Isolation transformer questions
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Author:  Pooge [ May 9th, 2020, 12:46 pm ]
Post subject:  Re: Isolation transformer questions

tomp wrote:

Your statement that on a balanced feed the noise would be equal on both lines and out of phase is only true if the transformer is passing a noise from the primary that is also equal and out of phase. If the noise on the primary is being received from an external source that is radiating into the lines depending on the frequency and the layout of the feed lines it would most likely be in phase on both lines. That is where balanced circuits especially with transformers reject common mode signals, that is signals that are in phase so that in phase noise would not make it through the transformer, ignoring capacitive coupling for this discussion. If they were out of phase, that is differential, the common mode rejection (CMRR) would not reject them because they are not common mode and they would pass to the secondary.

The out of phase voltages that would appear across the center tapped secondary would not truly cancel at the center tap. The voltage at the center tap is the middle of what ever total voltage appears across the secondary. Even if you ground that point the voltage relationship between the leads does not change and no current will flow to ground since the secondary is isolated from ground except at the point where you have made the connection. There is no circular path for any current to return to either lead of the secondary, again ignoring stray capacitance. All you are doing is referencing the center tap to a ground but that does not change the secondary lead voltages with respect to each other. If you connect a load across the whole secondary it would see the whole noise voltage on the secondary. If you connect from one lead to the center tap it would see half of the total noise. On the transformer, you cannot connect the two leads to each other or you would have a direct short. In that case the out of phase voltages would cancel along with the production of a lot of smoke.

If you look at the circuit diagram of the Corcom filter, there are only two lines plus a ground. They do not differentiate between a hot and neutral line. In fact, when the Corcom is used at 240 volts on a traditional split phase, ie balanced center neutral house feed both inputs are hot. The key to the figure is that both lines have inductors and capacitors in combinations that shunt a lot of the noise from either lead and either polarity to ground regardless where it comes from.

"Balanced" has nothing to do with equal voltages. Balanced is about equal impedances to ground. If the Corcom filter is fed from a balanced source, common mode signals of equal value would theoretically cancel to 0v at ground in the filter. If a Corcom is fed from a grounded-neutral 120v supply, the neutral is grounded at the source, where the hot side is not. (However, with the neutral grounded at the source, the common mode noise may not be equal on both legs in the first place.) If a "balanced" 240v w/neutral feeds the Corcom with equal impedances to ground, common mode noise can be cancelled at ground.

Author:  tomp [ May 9th, 2020, 3:28 pm ]
Post subject:  Re: Isolation transformer questions

OK. Let's try to get to the source of this discussion. Any AC powered equipment will have to, as I have said before, deal with two types of noise, common mode and differential. If we start with the utility power transformer on the pole which is single phase, it is unbalanced at the primary, that is one side of the winding goes to one of the three phase hot leads and the other side to ground. However, coming off the secondary, the power is balanced with a center tap to neutral which is connected to ground. Ignoring for the discussion the warts in the transformers, the transformer cannot internally put out a common mode noise signal on the hot leads because it is balanced. The two hot leads of the secondary are always 180 degrees out of phase with each other otherwise your house wiring would not work properly. Each hot to neutral reacts to the differential voltage of the whole secondary and creates two voltages that are each half of the secondary (240 volts) and 180 degrees out of phase.

The utility transformer can and in fact does put out both differential power and noise signals. That noise will be highest line to line and half either line to neutral. So, when you are dealing with your line connected equipment you will have to deal with either the full differential noise coming off the utility transformer if it is wired for 240 or half the noise if it is wired for 120.

Since the utility transformer cannot put out common mode noise, where does it come from. Generally, it is picked up as RF noise on the lines either coming into your house or on the wires inside your house. Remember that devices in your house like motors can create differential mode noise feedback to the house power lines and also radiated common mode noise. If your device has a power transformer it will also reject that common mode noise to a very high degree. If it has a switched mode power supply the incoming voltage will be converted and filtered to DC which will then feed the switching supply, also removing incoming common mode noise. To supplement this rejection of common mode voltages which get converted to common mode currents, most competent noise filters like the Corcom mentioned utilize common mode chokes that reject common mode currents. A link to common mode chokes is included. Also included is a better circuit diagram. If you look at the diagram you will see that the filter first has a line to line capacitor to remove a lot of differential noise. Following that is a common mode choke to remove common mode noise. Then there is another line to line filter. Next is a line to line filter center tapped to ground which could further filter common mode noise to ground. After that are a line to line resistor, two chokes and another lint to line filter which I believe, although am not sure act like a pi filter to reduce RF effects that may have gotten through.

So, looking at this you can see that a filter like the V series Corcom can do a very effective job at reducing both common mode and differential mode noise regardless of whether the source is balanced or not. Also, with some of the suggestions of connecting one or more isolation transformer secondary connections to utility neutral defeats the main purpose of an isolation transformer, that is to provide safety from the secondary to ground. If you connect any part of the secondary to ground you are as likely to get a life-threatening shock as without the transformer.

https://www.jensen-transformers.com/wp- ... ersion.pdf
https://www.jensen-transformers.com/wp- ... /an004.pdf
http://www.thatcorp.com/datashts/AES626 ... put_IC.pdf

Corcom V series line filter diagram.jpg
Corcom V series line filter diagram.jpg [ 65.36 KiB | Viewed 4247 times ]

Author:  Pelliott321 [ May 9th, 2020, 4:42 pm ]
Post subject:  Re: Isolation transformer questions

I finished the rebuild with ground isolated sockets. I made sure of polarity integrity throughout. I also separated the secondary's with one socket on one secondary for 3.5A AC and the other secondary supplying two sockets just in case paralleling the secondary was the problem.
I still have the open ground probably because I did not follow Stu's schematic. I just want to know if I understand it correctly. The schematic shows the neutrals and grounds on the secondary side are bonded to house ground.
Since all grounds are bonded together at the chassis, then all I have to do is connect the secondary neutral to the chassis.

Author:  Stuart Polansky [ May 11th, 2020, 3:36 pm ]
Post subject:  Re: Isolation transformer questions


Author:  Pelliott321 [ May 11th, 2020, 3:38 pm ]
Post subject:  Re: Isolation transformer questions

but do i still have isolation

Author:  Pelliott321 [ May 11th, 2020, 3:52 pm ]
Post subject:  Re: Isolation transformer questions

yes that works
i know you must think i am a bonehead here but
i will go through life plagued with these thoughts of why?????

Author:  Pelliott321 [ May 11th, 2020, 6:27 pm ]
Post subject:  Re: Isolation transformer questions

isolast_web.jpg [ 76.96 KiB | Viewed 4206 times ]

Author:  Pelliott321 [ May 11th, 2020, 6:28 pm ]
Post subject:  Re: Isolation transformer questions

thanks all
I will do a listening test before and after
install to see if I can hear anything different

Author:  Pelliott321 [ May 12th, 2020, 5:10 pm ]
Post subject:  Re: Isolation transformer questions

Ok put the iso back into the system
I carefully listen to a hires trak a Holly Cole
DSP 128 She has a deliciously smoky voice that tends to pull one into the song.
I listen "They Can't Take that away From Me twice through with no ISO, no 10ga power cords
then hooked up the iso with my 10ga power cords and listened again
if there was a difference I would lean towards the iso, but in a blind test I doubt I could tell you which is which.
ok as I was putting the iso back in I noticed this:
iso9_web.jpg [ 39.64 KiB | Viewed 4172 times ]

so the original unit was wired without the ground floating on the output side
and the unit probably would have shown an open ground.
confusion again
its dead quiet, probably better than it has ever been

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