email protected]> > cc: > Subject: Measuring Power Supply Output Current > > > > I have been asked by my TUV office to measure the actual output current > for each of the 5-7 voltages on my PC power supplies. The purpose of > this is to baseline the maximum output current in terms of maximum > power, not instantaneous current, for each of the various motherboards > we use. > > 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? > > 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? > > 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)? > > Thanks > > Rick Busche > Evans & Sutherland > [email protected] > > > ------------------------------------------- > 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 > 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
