Electronics > Projects, Designs, and Technical Stuff

Power Supply Line Filter with Trifilar Windings

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duak:
JDW, about your last question.  If the chassis is not connected to earth, it will tend to float towards half the AC voltage between line and neutral by virtue of C33 & C37 that form a capacitive voltage divider.

Also, regarding measuring LF2's value.  You shouldn't have to unsolder it if all the connections on its left are not connected.

JDW:

--- Quote from: duak on March 09, 2020, 05:00:21 am ---You shouldn't have to unsolder it if all the connections on its left are not connected.

--- End quote ---

I measured LF2 on 4 different Analog Boards (boards used inside the Macintosh 128k through the Plus) using my DE-5000 LCR Meter and TL-21 alligator clip adapter, with meter set to 1kHz (and I confirmed the same values for 100Hz and 120Hz frequency settings too):

1. 82µH (bifilar, no Green Earth GND, 1986 board)
2. 57µH (trifilar, 1983 board)
3. 47µH (trifilar, 1986 board)
4. 35µH (trifilar, 1983 board)

I measured the same value above for each winding on the same coil.

Note that the word at the very bottom of (3) & (4) above is COILCRAFT.  Also note the very confusing labels on (1) & (2) above, confusing because the labels are the same even though (1) is a trifilar and (2) is a bifilar!!

jonpaul:
Hello: Unusual to see two CM chokes in series.

Certainly the Japanese lack of any ground affects EMI compliance.

But in 1986, CISPR and other EMI compliance was not as strict.

I cannot understand the designers motivation of the Apple ii PSU, but I can say that all modern SMPS I have designed or seen use a 2 wire CM noise choke and that the ground (Earth) green is always direct to chassis. Just looping thru the trifilar means risk of interruption of ground path, e.g. bad solder on PCB.

Thus for safety compliance the mains green (or Green/yellow) wire  is directly to chassis.

Just the ramblings of an old retired EE

Jean-Paul

jonpaul:
All the inductances are about right for a CM choke: The exact values depend on line Zo, Y capacitors to earth and required compliance for conducted emissions, vs the PSU CM noise spectrum.

JP

poorchava:
This is sometimes needed. I recall one project in our company, where passing all 3 mains conductors (L, N, PE) through a CMC finally got us through conducted emissions. As is often the case with EMC: wo don't actually have a solid theory why this was so, but it worked :)

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