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Power Supply Line Filter with Trifilar Windings

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JDW:
Below are 2 photos of AC line filter "LF2" from two different Apple Macintosh 128k~Plus Analog/PSU Boards. The newest board is stamped 4192 (1986 PCB), which I assume means 42nd week of 1992, and uses the LF2 without the green Earth wire. The other PCB is a 1983-84 version stamped 0792, which I assume means 7th week of 1992, and you can see it has the green earth wire wound inside LF2. Indeed, most every Analog Board I've seen has the Earth wire wound in LF2 (i.e., trifilar windings). Search this PDF for "LF2" to see the schematic, which also shows it to be trifilar.
 
Why do you think the green Earth ground wire was not wound into LF2 on the 4192 board?
 
   

The 3-pin power connector mounts on the PCB, but the Earth pin has no PCB trace. Regardless of which board version we examine, the green Earth ground wire coming from the power cord screws down to the metal chassis of the computer so the chassis is Earth grounded. My question is centers on why that green Earth wire does or does not go through LF2.

jonpaul:
Hello: This is a common mode EMI choke, and cancells the currents flowing common to both line and neutral (balck/white)

The green ground isnot needs.

Jon

bsdphk:
The ferrite is a formidable impedance in high dV/dt scenarios so maybe the idea is to cause incoming surge energy to arc from the live/neutral to the pe-wire at the toroid, rather than further into the PSU, without expending a MOV or spark-gap ?

That could make a difference when a drunk driver hits the utilitypole and the HV falls on the LV and when utility poles get hit by lightning strikes.

JDW:
So basically jonpaul thinks the trifilar Earth is unnecessary, while bsdphk is arguing it is necessary for drunk drivers and lightning strikes.

The schematic shows the trifilar design, which implies the original designers thought it was necessary:



Also note the presence of the second LF1 filter in that same schematic, which means LF2 is not the only filter in this power supply.

Even so, we have a case where only a bifilar version (no green Earth ground wire wound inside) is used.  So was that use of a bifilar merely a cost savings measure, or could it really have been later decided that the triple winding was unnecessary?

fcb:
I would argue that LF2 isn't a common-mode choke. Just an RF choke.

The next choke LF1 is likely the common-mode.

The engineers deemed it necessary (probably after testing at EMC) - not seen it before in that configuartion.

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