Wall wart question

I'm tired of putting up with the interference on AM/FM radios that happen to sit too close to a switched-mode wall wart needed for another item. Is there any way I can suppress the interference, like putting ferrite rings on the DC output from the wart, or wrapping it in aluminium foil, or whatever? If no solution in that direction, how can I tell if a wall wart I might see on e-bay is switched-mode or the older transformer type that I assume doesn't create interference?
--

Chris

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Looping the output cable several times around a ferrite core or through a ferrite ring will give some reduction but see later.

probably won't as you couldn't earth it and could be potentially lethal if the foil or whatever came loose and in contact with the mains pins.

The switched mode will be a lot lighter so if postage is less it may give a clue. Most of the Ebay Chinese switched mode offerings are pretty dire and noise suppression is something which has to be designed in from the beginning, it can't really be added as an afterthought.
If you buy from the likes of CPC.co.uk it will cost a bit more but there is at least some chance the thing will actually meet the various standards marked on it. On the direct imports CE simply means "Chinese Export" and they make pretty neat broadband jammers. They are also amazingly optimistic with current ratings.
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On Tuesday, 9 August 2016 16:59:50 UTC+1, Peter Parry wrote:

Get a 2nd hand iron lump type wallwart. Be aware most are not voltage regulated, so offload voltage rises. Some equipment is not ok with that. More info needed.
There are other options.
NT
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Peter Parry wrote:

No, they are just using American Amps, these are about 3 times a UK Amp and achieved at unbelievably low temperatures, attached to an infinite heat sink! Most semiconductors are now always rated in American Amps because they look better on paper before they self destruct. I used to rate power semiconductors for reliability in the dim and distant past. International committee meetings were a hoot!
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On Tuesday, 9 August 2016 20:45:09 UTC+1, Capitol wrote:

Are they related to Argos Watts?
Owain
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On 09/08/2016 14:46, Chris Hogg wrote:

Decent well designed wall warts should be reasonably well behaved. Be surprised if they jammed FM except at very close range. AM tends to be super sensitive to badly designed switched mode PSUs.
You can get clamp on ferrite rings that will stop some of the interference but buying a better wallwart might be better.
http://uk.rs-online.com/web/c/passive-components/ferrite-cores/ferrite-sleeves/
How well they work is another matter. It might take the edge off something that is borderline but it won't work miracles.
Moving sensitive kit away from the source of interference is probably a better bet.
eBay Chinese imports you are lucky if they are electrically safe.
Any modern PSU will likely be switched mode these days (although the price of iron going through the floor might change that).
Is there a market for old ferrous transformer based wallwarts?
Regards, Martin Brown
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Martin Brown wrote:

I have seen production Asian switched mode psus, where the CE marked units had the suppression components removed after approval was obtained! All the mounting holes were in the PCB with shorting links inserted where necessary.
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I like this 'power supply': http://hackaday.com/2012/08/15/buying-cheaper-electronics-and-not-saving-money/
Though note that weight isn't a reliable indicator either: http://www.overclock.net/t/1246964/hardware-secrets-hantol-psu-fake-pfc-made-of-cement
Theo
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Nope, because any transformer will cost more to make than a switch mode wall wart now that the ics are so cheap.

Nope.
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e?

The wall wart I got boxed with a new Sony DAB/FM portable gave out bad interference on FM, probably they never tested it in a Japanese fringe area. Replaced it with an old heavy iron transformer PSU and no more problems.
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On Tue, 9 Aug 2016 17:20:07 +0100, Martin Brown

Unlikely - see below.

Linear external power supplies were effectively killed off by European Directive ErP (Energy related Products) 2009/125/EC (and similar American regulations). The related "COMMISSION REGULATION (EC) No 278/2009 of 6 April 2009 implementing Directive 2005/32/EC of the European Parliament and of the Council with regard to ecodesign requirements for no-load condition electric power consumption and average active efficiency of external power supplies" http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:093:0003:0010:EN:PDF
This requires external power supplies to have efficiencies and no-load power ratings which linear supplies cannot meet.
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Not by enough to matter.

Nope.

The size of it would normally indicate that and the weight if listed does too.
But it makes more sense to get proper branded wall warts that are more likely to have the interference suppressed properly and get them from amazon where you can send it back if it produces interference.
You shouldn't be getting interference on FM, give up on AM.
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See my post about this, some of use like to tune radios around the short waves and only powerline adaptors allow for some ham bands by filtering but most things seem not to bother at all and indeed quite a few routers are also pretty prone to crap all over the place. Digital and switch mode noise is relatively easy to contain if its designed in, but sadly this seems to be a lost art or maybe its penny pinching, I don't know. Brian
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On 10/08/16 12:50, Brian Gaff wrote:

The long and the short of it is that the spectrum up to around 30Mhz is now considered pretty much free for all, and is occupied by no end of devices and indeed interference all operating in those bands.
DSL operates up to several MHz.
SMPSUs in the 100Khz range will propagate muck up to several MHz or more as will anything arcy or sparky.
It doesn't matter that YOU want to listen to AM radio. No one else does, so the pressure to enforce is not there.
The best you could probably do is invest in one really good 12v screened PSU and hang USB sockets off it, and use those to drive equipment.
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The Natural Philosopher wrote:

Even when you have designed in the suppression and passed all the tests, you can still find in the field, a set of circumstances where the suppression does not work correctly. BTDTGTTS!
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On 10/08/16 14:23, Capitol wrote:

Oh tell me about it. Ive DESIGNED RF gear, RF gear in close proximity to PSUS and Shottky TTL.. The TTL was in a tinplate screened can but STILL the muck leaked out. The final suppression cap had to be soldered between the PSU pin and the tinplate case. And had to have wires precisely...so long. An there was still about 3UV of noise on a 75ohm VHF input, unless you attached a screened cable to it.
The AM noise came from the bridge rectifier. A little too fast to switch, Again capacitors right across the bridge terminals help. Not on the circuit board.
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It certainly hasn’t been lost. The best stuff like Apple chargers don’t produce any interference.
The others don’t bother with the best filtering now that so few listen to shortwave anymore.

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I've tried them all plugging it into a suppressed outlet, ferrites earthed biscuit tins etc, but the main problem is that they are badly designed and saturate the core on the tiny transformer used to step down the voltage via a free running oscillator connected directly to the mains via a crude rectification circuit. I've trawled around many peoples junk and found some of the old heavy normal type supplies and added conversion plugs on the end to fit the equipment, No not as efficient much bigger but oh, the bliss at least until the neighbour starts recharging their laptop or the power line internet adaptors two doors down are switched on, then its hash gurgle and buzz from long wave all the way up to over 50 megs in various bands and wandering burbling carriers and screeching noises from the ticking power line devices.
To my mind we should have an electronic equivelent of the noise abatement society. Short of moving into the middle of a field somewhere I doubt things will go back now. Too many Chinese imported crap generators. Brian
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On Wed, 10 Aug 2016 12:44:33 +0100, Brian Gaff wrote:

+1. They're making the radio spectrum into the equivalent of an unlicenced rubbish tip.
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