Ultra Low Noise Lab Power Supply Extender for your existing lab supply - Page 1
Forum section: electronics / projects

author: branadic
author_rank: Super Contributor
timestamp: August 07, 2018, 08:13:57 pm
content: Hi,I wanted to have an "Ultra Low Noise Lab Power Supply" for experiments with voltage references and instead of having a bunch of batteries (remember your green finger print on this earth) and instead of always using a low noise regulator for each quick and dirty voltage reference setup I decided to build a power supply for this and similar purpose.LT3042 and LT3045 are available for a while and page 23 of the datasheet gave me exactly what I needed. It took its time to grow this idea and meanwhile some guy onDIY audiocame across with a ready to go pcb for a first setup.The price of 12€ for an audiophile white looking board seemed okay to modify it to my needs. It's a two channel, fixed current board paralleling 3x LT3045 giving up to 15V and up to 1.5A per channel, more than enough. Instead of the 10way trimmpots on this board I added Bourns multiway pots in 100k and 100k with 100k in parallel for the voltage setting of up to 15V.Further I bought an50VA, 12Vac toroidal transformerand some panel meters forvoltageandcurrent. The panel meters are supplied by the rectified voltage at the input of the voltage regulators.The case was planed using Schaeffer front panel designer, exported as dxf and lasered at a local company. I decided to use a proMa EG2 aluminium profile (103mmx56mmx168mm) based chassis, but with wider front and back panel instead of standard 103mm and thus wider top and bottom plate.It works pretty well, even though I seem to have some minor issues with mains hum coupling somewhere into the output. I'm currently investigating on that and ordered some self-adhesiv mu-metal foil to wrap it around the transformer. Further I will replace cables by shielded ones.One the other hand without the transformer it is a pretty nice extender for existing lab power supplies, forming them into a low noise supply for the bench. So this is the idea. Let me ask, is there any interest in a group buy of bare boards and the aluminium chassis parts? All other parts can be easily bought in the bay and at your local electronic store or distributor. I plan to redesign the board with additional current limit setting, which is currently missing.Meanwhile the guy with the pcb cracked up and sells assembled boards with fixed output current to theaudiophile community for big bucks.EDIT: Added a first picture of my build.-branadic-

author: Henrik_V
author_rank: Regular Contributor
timestamp: August 08, 2018, 07:22:58 am
content: branadic, if you put some effort into a clean powersource, how about a better decoupling and guarding from mains?DMM power supplies transformers from Fluke/HP use two shieldings , one for PE and one for guard...

author: mzzj
author_rank: Super Contributor
timestamp: August 08, 2018, 08:04:23 am
content: Quote from: Henrik_V on August 08, 2018, 07:22:58 amThat what I was also thinking. Toroidal transformer is bad if you want to avoid coupling to mains borne noise and one without shield screens is extra bad.18650 Li-ion battery pack and LT3045 would avoid all the ground loops and conducted noise..

author: branadic
author_rank: Super Contributor
timestamp: August 08, 2018, 09:53:32 am
content: That's an effort I can spend, but I'm sure most of the people here on the board are not willing to pay the resulting price. Thus the idea of an ultra low noise power supply extender for existing cheapo lab suppliesBut noone said that I won't do both.-branadic-

author: prasimix
author_rank: Unknown
timestamp: August 08, 2018, 10:24:38 am
content: How are you going to shield noise that comes from cheapo power supply? Did you already try to connect LT3045 and check result? I presume that you are well aware how ambitious is your title (i.e. ultra low noise).

author: BravoV
author_rank: Super Contributor
timestamp: August 08, 2018, 10:54:46 am
content: Whats the low noise target in numbers or measurable metrics ?

author: branadic
author_rank: Super Contributor
timestamp: August 08, 2018, 05:03:48 pm
content: QuoteThe LT3045 has pretty good PSRR, so no worries whatsoever. You just connect the extender with banana jacks to your common cheap power supply.Yes, I already connected the board to my lab power supply (PPS5330) and measured noise (0.1Hz - 10Hz) in the order of 2µVpp @ 1V and 20µVpp @ 10Vdc. Yes, be sure that I got infected by metrology in almost every fashionQuoteSee answer before, take a look at the datasheet of LT3045 and remember that three are paralleled.-branadic-

author: prasimix
author_rank: Unknown
timestamp: August 08, 2018, 05:10:07 pm
content: Great, now I noticed in your signature that you're well equipped. I never been in position to measure anything below 440 uV that shows my "super-duper" Rigol DSO with input grounded.

author: branadic
author_rank: Super Contributor
timestamp: August 08, 2018, 05:26:17 pm
content: Simply add a low noise amplifier to convert your Rigol into a low noise measurement systemAdded a first picture to the first post.-branadic-

author: prasimix
author_rank: Unknown
timestamp: August 08, 2018, 05:38:05 pm
content: That make sense. First I have to find one which I can build (I believe there is a bunch of them someplace on this forum).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2018, 07:26:41 pm
content: I wonder how much emissions those LED displays give off.I have a few, I should check.In short: you can call it whatever you like, butit is not low noise unless you've MEASURED IT SO!Tim

author: branadic
author_rank: Super Contributor
timestamp: August 08, 2018, 07:51:51 pm
content: QuoteI'm sure you can read and I already stated that I measured ~2µVpp (0.1 - 10Hz) @ 1V and ~20µVpp @ 10V in configuration as an lab extender. I didn' state that I measured it in configuration as a stand-alone lab supply.I received some mu metal foil (0,1mm) today and wrapped it around the transformer (configuration as stand-alone lab suplly). I'm currently waiting for my low noise amplifier to load the input cap to 10V output.-branadic-

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2018, 07:57:52 pm
content: Quote from: branadic on August 08, 2018, 07:51:51 pmOh, in a reply lower down. Hard to see at a scan.What about 10Hz to 10MHz, say?Tim

author: branadic
author_rank: Super Contributor
timestamp: August 08, 2018, 08:02:43 pm
content: QuoteStep by step. Currently I measure it in the stand-alone solution, first 0.1 - 10Hz, next 10Hz - 100kHz as I have an amplifier for that too.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 08, 2018, 08:36:09 pm
content: First impression in stand-alone configuration (with transformer powered from 230V mains line) and 10V output voltage. One division equals 5µV (0.1x setting on the scope, 80dB noise amplifier).-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 11, 2018, 08:00:29 am
content: Did some further measurements in both configurations and it seems that there is no difference between Lab Supply Extender and Stand-Alone version, low frequency noise (0.1 - 10Hz) is in the order of2µVpp per volt.Next step is to measure 10Hz - 100kHz region.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 11, 2018, 09:49:05 am
content: 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.Attached is a picture of the inside and a picture of the preliminary front panel (a piece of printed paper laminated and attached with double-sided adhesive tape).-branadic-

author: jpb
author_rank: Super Contributor
timestamp: August 11, 2018, 12:00:10 pm
content: An interesting project.What is the position on the boards? I have more power supplies than I need really (an Agilent, a Hameg and a TTi) but I like the idea of being able to add an ultra low noise front end and perhaps use the same approach for dedicated power supplies to other projects (OTT but hobbyists don't need to comply with the constraints put on commercial engineers!)Given that the meters display down to less than a mV and the current can be up to 1.5A a useful addition might be sense inputs on the front panel but I guess for your purposes you don't plan to use the higher currents and don't particularly need the accuracy.

author: branadic
author_rank: Super Contributor
timestamp: August 11, 2018, 12:11:38 pm
content: The voltage meters measure the voltage at the output jacks and are just as accurate as they come from asia. Technical parameter are given as below:Weight: 16gDisplay mode: Five 0.36 LED digital tubeupdate rate: > 3s/timePhysical size: 48 x 29 x 22 mm (L*W* H)Opening size: 45 x 26 mm (L*W)Supply voltage: 3.5-30VLead wire length: 14 cm（4 lines）Measurement range: DC 0-4.3000-33.000VOperation temperature: -10 degrees~65 degreesMeasurement accuracy: (0.3‰+ 2words) {means the table worst case maximum error of less than 10mv normal within error 2mv}Red line 1:Power Supply + Black line 1:Power Supply -Red line 2:VIN + Black line2: VIN -They are powered from the rectified transformer voltage. Similar for the ammeter:Color: BlackDisplay: 5 digits 0.36" LEDLED color: BlueWorking voltage: DC 3.5-30VCurrent measuring range: 0-3.0000ACurrent measuring accuracy: ±(0.5‰+2digits)Working temperature: -10°C~+65°CDimension: 48 x 29 x 22 mmCable Length: 140mmIt measures the current from the negative jack to the regulator board. The regulator board is the white one on the left side with the blue heat sink.-branadic-

author: Wolfgang
author_rank: Super Contributor
timestamp: August 11, 2018, 01:08:45 pm
content: Hi,I have some suggestions for improvements of your design:- Make shielded compartments, one for the input filter, transformer, rectifier and first filter caps, and seperate ones for the other stuff- separate power supplies for the DMMs, also in a shielded box- User a preregulator before the final ones, also in an shielded box- Final regulators also in a shieled box, preferably with coaxial output- Use feedthru caps for the connection between boxesAm I paranoid ? Yes.I was trying to make absolutely quiet power supplies for noise measurement preamps. The problem I had is that most RFI is coming from outside, from LEDs, IOTs, Mobile phones, PLC, ... To kill all that is difficult and needs RF building techniques.As a last resort, I made battery powered supplies as well, they are the best so far ...https://electronicprojectsforfun.wordpress.com/power-supplies/battery-operated-power-supplies/

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 11, 2018, 01:29:52 pm
content: The photo of the insides of the enclosure look surprisingly conventional. I see no evidence of shielding or even wire routing that one would expect in a low-noise design. Aren't the digital displays noisy?

author: branadic
author_rank: Super Contributor
timestamp: August 11, 2018, 02:02:19 pm
content: Well, you can solve a problem from the direction of panic (top down), which ends up in an expensive piece of kit or you can solve a problem bottom up, build something and improve it until you reach your goal. I always choose the latterNoise from the panel meters can be easily filtered as it is high frequency. Couldn't find a problem till now, but if it turns out to be a problem I will perform some required steps on that.Up to now there was no need to use panic shielding. Well at least, as I'm not sure if the mu metal shield around the transformer was really necessary. But since this is a self-adhesive tape one I won't remove it anymore. Instead the classical way is:- to twist all AC cables- avoid ground loops, even shielded cables can create them, when shield is connected on both endsIf really necessary I will replace some cables by twisted shielded ones, but therefor I need to make further measurements to see if this is required here.-branadic-

author: Wolfgang
author_rank: Super Contributor
timestamp: August 11, 2018, 04:01:47 pm
content: Your approach is just as valid as mine. I have more effort, but your probably have more redesign steps.Hint: If you want rocket science or extreme performance, try to exclude as many errors as you can beforehand. There will be enough pitfalls left for you to fix. Murphy never sleeps.

author: branadic
author_rank: Super Contributor
timestamp: August 11, 2018, 04:15:34 pm
content: You are right Wolfgang, but what is critical for the stand-alone supply is less of an issue for the lab extender, as you don't have the 50Hz hum and the related problems as a source inside you device. But obviously there is not much interest in such a lab supply extender. Would have thought the resonance is much bigger.Finished the first measurements on my stand-alone solution in the 10Hz - 100kHz range and can now see the advantages that needs be addressed next.-branadic-

author: Wolfgang
author_rank: Super Contributor
timestamp: August 11, 2018, 04:28:04 pm
content: Hi Branadic,if you just want to finish off the noise and ripple of an existing PSU, have you seen the tricks from Charles Wenzel with his active noise killer circuits ?http://www.wenzel.com/documents/finesse.htmlMaybe this helps, when mounted close to the circuit to be powered.Maybe a comparison of voltage regulator chips is also interesting:https://www.bartelsos.de/dk7jb.php/rauschen-von-spannungsreglernAnother great source of tricks and infos is the reverse engineering of old HP power supplies with top performance.I am looking for one of those on eBay just to have a look inside. Manuals are probably available at the BAMA archive.RegardsWolfgang

author: branadic
author_rank: Super Contributor
timestamp: August 11, 2018, 04:43:23 pm
content: Hi Wolfgang,I do know both websites and alsomade some measurements and simulationusing ideas from both.-branadic-

author: Wolfgang
author_rank: Super Contributor
timestamp: August 11, 2018, 04:53:39 pm
content: Hi,I just had a look at it, but one questions remains - the LM317 is a really bad regulator regarding noise - the performance would probably a lot better if you had used an LM723 with PNP pass transistor and a Wenzel cleanup shunt afterwards - did you try this one ?RegardsWolfgang

author: Andreas
author_rank: Super Contributor
timestamp: August 11, 2018, 05:56:33 pm
content: Hello Branadic,I had a bad experience with 50 Hz hum.I was measuring 1/f (0.1 - 10 Hz) noise as usual of a battery powered reference with my 4th order band pass filter cirquit.The output looked quite plausible: around 4uVpp for a 10 V reference.But the signal looked somewhat "fuzzy" against similar measurements.So I did a FFT analysis which revealed a strong 50 Hz (and 150 Hz) signal.When zooming in you can also see a 0.8uVpp 50 Hz amplitude in the raw signal.(Ok I should have done a noise floor measurement before actually measuring noise.)Root cause was a switched on HP34401A near my "cookies box (tinned can)" where I put in my references and the amplifier.Usually I would think that a 6.5 digit DMM has enough shielding around the transformer but obviously this is not true.Also other transformers I have to put away for more than 0.5m to get no influence above noise floor.So I would use e.g. a 12V AC wall wart with enough distance to my low noise supply or split the design into 2 boxes.https://de.rs-online.com/web/c/stromversorgungen-transformatoren/netzteile/steckernetzteile/?searchTerm=12VACwith best regardsAndreas

author: carl0s
author_rank: Unknown
timestamp: August 11, 2018, 06:25:06 pm
content: That's a curious ruler. I'm trying to decide if I could use one of those, or if a tape-measure is better.

author: branadic
author_rank: Super Contributor
timestamp: August 11, 2018, 07:26:27 pm
content: Wolfgang, this is about a power supply extender with variable voltage setting based on TL3045, not LM317, LM723 or fixed voltage regulators. The measurements I performed in the linked thread was about LM317 and since I had the components and the noise amplifier laying around and was interested in the answers I simply did it.Andreas, thanks for the input. I haven't observed 50Hz hum in my 0,1 - 10Hz noise measurement yet, maybe due to 14s record time? I activated FFT on the scope, but saw nothing, so I will have a look on that. But I can see mains line hum in my 10Hz - 100kHz measurement and will need to improve the build for stand-alone configuration with the transformer in the same case as the regulator board. I do have some rf sheet metal from former rf projects to create some case seperators.About shielding of dc rail cables, what would you suggest? Twist positive and negative rails and add a shield connected to the case or would you go for coax? I was curious to see, that a high quality coax cable between dc output jacks and DSO picked up quite a lot of mains hum, while a twisted cable reduced this significant. So I tend to use twisted but shielded cables.However for the lab supply extender you only have to keep away mains line from it.-branadic-

author: Andreas
author_rank: Super Contributor
timestamp: August 11, 2018, 09:00:33 pm
content: Hello Branadic,I would go for twisted pair against 50 Hz (magnetic) hum.The reason is simple: The shield of a coax is simply not thick enough for magnetic shielding. So twisting is the more effective measure.Low frequent (magnetic) shielding is done by the skin effect: the field generates a current in the shield which produces a counter magnetic field.But at 50 Hz and copper you will need many multiples of 9.4 mm as shield thickness to get a effective shielding.https://www.adt-audio.de/ProAudio_Grundlagen/Skineffekt/Eindringtiefe.htmlOf course a additional shield is not completely useless: it helps against capacitive coupling.with best regardsAndreas

author: Wolfgang
author_rank: Super Contributor
timestamp: August 11, 2018, 09:53:01 pm
content: Hi,the plausible reason for the 50Hz is that the 34401A (and successors) are always averaging over an integer number of line cycles, so this influence is averaged out and a perfect transformer shielding is not needed. This is also found in the spec sheet of the meters.If line interference is cumbersome, you could try:- twisted wires so that magnetic influence cancels out (works better than coax)- maximize distance between the source of magnetic fields and the sensitive circuitry- you could optimize stray fields by changing transformer mounting and direction- never load transformers to the max because stray fields rise when cores approach saturation- use toroids instead of classic transformer cores- check rectifiers for no snap effects (use schottky or soft-recovery, snubber circuits, ...)- soft iron boxes (or mu-metal) can shield some magnetics fields quite well.To branadic,OK, the LT3045 is a regulator with impressive data, I never used one of them due to the max voltage being too low. This plus a shunt noise canceller could yield good results, why not. I can try when I am back home at my lab in September.Much luck !

author: branadic
author_rank: Super Contributor
timestamp: August 12, 2018, 12:03:06 pm
content: Twisted all cables and also added MKS2-100nF on the output jacks, this gave a slight improvement in noise, but not good enough.A seperate compartment for keeping out 50Hz hum would require at least a 1.5mm steel case. The rf sheet metal I have is only 0.5mm, not the perfect choice. So currently I stuck and can't go on.-branadic-

author: Echo88
author_rank: Frequent Contributor
timestamp: August 12, 2018, 12:26:03 pm
content: The only cheap source of shielded transformers i was able to find were transformers for tube-amplifiers, for example:https://www.ebay.de/itm/Rohrengleichrichter-Netztrafo-355/123279036514?hash=item1cb400d062:g:3EgAAOSw7t1a-pQzCan someone suggest a source for double shielded transformers,preferably toroids, like Henrik mentioned?

author: branadic
author_rank: Super Contributor
timestamp: August 12, 2018, 01:13:15 pm
content: The only thing I know are R-core transformers, but can't say nothing about their performance.-branadic-

author: Messtechniker
author_rank: Frequent Contributor
timestamp: August 12, 2018, 01:17:21 pm
content: I'd put the transformer, then not necessarily a torroid, with mains input filter, bridge rectifierand a large cap + a small cap and a bit of series inductance in a separate metal box you can keep nicelyfar away (2 to 3 m or more, use shielded cable) from your low noise regulation box. Saves youall that possibly expensive shielding effort which even so may be insufficient.If still not satisfied you might try two back-to-back transformers with carefulpassive filtering (100 Hz low pass) in between to get rid of high frequency mushcoming from the mains.A power transformer with a shield between primary and secondary would be preferable,but these are expensive custom designs* often for medicinal equipment.And might be salvaged from such..... Good of a one off but not so good if otherswant to build your circuit.Just a few ideas.*) just ask Pikatron or Haufe in Germany for a quotation

author: branadic
author_rank: Super Contributor
timestamp: August 12, 2018, 01:57:11 pm
content: Thanks, but I will go a different way:First, because it's relative easy for me to manage, make a steel tube (1.5 - 2mm thickness) with steel washers for both ends and magnetically encapsulate the toriod transformer. Yes I know, there are some more cables carrying the mains line hum, thus...Second, design a steel case (1.5 - 2mm) for the regulator part, laser it, weld the single parts together (for such reasons I bought kweld) and install it inside the aluminium chassis.Third, I already orderedR-core transformerfor comparison and will measure differences in different orientations, which could also be interesting for further projects.Normal transformershave the disadvantages that they are big and don't fit into my chassis.This will take some time for the material to arrive, but it's worth learning.-branadic-

author: Wolfgang
author_rank: Super Contributor
timestamp: August 12, 2018, 05:33:08 pm
content: Just a thought - tinplate (Schubert) boxes could be easier to handle; and they have good RF shielding properties.

author: branadic
author_rank: Super Contributor
timestamp: August 12, 2018, 05:46:29 pm
content: Quote from: Wolfgang on August 12, 2018, 05:33:08 pmThat's correct, but since it's only tin plated steel you need a certain thickness to surpress 50Hz/60Hz. Following the picture below this gives 2mm @ 100Hz.The curve in the above diagram uses μr=460 for steel AISI 1010 and a specific resistance of 6.9e-7 ohm*m. Other formula indicate at least 1.5mm for ST37. Now you might understand why the cases of good instruments are that massive around the transformators.-branadic-

author: mzzj
author_rank: Super Contributor
timestamp: August 12, 2018, 07:09:47 pm
content: Quote from: branadic on August 12, 2018, 05:46:29 pmWrong pic or you extrapolated from graph? Your graph shows 60 micrometers at 100 kilo-hz?AFAIK its even worser than that when you are trying to suppress (magnetic) near-field:

author: Wolfgang
author_rank: Super Contributor
timestamp: August 12, 2018, 07:27:15 pm
content: Hi branadic,I am not sure if the Schubert boxes are really made of steel. To me it looks like a very soft iron alloy (Weißblech). They can make steel ones, too, but you have to order them explicitely. How well do your curves fit to soft-iron alloys ?

author: branadic
author_rank: Super Contributor
timestamp: August 12, 2018, 09:02:41 pm
content: Correct, I extrapolated to 100Hz but also checked with a µr of 250 for steel. Maybe an optimistic assumtion, there is not much on the net about it. I don't want to surpress near field but low frequency from mains line.As far as I know tinplate is just tin plated steel, nothing fancy and I couldn't find any hint on carefully treatment, which would indicate some special grain orientation of the steel.Meanwhile I started to upgrade my 10Hz - 100kHz 80dB AN83 amplifier. Since now I had 330µV/10V SMD tantalum installed, which was fine for measuring voltage references. I now upgraded to3x AVX Corporation FFB54D0117KJC, so that I'm able to measure over the full output voltage range of the power supply and up to 75V.-branadic-

author: Harm314
author_rank: Newbie
timestamp: August 14, 2018, 06:55:18 pm
content: Hi Branadic,The power supply extender sounds interesting, but I'm not certain from your posts what you intend to deliver: PCB only, PCB + parts, or fully built module.If I read the demo board sheet correctly it is built using a four-layer PCB. That is beyond my design skills.

author: branadic
author_rank: Super Contributor
timestamp: August 14, 2018, 08:25:32 pm
content: QuoteWrote the answer in the first post. It will be a dual layer board only. The aluminium chassis parts get cheaper the more people are coming together. But up to know only two persons are interested.Back to the stand-alone solution:Found some shield from industrial cables today. Which cables should be shielded? DC cables? AC cables? What to connect shield to? GND (for DC cables) or earth (for both)? Do we have some experts on that or is it just try and error by measuring the resulting noise?-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 22, 2018, 05:38:39 pm
content: Received some goodies today-branadic-

author: BravoV
author_rank: Super Contributor
timestamp: August 22, 2018, 10:50:32 pm
content: Quote from: branadic on August 22, 2018, 05:38:39 pmWhats the SCN wire stands for ?Btw where did you buy it ?

author: Cody Turner OKC
author_rank: Contributor
timestamp: August 22, 2018, 10:58:54 pm
content: woah, intresting production date on those... hahah

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: August 22, 2018, 11:54:16 pm
content: That means 9th of April in Europe.I'd like to know if there are more to be sourced?I have just put an 650 VA isolation transformer into a 4 height units 19" boxwith some outlets and lots of filters to feed my scope and a NGT20power supply or two. There is no safety ground on the secondary,but lots of ferrite. It seems, my worst ground loop problem is gone.Cheers, Gerhard

author: 2N3055
author_rank: Super Contributor
timestamp: August 23, 2018, 09:16:35 am
content: Quote from: BravoV on August 22, 2018, 10:50:32 pmThat is the screen to minimize capacitive coupling.I'm interested how this transformers will perform...

author: branadic
author_rank: Super Contributor
timestamp: August 23, 2018, 10:48:11 am
content: Quote from: BravoV on August 22, 2018, 10:50:32 pmSCN (green-yellow) means, as 2N3055 already mentioned, screen and has to be connected to earth.It's already all there and linked in the#msg1740833-branadic-

author: mzzj
author_rank: Super Contributor
timestamp: August 23, 2018, 11:45:36 am
content: 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.

author: splin
author_rank: Frequent Contributor
timestamp: August 23, 2018, 01:58:26 pm
content: 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~SlbHIxmI 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?

author: mzzj
author_rank: Super Contributor
timestamp: August 23, 2018, 05:00:48 pm
content: Quote from: splin on August 23, 2018, 01:58:26 pmAFAIK 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 turnFor 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.

author: splin
author_rank: Frequent Contributor
timestamp: August 23, 2018, 07:57:52 pm
content: Quote from: mzzj on August 23, 2018, 05:00:48 pmThe 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?

author: branadic
author_rank: Super Contributor
timestamp: September 02, 2018, 02:53:13 pm
content: Time to go on...Attached a comparison of the new R-core transformer (65VA) withring core transformator (50VA) type RKT 5012with additional mu metal shield.-branadic-

author: splin
author_rank: Frequent Contributor
timestamp: September 03, 2018, 02:19:23 pm
content: 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?

author: branadic
author_rank: Super Contributor
timestamp: September 03, 2018, 03:27:05 pm
content: post 12bitQuote from: splin on September 03, 2018, 02:19:23 pmI 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-

author: branadic
author_rank: Super Contributor
timestamp: October 01, 2018, 05:14:58 pm
content: Measured capacitance of the toroidal vs. R-core transformer...toroidal transformer with additional mumetal shield- 99.7pFR-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: 82.5pFMeasurements were proven to be reproducible and performed @ 1MHz.-branadic-

author: Zorc
author_rank: Contributor
timestamp: October 13, 2019, 09:51:05 am
content: 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)

author: Wolfgang
author_rank: Super Contributor
timestamp: October 13, 2019, 10:23:04 pm
content: Quote from: branadic on September 03, 2018, 03:27:05 pmHi 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 ?RegardsWolfgang

author: branadic
author_rank: Super Contributor
timestamp: October 15, 2019, 10:22:21 am
content: Hi,didn't make any further modification to the setup, but am using the power supply now and then.-branadic-

author: niner_007
author_rank: Frequent Contributor
timestamp: July 12, 2020, 10:30:22 pm
content: 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

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 13, 2020, 05:00:14 pm
content: These LED pane meters powered from the same line will make output noisy (somewhere at kHz or tenth on kHz).

author: niner_007
author_rank: Frequent Contributor
timestamp: July 13, 2020, 09:41:32 pm
content: 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.

author: branadic
author_rank: Super Contributor
timestamp: November 29, 2020, 10:22:30 am
content: 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-

author: EC8010
author_rank: Frequent Contributor
timestamp: May 26, 2024, 11:47:43 am
content: 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.

author: Echo88
author_rank: Frequent Contributor
timestamp: May 26, 2024, 01:51:40 pm
content: 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?

author: EC8010
author_rank: Frequent Contributor
timestamp: May 26, 2024, 03:00:55 pm
content: 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,Ithink 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 MSO54I 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!

author: 5U4GB
author_rank: Super Contributor
timestamp: May 28, 2024, 01:37:27 pm
content: Quote from: branadic on August 11, 2018, 09:49:05 amI 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.

author: Echo88
author_rank: Frequent Contributor
timestamp: June 08, 2024, 09:32:21 pm
content: 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/#msg5535891Apparently 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.

author: harerod
author_rank: Frequent Contributor
timestamp: June 12, 2024, 07:28:43 am
content: 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.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: June 12, 2024, 09:18:28 am
content: 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 versionwith 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 / SMAfeedthroughs. 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/>