How to select a "wall wart" for lowest emissions? - Page 1
Forum section: electronics / projects

author: cvanc
author_rank: Frequent Contributor
timestamp: March 03, 2024, 09:37:39 pm
content: So I need a 5V wall wart minimum 2 Amps that is as quiet as possible in terms of RFI/EMI. I'm not sure what specific standards help me for this shopping?I've attached a couple grabs from Digi-Key, do any of these parameters guide me? All suggestions welcome; thanks very much.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 03, 2024, 09:54:09 pm
content: One good bet is to use a "medical-grade" power supply (as long as the certification is genuine), as manufacturers normally have to prove conformance to be able to label them "medical", while just a CE or FCC mark doesn't necessarily mean that the manufacturer has done anything to even check conformance through tests (self-certification). Medical-grade power supplies also have increased isolation.One reliable manufacturer is FRIWO if you're looking for quality supplies that won't cheat certifications. Even their industrial-grade supplies should be perfectly fine for your use, something like this:https://www.friwo-shop.de/en/power-supplies-industrial-ite/interchangeable-plug-power-supplies/9/fox12-x

author: Whales
author_rank: Super Contributor
timestamp: March 03, 2024, 11:43:38 pm
content: Quantity? Is this a one-off or for mass manufacture?What parts of the frequency spectrum is your use case sensitive to? If you can tolerate spikes in the hundreds of hertz region then a traditional mains transformer and rectifier might be better. If 1MHz noise is a problem then you'll want to pay attention to the switching freq of your switchmode supply.

author: amyk
author_rank: Super Contributor
timestamp: March 04, 2024, 12:13:42 am
content: Mains frequency transformer with a linear regulator.

author: Konkedout
author_rank: Frequent Contributor
timestamp: March 04, 2024, 12:41:23 am
content: Yes amyk nailed it. A line frequency transformer and linear regulator will create negligible noise. I think you said you want 5V @ 2A so that is not very high, but do you really need the power supply to hang from the wall outlet?Some sort of desktop enclosed power supply might be more do-able due to size and weight.

author: 5U4GB
author_rank: Super Contributor
timestamp: March 04, 2024, 01:44:03 am
content: In particular for that voltage and power rating there's a ton of vendors selling linear supplies for people to power their Raspberry Pis full of DC/DC converters and oscillators and whatnot to improve the purity of the precious audio fluids.Teradakdo reasonably decent products at non-audio-woowoo prices, and sell through the usual crapvendors.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 04, 2024, 03:55:17 am
content: meanwell has very good medical grade psu, some tolerate 4000v leakage tests ....AC to DC, DC - DC etc ...

author: cvanc
author_rank: Frequent Contributor
timestamp: March 04, 2024, 12:36:59 pm
content: Thank you all. This is quantity one and a portable application, so the wall wart style is best here.I've no interest in building anything up, want to buy off the shelf. I'm not sure anyone even makes linear wall warts anymore?But the key question I had is "which of these dozens of standards directly specify low emissions?"Appreciate your input, thanks and regards.

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 04, 2024, 01:57:00 pm
content: Linear wall warts are still available, something like $20-30 for ~10VA though, saw them a few months ago on Mouser/Digikey. If you just need one finding the local thrift store with a big box of cords to pick through can be a valid option.

author: Majorassburn
author_rank: Regular Contributor
timestamp: March 04, 2024, 02:35:31 pm
content: Quote from: cvanc on March 03, 2024, 09:37:39 pmAnswer:1) None of those "grabs" will specify emi/rfi emission levels because they don't have to.2) Emission levels set by the (FCC/US Govt/UL) are specified "not to exceed" rather than "what level are they".3) All modern 5V@2A wall warts are switchers which means they are not only emi/rfi noisy but also somewhat noisy on their DCV output.4) Most ITE wall warts are designed to power well-filtered devices.5) Such wall wart switchers are a convenient way for device-makers to side-step UL, regulatory requirements and product liability concerns that they would otherwise be exposed to if they had120V mains-connected internal power supplies. That's why most everything has an external wall wart power supply nowadays.6) Finally, if you want to select the quietest wall wart switcher of a particular bunch, you'll have to conduct your own comparison tests if you have the proper equipment and environmental setup to do so.By the way, any normal ambient environment will have so much emi/rfi/earth magnetic influence prevalent that no amount of emission from any wall wart will be measurable unless its inside a Faraday Cage or deep within the Rocky Mountains!