Type of transformer to use for low noise?  - Page 1
Forum section: electronics / projects

author: ftransform
author_rank: Frequent Contributor
timestamp: January 15, 2013, 10:30:49 pm
content: What type of transformer should I use for a low noise power supply?Right now I am considering a simple radioshack transformer,http://www.radioshack.com/product/index.jsp?productId=2102702&filterName=Price&filterValue=%2410.00+-+%2449.99I have one of these and it is wrapped in metal but I have been doing a little research about transformers and I am not sure if its the best choice for a low noise system.Some people recommend using torrodial transformers for low noise stuff, but other people advise against it.http://www.soundstage.com/maxdb/maxdb071998.htm< this website claims that E core ones are lower noise..What transformer should I use? If anyone could recommend a transformer which is less noisy then the radioshack one that is less then 40$ I would appreciate it.How much of a difference will it make?This is meant to be a 110 -> 12-0-12 step down transformer.I am not interested in winding my own. It is difficult for me to find anything online because I am swamped by audio websites (which I do not trust). This is meant to be the power supply for a sensor system. I don't think it will draw more then 3-4 amps (with all the micro and control system stuff).Also, can any benefit be had from filtering mains power through say a common mode choke prior to a 60 hertz transformer or is this just silly. I'd imagine the transformer would handle it but I don't really know.I know they sell those little metal can EMI filters you plug into, are they any good? Any good brands to consider?Looking at some TDK-LAMDBA and DELTA EMI filters I see that you can get them for specific frequencies. What should I focus on? I figure most noise occurs at low frequencies *due to switch mode psu* so I thought this was a good choice (150khz - 1mhz)http://www.tdk-lambda.com/products/sps/nf/ma12/indexe.htmlOr should I go for the 150khz - 30mhz?

author: penfold
author_rank: Frequent Contributor
timestamp: January 16, 2013, 09:15:22 am
content: Toroids are renowned for low leakage and thus lower fields coupled into the other elements of the system. I've never heard of e-cores being particularly low for noise.Mains input filters are more often than not intended to reduce noise from your system coupling back to the supply, I'd advise you just use any transformer and keep the supply elements segregated in their own metallic enclosure and filter the dc to within an inch of its life both at the supply side and on the input to the system

author: Kremmen
author_rank: Super Contributor
timestamp: January 16, 2013, 10:25:40 am
content: Mains transformers don't really radiate noise. Traditional cores may have more stray fields due to worse flux path containment than toroids do but this is not noise in the sense you seem to be discussing.Now if the transformer is supplying an SMPS then that one may radiate whatever depending on the specifics. Another mechanism is if the transformer supplies a heavily loaded rectifier/cap combo. Then the current waveform will be quite spiky ruining the power factor. At high enough currents the supply voltage starts to distort too, because of impedances in the grid wiring.The high frequency stuff will originate from switching events in the SPMS, not in the transformer.Use a toroid.

author: ftransform
author_rank: Frequent Contributor
timestamp: January 16, 2013, 06:29:36 pm
content: OK, how about this?http://cds.linear.com/docs/Application%20Note/an124f.pdfJim Williams uses aLOW SHUNTCAPACITANCE ISOLATIONTRANSFORMER(LOCATE ?3 FEETFROM SHIELDED CAN)Does low shunt capacitance have anything to do with it? Why would he need a low shunt capacitance isolation transformer in addition to a high end bench power supply?The app note seems to imply that it will somehow reduce noise.ANd I have even read on this forum that toroid transformers tend to radiate a field through the middle hole.Could I achieve better noise reduction if I used two or even three ordinary transformers feeding into each other? It seems this is exactly what jim williams does.Also, what do you think of that link in the first post about torroids Kremmen?

author: jeroen74
author_rank: Frequent Contributor
timestamp: January 16, 2013, 09:24:28 pm
content: A quick scan through that appnote tells me it's about nanovolts. Are you dealing with that kind of levels? You do not worry about noise coupling into the wires to the sensors?

author: ftransform
author_rank: Frequent Contributor
timestamp: January 16, 2013, 10:16:59 pm
content: Quote from: jeroen74 on January 16, 2013, 09:24:28 pmI'm still curious.

author: free_electron
author_rank: Super Contributor
timestamp: January 16, 2013, 10:22:23 pm
content: -edit- brainfart corrected :Toroid with a shield inbetween hv and lv part.orR-core transformers (best)

author: ftransform
author_rank: Frequent Contributor
timestamp: January 16, 2013, 11:05:03 pm
content: Quote from: free_electron on January 16, 2013, 10:22:23 pmCan you elaborate on this shield? Perhaps post an example of such a unit?

author: Kremmen
author_rank: Super Contributor
timestamp: January 16, 2013, 11:17:03 pm
content: That kind of a shield is a grounded foil between the primary and secondary coils of the transformer. You can't add it to a transformer, it is either there or not. In case you roll your own it is vitally important not to short the foil ends, or you will have a short circuit.

author: GK
author_rank: Super Contributor
timestamp: January 16, 2013, 11:25:59 pm
content: Toroidial mains transformer have a high primary-secondary capacitance due to the tightly coupled windings, so any high frequency disturbances on the mains side tend to get coupled to the secondary side quite effectively. The shield is typically called an electrostatic screen and can very effecitvely isolate the secondary from HF interference on the mains side. It is electrically connected to earth/chassis.Toroidal mains transformers can be wound with an electrostatic screen between the primary and secondary windings. That way you get the best of both worlds - low radiated magnetic field and high pri/sec isolation. I have a 2kVA example pulled from some medical equiptment under the bench.

author: free_electron
author_rank: Super Contributor
timestamp: January 16, 2013, 11:29:10 pm
content: Correct, toroidals have large capacitve coupling between primary and secondary. That's why you want that shield.R-core solves that as the windings are NOT on the same bobbin but reside on the opposite leg of the ring.i posted a brainfart previously. i meant to write : toroids with a screen, or , r-core transformershttp://www.custommag.com/products/r-core.shtmlhere is an overview :http://www.custommag.com/products/r-core_compare.shtml

author: ftransform
author_rank: Frequent Contributor
timestamp: January 16, 2013, 11:44:37 pm
content: Ah hmm. Now to begin making purchasing decisions.I'm going to have to do more research in order to decide between an R-core or a electrostatically shielded toroid.Is it still advisable to use some cheapo isolation transformer prior to the groovy high tech one?

author: jnd
author_rank: Regular Contributor
timestamp: January 16, 2013, 11:45:39 pm
content: Interesting, never heard the term R-core. I thought C-core, as it is referred to around here were one of the best, they do look similar. C-core transformers were used in many precision Czechoslovak instruments, there were no toroids at the time.

author: GK
author_rank: Super Contributor
timestamp: January 17, 2013, 12:02:50 am
content: Quote from: ftransform on January 16, 2013, 11:44:37 pmOnly if you want to waste some money. I doubt you'd have any luck finding a supplier of R-core transformers these days either; they are old hat. Even an electrostatically screened toroid will be difficult to source, save for a custom order.However if isolation from mains borne high frequency interference is your concern, the most simple fix using off-the-shelf components is to prevent the HF interference making it to the transformers primary winding in the first place. There is a plethora of mains-side RFI filters to choose from.What is your application?

author: free_electron
author_rank: Super Contributor
timestamp: January 17, 2013, 01:51:37 am
content: if you rip apart the really pricey Denon , Onkyo , yamaha equipment you find r-core transformers. Not exactly old-hat as they were only invented a decade or so ago...Don;t confuse these R-cores with the old style split bobbin trasnformers that use a stack of plates ! The trick sits in the alloy used for the core material.shielded toroids are no problem either.

author: GK
author_rank: Super Contributor
timestamp: January 17, 2013, 02:29:36 am
content: None of the major distributers (especially local) that I know of sell a range of toroidial mains transformer with electrostatic screens. For such things I've always had order directly from a manufacturer; then seldom available in a quantity of one for the specification desired.If money and MOQ isn’t an issue I can of course readily get R-cores and shielded toroids of just about any specification; especially from overseas. International shipping costs on heavy items such as transformers sucks too. That does generally make both types a bit of a problem for a one off hobby project, in my experience.If the OP is just building a general purpose bench supply for I think he's worrying much about nothing.

author: GK
author_rank: Super Contributor
timestamp: January 17, 2013, 03:59:35 am
content: Quote from: ftransform on January 15, 2013, 10:30:49 pmSilly me; paying more attention to the replies I missed this line the first time round. What type of sensors? Does the transformer actually need to be mounted, due to limited space, in close proximity to sensitive, high-gain sensor/pickup circuitry? Hard to offer any real practical advice without somewhat more detailed information (as opposed to theoretical discussions on why one type of transformer is better in some aspect than another, which for all sakes and purposes may very well be irrelevent!).

author: ftransform
author_rank: Frequent Contributor
timestamp: January 17, 2013, 06:06:33 am
content: Quote from: GK on January 17, 2013, 03:59:35 amNo it does not need to be mounted near the sensor system. I was just curious because that jim williams app note got my mind going. The plan is to just use a rectify a step down transformer into a LM3080 and perhaps try to follow it by one of these filters to see if it gives an improvementhttp://www.wenzel.com/documents/finesse.html(they use it for a LM317 but the LM3080 is quite a bit better so I don't know if those circuits can really improve it any) and also experiment with other types of low pass filters (idk what will give better performance).I want to test it with a few different sensors. Scale and RTD first.