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Guru
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Low Capacitance Coupling in Mains Step-Down Transformers

01/21/2017 12:22 AM

Hello friends,

I am looking for some help from manufacturers of "Low capacitance coupling in mains step-down transformers" for instrumentation 10W to 100W power range. I find huge RF signal 2MHz to 20MHz range entering into secondary coil. It is perhaps from nearby air force station from their radars and city radio station communication and does not look like from mobile towers. It is being picked by the power transmission lines.

Similar problem also come from instrumental noise like drill machine having carbon contacts sparking and generating noise.

I am looking for very low noise in the DC supply <1mV peak to peak but this RF is ruining out everything by injecting greater than 800mV peak to peak burst noise.

One good idea looks like having a low capacitance transformer. Another idea is to use mains filter. Transformer shield may give capacitance coupling so nor sure is it will be of advantage. Some people use only two wires in power line and Earth may not be available.

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Guru

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#1

Re: Low capacitance coupling in mains step-down transformers

01/21/2017 2:13 AM

Use capacitance multiplier to get less than 1mV ripple.

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Guru
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#6
In reply to #1

Re: Low capacitance coupling in mains step-down transformers

01/22/2017 2:36 AM

Capacitance of the transformer couples RF to output for which normal rectifiers can't respond, pass this RF through diode capacitance. Common line gets it directly from transformer.

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Guru

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#2

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/21/2017 1:00 PM

Hell just insert a line (RFI/EMI) filter between the mains and the power-supply transformer, preferably as near the transformer as you can, and use any transformer you like. At those frequencies (2-20 MHz) EMI can enter by any number of means, not just the mains, but also through long leads which can act as aerials whether connected or not. Put everything in a metallic enclosure and use RFI-suppressing feedthroughs as applicable. Ferrite beads also help, especially against higher frequency RFI.

Nearby radars and mobile (cellular radio) towers operate at much higher frequencies than you cited and may require more tightly-shielded enclosures including RF gasketing on enclosure access covers and so forth.

At the other end, for very-low-frequency or inductively-coupled EMI, use mu-metal/supermalloy enclosures designed for the purpose.

There are tons of material online covering every aspect of RFI shielding/suppression, everything from elementary principles to Tempest compliance. Maybe start here?

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Guru
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#7
In reply to #2

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 2:47 AM
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Guru

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#9
In reply to #7

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 3:09 AM

I don't get it: you tell me that line filter in my photo doesn't reject CM noise but then show me this nearly identical design where it clearly does, the only difference being that centre-tapped capacitor to ground?

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Guru
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#10
In reply to #9

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 3:19 AM

That was a mistake. In fact the inductance coil is to reject common ode noise and capacitor is to reject differential noise. You are right. Of course there are two separate types of inductance coils for common mode and differential both or the same can be wired both ways.

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Guru

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#11
In reply to #10

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 3:35 AM

Insert one in the mains immediately ahead of your power transformer and measure the noise on your secondary. What effect does it have on the noise level? If the noise is still there it's sneaking in through another path.

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Guru
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#17
In reply to #11

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 10:45 PM

The other point of noise getting in is the oscilloscope itself. I am using TDS2024 Tektronix DSO which has input filter but not sure if earth is connected to signal Ground of the probes. I will check its continuity by a multimeter.

Yes, I will try different combinations of the input filters. I will also test with and without using the Earth line filter. Oscilloscope is wired with the Earth line in the filter with two capacitors from live and neutral for sure.

I also have to see if capacitors have fast response as frequency of the noise is very high. 1nF, 10nF and 100nF ceramic capacitors will be tried. I can also use series inductors to slowdown the frequency response before common mode coil is inserted. It may also work against if it causes mismatch in inductance values. Line inductance are not alike and hence common mode noise may not be exactly alike or deferentially matched. For common mode coil to work, it must be identical in phase.

Capacitors do the averaging but don't kill the noise of unipolar type which often happens with impulse noise. Reduction i peak noise voltage is expected.

I have to do lot of work for this before I can present any results. Oscilloscope has minimum 2mV/div recording and lowest detectable change may be 0.2mV.

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Guru

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#21
In reply to #17

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 11:31 PM

"Line inductance are not alike and hence common mode noise may not be exactly alike or deferentially matched. For common mode coil to work, it must be identical in phase."

It is nearly always a superposition of both differential and common-mode noise. Some part will be CM even if the sum is not. The part that is not CM is by definition differential and is addressed by that part of the filter. The part that is CM is addressed by its respective part.

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Guru

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#3

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/21/2017 11:39 PM

"I find huge RF signal 2MHz to 20MHz range entering into secondary coil."

How do you detect an RF signal "entering into" the secondary coil? Do you really mean you find that RF signal when viewing the output of the secondary coil?

If it is an unshielded transformer in an unshielded enclosure, the secondary may be acting as a magnetic pickup. If this is the case, no amount of line filtering prior to the transformer will do any good.

A line filter after the transformer should help, as long as everything beyond that filter, including any wiring between the filter and the shielded enclosure, is shielded, as indicated by AW's post.

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Guru

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#4
In reply to #3

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 2:06 AM

Put the whole monty inside a shielded enclosure, transformer included and put a line filter ahead of the transformer, one of these for example. Cheap, effective, and readily available.

At those frequencies I seriously doubt it's magnetic coupling per se. That kind of coupling, if it's going to be a problem, will typically be one at very low frequencies on the order of kHz and below.

It would be simple enough to test: just stick a line filter ahead of the transformer. If it seriously attenuates the noise on the secondary, it's coming in through the mains. If it doesn't, it's coming in via another path and he'd best put the whole thing in a shielded enclosure.

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Guru
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#5
In reply to #4

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 2:31 AM

Hello Andrew Westman:

This filter design is a nice idea for differential noise but does not work on common mode noise picked on Live and Neutral together and gets through transformer capacitor.

I have not seen any mains filter which has both differential and common mode inductive filter even though such filters can be constructed using discrete components.

In your circuit image, there is no filter capacitor to Earth line and it looks like of an optional use. I usually get filter capacitors wired to Earth from both Live and Neutral lines inside filters which is sometime good for creating problem when Earth is faulty. In medical use equipment Earth is totally isolated for safety.

Thanks for good information.

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Guru

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#8
In reply to #5

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 2:58 AM

The filter shown rejects both differential and common-mode noise. It wouldn't be a commercially-viable product if it didn't.

Rejecting common-mode noise is exactly what that transformer does in the centre of the filter diagram shown in the photo. That is why Live and Neutral are co-wound on the same core. The capacitors across the Live and Neutral cancel differential-mode noise.

Murata explains it rather well (pdf)

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Guru
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#18
In reply to #3

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 10:47 PM

I use Tektronix TD2024 DSO (oscilloscope) 2GSPS.

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Guru

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#20
In reply to #18

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 11:30 PM

I assumed you were using an oscilloscope to see the RF.

I suspect it is a matter of semantics; thinking in one language and writing in another. I was bothered by your statement: "I find huge RF signal 2MHz to 20MHz range entering into secondary coil." [my emphasis]

I presume you connect the 'scope across the two terminals of the secondary of the transformer. In that case you will be observing the voltage/current generated by that secondary coil; I would describe that as a signal leaving the coil, not entering it.

Of course the signal measured across those secondary terminals may be modified by activity downstream of the transformer, which could indicate some oscillation somewhere in subsequent circuitry.

As others have indicated, placing a line filter at various locations before and after the transformer, and observing the resulting signals, should make it possible to isolate the source of the RF.

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#22
In reply to #20

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/23/2017 12:05 AM

Yes, signal was measured at the secondary of the transformer and added with more circuit of bridge rectifier, 1000uF filter cap LM7805 regulator with another 10uF filter cap. The 5V DC usually has <2mV peak to peak noise but burst of noise comes to it from mains. Current was loaded from 10mA to 100mA range by a resistive load.

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#24
In reply to #20

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/23/2017 7:55 AM

As I have stated earlier, RF enters through mains wires from various sources and is input to the transformer which gets coupled through transformer capacitance nd not magnetically.

RF is also there as EM wave as field from some Radiation transmission sources. There are radio stations, Mobile towers and LF, MF, HF Radars of high power within few km range. They generate burst of RF of definite frequency and have signature.

Some HF noise comes from bad equipment attached to the mains power source whenever they are operated. This is well known source.

My concern is all EM and RF noise above 1MHz to 20MHz range only. I am not worried about LF ripple.

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#12

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 10:07 AM

While installing a line filter will help, your solution lies with a DOUBLE SHIELDED ISOLATION TRANSFORMER. Such isolation transformers can be built with less then 1pF capacitance between the windings and offer 140db isolation.

Such transformers have a shield around the primary winding connected to GROUND, and a shield around the secondary winding connected to your COMMON.

Once you get a proper isolating transformer, you will find that your noise has not reduce by as much as expected, . . . and this is caused by the T&M equipment you are using. ALL of your T&M equipment MUST ALSO BE POWERED FROM THE SAME, OR A SIMILAR ISOLATING TRANSFORMER so as to not act as noise sources.

And now that you have the experiment and the T&M equipment all powered from proper isolating transformers, you will find that you noise is till higher than you desire. And this is turn will force you to conduct your experiment inside a shielded room, preferably a "double shielded room", where everything inside the room is powered from a proper isolating transformer. Low and behold, . . . your noise is well below 1 mV peak.

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Guru

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#13
In reply to #12

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 10:40 AM

Don't forget the part where he ships the double-shielded room with every unit.

At some point this thing will have to work in the real world.

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#14
In reply to #13

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 1:36 PM

Good point Andrew, . . .

Although I have seen "transportable shielded enclosures", . . . when working with sub-millivolt signals over the frequency range mentioned here, the "WORK" is shipped to the shielded room facility.

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Guru

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#15
In reply to #14

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 2:57 PM

Apart from using a shielded transformer, he makes no mention of using shielding anywhere else.

When the engine light comes on in my car, my first impulse is not to replace the engine. Maybe it's an oxygen sensor that needs replaced (which was, in fact, the case). I.e., start with the cheapest candidate solution first and work up in order of expense/complexity. For example, he might stage it thus:

1) Insert line filter between mains and power transformer and measure the noise (taking care his T&M gear isn't actually contributing to the problem, as you said earlier). How's the noise now? Reduced sufficiently for his purposes? If not...

2) Shield everything in an enclosure. He could start with an aluminium econo-box and see how it goes. It may be sufficient but he won't know this until he tries. How's the noise now? Reduced sufficiently for his purposes? If not...

3) Use an isolation transformer as you described. How's the noise now? Reduced sufficiently for his purposes? If not...

4) Shield everything in a double-shielded enclosure, possibly even one magnetically shielded (mu-metal) in addition to its usual shielding function if he's also concerned about ELF coupling. How's the noise now? Reduced sufficiently for his purposes? If not...

5) Tell the customer he needs more money. Lots more money.

6) Buy a Lambo Veneno and nice place on the French Riveria. FTW

At each step he's going to have to test this in a noisy environment; else how would he know it's doing the job? If his circuit isn't actually generating noise itself, testing it in a shielded room will only tell him how well the room protects his project from RFI, not how well his project protects itself from same.

Now he could hire the appropriate T&M gear and a suitable RF chamber to test against a wide range of frequencies under controlled conditions and this is also done, say, to meet certain certification requirements, but it is very expensive.

He also mentioned something about a missing ground connection (ie, 2-wire mains power) and also isolation typical of some medical electronics.

Perhaps if the OP were more forthcoming as to the actual application? I await with bated breath.

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Guru
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#19
In reply to #15

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 11:01 PM

Points well taken. I use 3mm thick Aluminum enclosure but want to switch to 1mm thick MS enclosure. I will try to give some space between transformer and other circuits to avoid magnetic field interference.

Mains current is less than 500mA and I am going to select filter which is rated for less current.

I have torroidal transformers with shied between primary and secondary as single shield. I may have to ask for double shielded one specially manufactured for this application.

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#16

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/22/2017 4:02 PM

"Some people use only two wires in power line and Earth may not be available."

So is it for you or somebody else? The filters shown need an earth ground to work properly. The common-mode aspect does not include the safety ground, though. You can wrap the whole power cord through a large toroid, providing common-mode rejection for all 3 wires. Tape-wound toroids are for low frequencies, so you need a ferrite one.

A shield in the transformer between the primary and secondary (tied to earth ground) should get rid of the RF signal too. Low capacitance transformer is not likely to work at those frequencies.

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#23
In reply to #16

Re: Low Capacitance Coupling in Mains Step-Down Transformers

01/23/2017 12:14 AM

It is our own experiment.

Not always I use Earth wiring for signal filtering. Experiments are for medical use and isolation is considered better. However, I can use Earth line to test the advantages. I have Earth rod and a Copper Plate of 1 sq. meter at about 2m depth. I do not load the earth line.

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