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Electronics => Projects, Designs, and Technical Stuff => Topic started by: ezalys on March 20, 2022, 01:04:38 am

Title: Off the shelf double-shielded transformer
Post by: ezalys on March 20, 2022, 01:04:38 am

Is there anywhere that sells a double-faraday-shielded 120V -> +15/-15 transformer? 100 mA current minimum on both rails, ideally 200 mA or more. I'm working on an application that's particularly sensitive to common mode noise.

Title: Re: Off the shelf double-shielded transformer
Post by: bdunham7 on March 20, 2022, 02:05:25 am

I'm not sure what 'double Faraday shielding' would look like on a mains transformer, but the if you want low capacitive coupling and low common mode noise you might look at R-core transformers.

https://www.ebay.com/itm/222539735477 (https://www.ebay.com/itm/222539735477)

Title: Re: Off the shelf double-shielded transformer
Post by: NiHaoMike on March 20, 2022, 04:06:27 am

Most likely you'll have to custom engineer a solution.
https://www.youtube.com/watch?v=9JinSfCKuNQ (https://www.youtube.com/watch?v=9JinSfCKuNQ)

Title: Re: Off the shelf double-shielded transformer
Post by: jonpaul on March 20, 2022, 08:43:04 am

Hello: Ebay Chinese lot NOT shieled but has external shading band to reduce mains H filed radiation.

Torids and R type are worst on isolation C due to geometry.

Check the stock transformer houses like CoilCraft, Signal Transformer, Renco in USA for a SINGLE shielded (Faraday Shield) NOT "double shielded"

Only costly instruments like seismographs need double or triple faraday shields.

In HiFi it makes no difference and the CM noise burden is not great and the design of the audio ckt should have good PSRR and CMRR

See the excellent app notes of the late and great Deane JENSEN
https://www.jensen-transformers.com/application-notes/ (https://www.jensen-transformers.com/application-notes/)

Bon chance


Jon

Title: Re: Off the shelf double-shielded transformer
Post by: ezalys on March 20, 2022, 02:28:29 pm

I've managed to find plenty of split bobbin transformers without faraday shields, and plenty of toroidal transformers with faraday shields, but no split bobbin transformers with faraday shields! Super frustrating.

Title: Re: Off the shelf double-shielded transformer
Post by: jonpaul on March 20, 2022, 05:40:17 pm

Hello again  ezalys

 Reason for the availability is little market for    faraday shield in  commodity split bobbin transformers.

Can you precice your exact application, give a schema or block, to better understand the requirement for Faraday shield single or double?


Since the chassis to   line  and  secondary stray C is  ~> 10 pF the shield is not effective in most situations.


Very sensitive instruments may benefit but doubt if any difference in hifi, ADC, DAC, consumer audio stuff


Jon

Title: Re: Off the shelf double-shielded transformer
Post by: djerickson on May 26, 2022, 02:05:59 pm

I'm with ezalys 100%. Virtually every high-end, isolated instrument uses a multiply (dual or triple) shielded power transformer to reduce AC leakage currents. I wonder about high-end audio equipment. If you design your own transformers and have them built by a transformer manufacturer, getting shields is no big deal. But no off-the shelf transformers have quality shielding. This leaves us DIY types hanging.

I'd love to see some papers, articles, web sites, videos, etc. on how to fabricate AC and/or switching shielded transformers. 

I use a medical-grade, shielded toroid on my DIY-SMU project and the AC leakage currents are about 10-20uA. Not great.

Dave
http://www.djerickson.com/diy-smu (http://www.djerickson.com/diy-smu)