Rick, I like to use a clamp-on DC current probe and an oscilloscope. Tektronix makes these probes. If the 'scope has an RMS feature it will calculate the RMS current over the length of the trace. In this way you can adjust the length of the RMS operation. If your 'scope does not have this feature you can do an RMS calculation on paper. It is the square root of the mean of the squares. I divide the trace into 10 segments and it is quite accurate. If you don't have a clamp-on DC probe you can do it with a shunt resistor. You will want something lower than 5 milliohms to keep the burden voltage low. Fluke makes a nice 1 milliohm shunt or you can buy a resistor and do it. You will have to cut the wires. How to connect the 'scope to a floating current shunt? You can translate the signal to ground using an op amp and a FET (not a diff amp circuit). But there is a much simpler method that is sure to upset product safety guys. Cut the ground prong off of the 'scope AC power (use a battery operated 'scope). The 'scope chassis is now DC referenced to one end of the shunt resistor and you can make your measurement across the shunt. I have done this, with an isolation xfmr, to 1 kV. After a couple of good shocks I was trained not to touch the 'scope chassis and earth ground.
Dave Cuthbert Micron Technology From: rbus...@es.com [mailto:rbus...@es.com] Sent: Monday, February 17, 2003 1:19 PM To: emc-p...@majordomo.ieee.org 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 rbus...@es.com 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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
