Well of course the switching device cycling the power is between you and the electrical system. Where else would it be? The point is switching is done between the common impedance of the bus (modeled by the LISN) and the switched load.
You cannot switch on the power input side of the LISN (unless of course the spectrum of interest is below that covered by the LISN, so below typically 150 kHz, or maybe 10 kHz). If you try energizing the LISN along with its load, you are measuring the effects as filtered though the LISN, which is definitely not correct. With the LISN at 12 Vdc, or 28 Vdc, or whatever the nominal bus potential is, the switching device alternately connects that potential to the load, or disconnects it. The load then draws a transient current until it reaches steady-state potential, whereupon it generates the emissions typically covered by frequency domain requirements spanning 10/150 kHz to 10/30 MHz, or sometimes beyond. Someone mentioned using a voltage probe, implying don¹t use a LISN. That may be acceptable if the intent is to measure inrush current, and the source is nearby and stiff. But if you are in a screen room and the power is run through facility EMI filters, then the LISN absolutely needs to be there to provide a controlled impedance to work against. Or, if the bus is dc, you can bypass the power with enough capacity that the voltage sag during the inrush event is minimal, guaranteeing the measured inrush current is worst case. Measuring inrush current, or transient potential drop without a controlled source impedance is every bit as unacceptable as measuring either steady-state frequency domain current or voltage ripple in the absence of a LISN or feedthrough capacitor. It is an uncontrolled measurement. Ken Javor Phone: (256) 650-5261 From: Douglas Smith <d...@emcesd.com> Reply-To: Douglas Smith <d...@emcesd.com> Date: Fri, 10 Dec 2021 04:01:42 +0000 To: <EMC-PSTC@LISTSERV.IEEE.ORG> Subject: Re: [PSES] unusual conducted emissions question There is a device between us and the electrical system that does the power cycling. We have no control over that device, part of the vehicle. Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108 Email: d...@dsmith.org Website: http://dsmith.org From: Ken Javor <ken.ja...@emccompliance.com> Sent: Thursday, December 9, 2021 7:55:05 PM To: EMC-PSTC@LISTSERV.IEEE.ORG <EMC-PSTC@LISTSERV.IEEE.ORG> Subject: Re: [PSES] unusual conducted emissions question The purpose of a LISN when testing for vehicular use is to simulate the common impedance between battery and fuse block. There should be no power cycling ³behind² the LISN, because in the vehicle there is no switch between battery and fuse block other than the ignition switch itself. Therefore the power cycling device goes between the LISN output and the switched load. Ken Javor Phone: (256) 650-5261 From: Douglas Smith <d...@emcesd.com <https://urldefense.proofpoint.com/v2/url?u=http-3A__doug-40emcesd.com&d=DwM FAw&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=c9NR2mGfldry-2pM9Bbuww&m =kavBu9H9umaS2l7Zwzp81cdwEgTt9UJa3f56hflvA8A&s=OVcQCPr1r2j4G99wMQoHdTA8Uh-Ec shq2eAiFJgY7Xc&e=> > Reply-To: Douglas Smith <d...@emcesd.com <https://urldefense.proofpoint.com/v2/url?u=http-3A__doug-40emcesd.com&d=DwM FAw&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=c9NR2mGfldry-2pM9Bbuww&m =kavBu9H9umaS2l7Zwzp81cdwEgTt9UJa3f56hflvA8A&s=OVcQCPr1r2j4G99wMQoHdTA8Uh-Ec shq2eAiFJgY7Xc&e=> > Date: Fri, 10 Dec 2021 00:16:51 +0000 To: <EMC-PSTC@LISTSERV.IEEE.ORG <https://urldefense.proofpoint.com/v2/url?u=http-3A__EMC-2DPSTC-40LISTSERV.I EEE.ORG&d=DwMFAw&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=c9NR2mGfldr y-2pM9Bbuww&m=kavBu9H9umaS2l7Zwzp81cdwEgTt9UJa3f56hflvA8A&s=RMadKU3rlYA8dxMr L249JcSU5tJZbNiPKINvJSO5bwo&e=> > Subject: [PSES] unusual conducted emissions question Hi All, I have come across an unusual conducted emissions issue with a device where the impedance stabilization network for automotive testing itself causes a problem that will not let a class of equipment ever pass because of the network not the equipment itself. In this case, the power is cycled every second and the device that does that is behind the network as that is not being tested. The interrupted power goes through the network (12 Volts) and to the EUT. We got to the point where we replace the EUT with a DC load, 10 Ohm resistor in parallel with a 10 uF cap, to simulate the inrush current. That combination fails at the low end of the frequency spectrum a lot because the network, just a collection of L, C, and R is ringing at each power transition. Any thoughts? Seems like an artifact of the standard. The device cycling power is a lab circuit and not part of the test and so should be behind the network. Doug - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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