EMRFD Message Archive 8807

8807 2013-07-13 17:35:37 101 Common Mode Current on DC line? Message Date From Subject Hi, Normally we find literature discussing about Common Mode Current on AC lines. Is there such thing as Common Mode Current on DC lines as well? Regards, Amin Amin: Think about the AC line for a moment. You have two wires delivering power. Imagine a 3rd wire, a reference voltage, or "ground," which is centered between the two. This is best illustrated by a 220VAC line in a typical home (in the US). With this illustration, the AC power is being delivered by V_ac on one wire and -V_ac is on the other wire at that instant moment in time. "Alternating Current" means each signal is on a sinusoid, continually changing. The "common mode" on these wires would be a voltage which is delivered identically by the two wires. If this is Vn, you would see +Vn on one of the two wires and +Vn on the other two wire. "In common." Sometimes we may have a reason to superimpose something common onto a differential pair. In general though, a differential pair of lines is run to allow signal (or power) delivery with anything in common to be cancelled. Noise that is picked up due to an interferer can be rejected pretty easily, at least the portion that the two lines have in common. So when you ask about common mode current on a DC line, we have to ask about what we really mean here. If you are talking about a single power supply line, no, the term doesn't really mean anything. If you have numerous power supplies in a system, and each of them have the same interference applied to them, well, I guess those would be in common. But in general the term does not make much sense. Books to consider reading on common-mode and differential mode operation include "High Speed Digital Design" by Johns > Normally we find literature discussing about Common Mode Current on AC > lines. > > Is there such thing as Common Mode Current on DC lines as well? Sure. You can have common-mode RF current on any set of wires, whether they were designed for AC or for DC. You can have common-mode DC current too, but that's not a concern for radiation. Let's say you have a pair, and the current on one wire is i1, and the current on the other (in the same physical direction) is i2. (i1+i2) is the common-mode current. If the same current goes out as what comes back, then i1=-i2, so (i1+i2) = 0. If the currents aren't equal and opposite, then you have some common-mode current. That of course requires another current path somewhere else, since even the common-mode current should make a complete loop, if you don't want the electrons to build up on one end. I am making a distinction between common-mode current, and common-mode voltage. Andy > Is there such thing as Common Mode Current on DC lines as well? > > Regards, > Amin The word "lines" in the question is plural, and won't multiple lines carrying DC current have an average value that is functionally like a common mode current for AC? For example a red wire carrying the +12 to a mobile rig, with current returned to the minus side of the battery with a separate black wire: if there is no other path for rig current to return to the battery the current in the red and black wires will be the same value, flowing in opposite directions. Taken as a pair, the common mode current is zero. So those two wires could be wrapped through a ferrite core can isolate RF on the rig from the car's DC system without concern that when the rig draws 20A the ferrite will saturate: it won't, because the net current is zero, and there is no net flux in the ferrite. RF from the rig, in this case, could be coupled to the grounded antenna with a 1:1 transformer and the rig connected to the car chassis with a large bypass capacitor. W7AAZ In the EMC world common mode currents are typically what make cables radiate. These are unintentional currents that get coupled into the cables. The cables can have AC, DC or no "intentional" current. ================================================================== Mike Mayer mwmayer@tds.net -----Original Message----- For DC, a power supply might be called a "floating" supply. A battery is an excellent example, where the DC current supplied from one terminal exactly returns to the opposite terminal. No common-mode current flows in connections to a perfectly floating battery. Floating sources are required when you want to stack them in series to get higher voltage. Hi Roger, Andy, Bill, Mike, Glen & Others. Thank very much for your supports. Sorry for the late reply. Just come back from outstation. Best Regards, Amin