I wasn't going to weigh in because I knew there would be people with more experience with dc current probes than myself, but this posting requires an input.
My understanding is that this is a dc measurement, hence the discussion of waveform potential and phase doesn't apply. If it were an ac measurement, this would be a very important consideration, but it doesn't bear on the current probe shunt selection. The problem with most dc probes is a small window that isn't suitable for a 100 Amp current carrying conductor. To my way of looking at things you ought to be able to specify a resistive shunt that would not compromise measurement integrity. A typical scope can measure 10 mV with reasonable signal to noise ratio. If the minimum current to be measured were 10 Amps, this would imply a 1 milliohm shunt. That is less than the wire resistance would be in any real world installation. If you were worried whether the shunt was impacting the inrush current, you could get two of them and put them in parallel and see if you get a different result. If you don't you have a good measurement. If you get more current, time to build a lower resistance shunt. BTW, this all assumes the power source itself is stiff. You would need to measure the output potential when the switch is closed. Ideally the output potential measured with a scope would not sag at all. In reality, the allowable sag would be specified. > From: Rich Nute <[email protected]> > Reply-To: Rich Nute <[email protected]> > Date: Tue, 17 Feb 2004 09:04:34 -0800 (PST) > To: [email protected] > Cc: [email protected] > Subject: Re: DC Current Probes > > > > > > Hi Ed: > > >>> I am looking to source a DC current probe to measure >>> steady state as well as inrush currents for a modular chassis >>> up to 100 Amps DC. > . > . > . >> Do you REALLY need a current probe? Can't you do this with a resistive shunt >> and an oscilloscope, using either differential inputs or simply floating the >> scope? > > Inrush current is not an easy measurement. > > In addition to a suitable current measuring > device, the source impedance is critical as well > as the control of the turn-on (which should be > something like 80 degrees to get maximum inrush > current). > > Depending on the magnitude of the inrush current, > a resistive shunt may significantly reduce the > inrush current. In general, a resistive shunt > should not be used for measuring inrush currents. > > Tektronix makes a high-current version of their > dc current probe that easily makes the inrush > current measurement. This current probe at > least assures an accurate measurement so that > one can concentrate one's efforts on the source > impedance and switching. > > > 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.ieee-pses.org/ > > 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] > > 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.ieee-pses.org/ 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
