Author Topic: Ultra Low Noise Lab Power Supply Extender for your existing lab supply  (Read 25796 times)

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Offline mzzj

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #50 on: August 23, 2018, 11:45:36 am »
I have been eyeing(eying?)  those small R-core transformers also for similar projects.

Would be interesting if you measure pri-sec parasitic capacitance for the R-cores and your toroid.
WAG that toroid is 10 times worse.
 

Offline splin

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #51 on: August 23, 2018, 01:58:26 pm »
Are you sure the screen is a proper electrostatic screen? Looking at this similar transformer:

 https://www.ebay.co.uk/itm/115V-230V-30W-R-Core-Shield-Transformer-Audio-Amplifier-DAC-12V-12V-9V-9V/323400748432?hash=item4b4c2fcd90:g:xmsAAOSw~SlbHIxm

I wouldn't be surprised if the 'screen' connection was to the overall copper wrap which would be worse than useless by increasing the inter-coil capacitance.

How would a proper screen be arranged on an R-core transformer given that it would need to be placed between each coil and the core as well as between the cores. There clearly isn't a screen fully encapsulating each winding as the ends of the windings are clearly visible in the photos.

Is it not usual to have independent primary and secondary screens?
 

Offline mzzj

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #52 on: August 23, 2018, 05:00:48 pm »
Are you sure the screen is a proper electrostatic screen? Looking at this similar transformer:

 https://www.ebay.co.uk/itm/115V-230V-30W-R-Core-Shield-Transformer-Audio-Amplifier-DAC-12V-12V-9V-9V/323400748432?hash=item4b4c2fcd90:g:xmsAAOSw~SlbHIxm

I wouldn't be surprised if the 'screen' connection was to the overall copper wrap which would be worse than useless by increasing the inter-coil capacitance.

How would a proper screen be arranged on an R-core transformer given that it would need to be placed between each coil and the core as well as between the cores. There clearly isn't a screen fully encapsulating each winding as the ends of the windings are clearly visible in the photos.

Is it not usual to have independent primary and secondary screens?

AFAIK r-core transformers are usually/often wound with primary and secondary on separate legs/bobbins. This gives good isolation and low interwinding capacitance but larger leakage inductance and stray magnetic fields.
Usually you have to select balance between interwinding capacitance and leakage inductance, on 50 Hz operation and low noise measurement equipment I rather have low capacitance.

Super easy to also add your own static screen between bobbins, just don't make it shorted turn  ^-^
For best results you can also consider grounding the core.

OT: didn't know how the R-core transformers are wound, pretty cool:

edit: after more googling I think the primary is split between both bobbins. Shielding between windinds  is still easy compared to toroid transformer.
« Last Edit: August 23, 2018, 07:01:42 pm by mzzj »
 

Offline splin

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #53 on: August 23, 2018, 07:57:52 pm »
AFAIK r-core transformers are usually/often wound with primary and secondary on separate legs/bobbins. This gives good isolation and low interwinding capacitance but larger leakage inductance and stray magnetic fields.

The direct interwinding capacitance might be quite low due to the bobbin spacing but both coils will have relatively high capacitance to the core so they are in series. Grounding the coil will help stop mains noise coupling to the secondary but not mains earth borne noise.

However, inserting screens between each coil and the core might not be too difficult? As I understand it the primary shield is normally grounded and the secondary is connected to guard?
 

Offline branadicTopic starter

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #54 on: September 02, 2018, 02:53:13 pm »
Time to go on...
Attached a comparison of the new R-core transformer (65VA) with ring core transformator (50VA) type RKT 5012 with additional mu metal shield.

-branadic-
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Offline splin

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #55 on: September 03, 2018, 02:19:23 pm »
Hi branadic, what is the screen wire connected to on the R-core? Is is to a copper foil wrapped around the transformer or does it have a proper electrostatic screen(s)?

Are you able to measure the primary to secondary capacitance easily?
 

Offline branadicTopic starter

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #56 on: September 03, 2018, 03:27:05 pm »
post 12bit

Hi branadic, what is the screen wire connected to on the R-core? Is is to a copper foil wrapped around the transformer or does it have a proper electrostatic screen(s)?

Are you able to measure the primary to secondary capacitance easily?

I can't tell you what the screen wire is connected to. I would need to remove all tapes to see that, as it's not visible and would thus damage the transformer.
The only thing I currently have at hand to measure capacitiance is a VNWA. If you give it a few days I can measure it at work at a reasonable instrument.

-branadic-
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Offline branadicTopic starter

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #57 on: October 01, 2018, 05:14:58 pm »
Measured capacitance of the toroidal vs. R-core transformer...

toroidal transformer with additional mumetal shield
- 99.7pF

R-core transformer
- black to brown1:37.4pF
- black to brown2: 63.1pF
- black to yellow1: 41.7pF
- black to yellow2: 60pF

- red to yellow1: 43.6pF
- red to yellow2: 71.6pF
- red to brown1: 43.5pF
- red to brown2: 76.1pF

- black to screen: 66.9pF
- red to screen: 82.5pF

Measurements were proven to be reproducible and performed @ 1MHz.

-branadic-
« Last Edit: October 11, 2020, 05:30:51 pm by branadic »
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Offline Zorc

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #58 on: October 13, 2019, 09:51:05 am »
Very interesting project, thank you!
Have you made any more modifications and are you satisfied with the final result?

I've been considering building a power supply based on the LT3045 as well with a few fixed voltages but with adjustable current limit (and current display)
 

Offline Wolfgang

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #59 on: October 13, 2019, 10:23:04 pm »
post 12bit

Hi branadic, what is the screen wire connected to on the R-core? Is is to a copper foil wrapped around the transformer or does it have a proper electrostatic screen(s)?

Are you able to measure the primary to secondary capacitance easily?

I can't tell you what the screen wire is connected to. I would need to remove all tapes to see that, as it's not visible and would thus damage the transformer.
The only thing I currently have at hand to measure capacitiance is a VNWA. If you give it a few days I can measure it at work at a reasonable instrument.

-branadic-

Hi Branadic,

For an ultrastable, ultra low phase noise oscillator project I needed a *really* quiet PSU. After some homebrew attempts, I ended up with a Keysight B2962 SMU with ultra low noise filters. This part does make a difference, but it is expensive.
Maybe you can clone the output filters to improve your design ?

Regards
  Wolfgang
 

Offline branadicTopic starter

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #60 on: October 15, 2019, 10:22:21 am »
Hi,

didn't make any further modification to the setup, but am using the power supply now and then.

-branadic-
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Offline niner_007

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branadic, can you share pictures of your bare LT3045 PCB? I’m currently working on a low noise supply based on LT3042, the topology is inout filtering and EMI - transformer - rectifier - capacitors- LT3081 pre-regulator - LT3042 regulator; one PCB with input filtering, pre-regulator, another PCB, stacked on top, with filters and connectors for the two independent LT3042 PCBs, eventually I plan separate metal cans for transformer, pre-regulator, and LT3042; attached is two of the PCBs
« Last Edit: July 12, 2020, 10:44:50 pm by niner_007 »
 

Offline Vovk_Z

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These LED pane meters powered from the same line will make output noisy (somewhere at kHz or tenth on kHz).
 

Offline niner_007

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I believe the adjustable resistor and long wires to it, also introduces noise. The reason I asked for the bare PCB, was because LT3042 requires special layout considerations, LT/AD, documents the layout guidelines in the data sheet, if you treat it as a normal regulator it won’t have the advertised performance.
 

Offline branadicTopic starter

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Re: Ultra Low Noise Lab Power Supply Extender for your existing lab supply
« Reply #64 on: November 29, 2020, 10:22:30 am »
Just some update.
I noticed quite large noise injected by the blue LED panel meters, they are based on ADCs and powered by the rectified and unregulated input voltage to LT3045, not by their regulated output voltage. Thought, PSRR should do it, but the ADCs themself introduce noise.
Thus, I've replaced them with analog panel meters. It looks ugly, but that's how you're doin' to achieve low noise.
Now, even a DIY 10V reference based on LTZ1000CH shows low noise.

-branadic-
« Last Edit: November 29, 2020, 10:25:25 am by branadic »
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Offline EC8010

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I have an interest in a low noise power supply, but I looked at the problem from the other end. This thread has primarily been concerned with the regulator and achieving low differential mode noise, whereas I looked at the transformer and concerned myself primarily with minimising common mode interference from the mains. To minimise common mode interference from the mains, you need to minimise Cps (mains transformer primary to secondary capacitance), then use that Cps in a potential divider by adding (say) 10nF from each end of the secondary directly to chassis. Nothing new there. So it comes down to minimising Cps...

Traditional mains transfomers using EI stampings often had an electrostatic screen between primary and secondaries. This wasn't to pander to low noise electronics but to interpose an earthed conductive barrier between mains and the user. Thus, unless you asked explicitly, your E/S screen would be a layer of wire, not foil. With foil, you could expect Cps anywhere between 2pF and 10pF for a 50VA transformer. Without an E/S screen, about 250pF.

Once upon a time, there was a company called Topaz who made low Cps mains isolating transformers. They claimed capacitances in single-digit fF. That gives you a lot more attenuation. Very occasionally, a Topaz transformer turns up on eBay. Unfortunately, they're expensive and usually in America. Many people have no idea of how to pack a heavy but fragile transformer - one arrived damaged and required a day's work to repair.

Now to the point of my post. You can make a fairly low Cps transformer yourself. I started with a 100VA DIY transformer kit that had a split bobbin already wound with its primary to four turns per volt. If you make a splittable former, you can wind your secondary, split the former (and before the coil moves) wind polyester tape tightly around it. You then cover it entirely in self-adhesive copper tape as sold to gardeners for deterring slugs, making sure you don't create a shorted turn. You now cut the outer flange of the kit's bobbin away on the empty chamber and slide your screened coil on. Make and fit a new flange, fit stampings, assemble transformer, adding copper tape from screened coil to transformer chassis/stampings. I achieved Cps 0.06pF. Call it 0.1pF because at that point my measurement technique was not good.

I later improved construction and measurement (multifrequency rather than single frequency) and achieved (with considerable difficulty) Cps = 8fF +/- 60% (yes, I know, a large uncertainty, but fF are difficult to measure!). But what I also discovered is that the external wiring is crucial. There's no point in making a beautiful transformer if there's the slightest sniff of capacitance between input and output wiring. Mains wiring has to be totally screened, with copper tape over any gaps in the screen. To conclude, it's worth fitting a screened secondary to a split bobbin mains transformer to get Cps <0.1pF, but you must also totally screen the mains wiring with no gaps whatsoever. And yes, analogue meters - no ADCs and digits to clatter.
« Last Edit: May 27, 2024, 08:41:37 pm by EC8010 »
 
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Online Echo88

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So far i cant recall that someone achieved and measured this low Cps in a DIY setup, very nice.
I have a few questions:
Can you get into more detail on your Cps-measurements/what gear you use to reliably measure in the fF-range?
Do you also do CM-measurements? I found those at least in my low CM-noise measurements of specialised DCDC-converters (LT1533/Pickering-Design/Koax-shielded Keithley transformer) very difficult at very low ranges.
Can you share some pictures to showcase how you avoid shielding gaps?
Did you also do Cps-Measuremens on your topaz transformer, to check its fF-claim?
 

Offline EC8010

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As you hint, measuring down to fF is not trivial. Measuring Cps is effectively a common mode measurement. After quite a bit of shoddy experimentation, I found that applying between 10MHz and 100MHz common mode was the way to do it. You don't get a sensible result <10MHz because your signal is too small. I used the 'scope in FFT mode (and externally triggered) in order to get a nice narrow bandwidth at the frequency of interest and reject noise/interference in my measurement. A spectrum analyser with tracking generator would be even better. The interesting thing (well, I think it's interesting) is that up to 100MHz Cps is a very good model of the common mode coupling between primary and secondary of a transformer. But you do need to do measure at many frequencies in order to put a line through the results and ignore all the peaks and dips due to RF misterminations and reflections. I shorted primary and I shorted the secondary. I applied the RF via a through termination between chassis and shorted primary. I used 300MHz oscilloscope to measure between shorted secondary and chassis, with cable terminated at oscilloscope (oscilloscope is reasonably flat up to 100MHz, so that's what set my 100MHz limit). I folded a pair of aluminium right angles and bolted them over the transformer so that they (just about) touched to form a box with open ends. I fitted an input BNC on one side of the transformer, connected to the shorted primary by as short a wire as possible. I did the same on the opposite (secondary) side. The folded aluminium was quite long and overhung the transformer by about 70mm. But there was still a big hole at each end, so I folded some thick aluminium foil over each end and taped it down with self-adhesive copper tape. I taped copper tape over every gap. I learned about copper tape going through EMC compliance testing. That dealt with the practical side of the measurement. Generator was Agilent E4420B and oscilloscope was Tektronix MSO54

I plotted received voltage against frequency on a graph. Also on the graph, I plotted the expected signal from a capacitor feeding a 50R load. I adjusted capacitor value until model and measurement overlaid. (Well, "overlaid" is perhaps an exaggeration, but I told the Excel Solver to twiddle C for a least squares fit.) To get the uncertainty, I calculated standard error of the fit between model and measurement.

As for getting an 8fF transformer, that was a lot of work. The Topaz transformers individually screen their primary and secondary with copper foil, but that's not good enough for a safety barrier, so they also add a sheet metal screen between primary and secondary. That would become a shorted turn, so they use a pair of earthed sheet metal fingers coming from either side, but with the fingers insulated to avoid a shorted turn. It's effectively a third screen (albeit not terribly effective). To make my 8fF transformer, I didn't have a coil winder at the time, so although I was happy to hand wind a secondary on its bobbin (on a stick) and count turns, I wasn't happy to do a primary. So I improved the finger screen. I found that tiny little gaps were enough to increase coupling between primary and secondary, so I kept adding copper tape everywhere I could see daylight looking from one side to the other. I had to add a lot of insulating shims to ensure that I didn't form a shorted turn between finger screens and stampings. I can't show you a photograph because after all that work, the transformer had to do something useful, so it was put in a die-cast aluminium box with a folded aluminium screen over it and copper tape over gaps to prevent incoming mains being coupled to the output. I'm afraid an LM317 regulator was then used - my aim was to squash common-mode. However, my technique of improving the finger screen was a poor way of attacking the problem. The way to do it is to do what Topaz did and individually screen primary and secondary. Now that I have a coil winder with turns counter, I could do that. But what stops me are the safety issues. In modifying a transformer kit that already had reinforced insulation between primary and secondary, my modifications didn't add any safety issues. But winding a primary, insulating it, and screening it very definitely does. So that's why I'm suggesting that you only screen the secondary and can expect to achieve < 0.1pF, but even at that capacitance, you need very careful screening of the mains wiring.

I'll dig around to find the 8fF spreadsheet results graph and post it later. Found it! The "Thing" transformer was a commercially made 50VA transformer with foil E/S screen. You can see I was able to get reliable measurements down to quite a low frequency with 2pF of capacitance. With the "Copper foil wonder", 8fF made reliable measurements a lot harder, and they only appear > 10MHz. Note all the peaks and dips for both sets of measurements due to RF misterminations. It's Version 3 because it took me three attempts refining my measurement and construction before I got a good result.

On the foil round each winding, when I came back to the start, I put a turn of polyester tape over the copper so that I could overlap the foil without shorting. But I then put another turn of polyester tape over that overlap and covered it with more copper tape to create a radio frequency maze. Otherwise, RF would have leaked through the thickness of the polyester tape that prevented the shorted turn. Yes, I know polyester tape is thin, but I repeatedly failed EMC compliance due to an unregarded 0.1mm gap. Once spotted, a little copper tape allowed a pass, so I now know to be very careful about gaps. If (using a powerful torch) you can see light, you have a gap and it needs copper tape - it's as simple as that.

I didn't test the Topaz and I should have done. Why not? Because I needed to use it and I'd already added an MOV across the input, and 4n7 Y capacitors from the end of each winding to shell. But from the measurements I've made I see no reason to doubt their claim - initially surprising though it is!

« Last Edit: May 26, 2024, 04:01:28 pm by EC8010 »
 
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Offline 5U4GB

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It turned out that the hum that I measured was due to the cable between DSO and ULNPS supply I used for measuring. Replaced RG58 coax by twisted teflon cable and picked up hum is now much smaller but not fully vanished.

I posted some screen shots a few weeks back of a similar problem, a decent-enough linear power supply with noise coupled onto the output cable from a nearby SMPS.  So I think your limiting factor isn't going to be the power supply itself but how much noise is present in the environment and how well you can keep it out of anything you're working with.
 

Online Echo88

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For those considering r cores for their shields and low coupling heres a teardown of one specimen: https://www.eevblog.com/forum/chat/are-r-core-transformers-a-thing/msg5535891/#msg5535891
Apparently in my case the manufacturer thought: The buyer wont notice a proper done shield, so why bother when its not even visible and one can cut cost.
 

Offline harerod

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branadic, thanks for the inspiration.
 
Does anybody have a recommendation for a symmetrical version of this, i.e. a low noise opamp supply?
 
A device like this would come in handy as a filter front end for my bench power supply, a TTi EX345RT, which is rather versatile and adequate for my usual work.
 

Offline Gerhard_dk4xp

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Another factoid on commercial power supplies and pasive filtering:

<     http://www.hoffmann-hochfrequenz.de/downloads/Noise_Measurements_On_Some_Laboratory_Power_Supplies.pdf     >

The usual 3042 goes down to 1-2 nV/rt Hz, also the high current version
with external power transistor from the data sheet, page 17 or so.


Own output noise of some regulators, measured with Agilent 89441A,
20*ADA4898 preamp and an aluminium children's coffin with BNC / SMA
feedthroughs. Measurements over 1Hz to 1 MHz are really 1 per decade,
collected over LAN, combined and printed with gnuplot.

https://www.flickr.com/photos/137684711@N07/24070698809/in/album-72157662535945536/lightbox/    >

« Last Edit: June 12, 2024, 09:46:08 am by Gerhard_dk4xp »
 
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