Author Topic: How to pick a quiet ac adapter  (Read 1185 times)

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Offline bobcat2000

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How to pick a quiet ac adapter
« on: April 11, 2021, 04:14:23 pm »
Hi Guys,

I picked up a bunch of used 5V and 12V household AC adapters.  Some of them are very noisy.  Noisy means when I plug it in, I hear noise coming from my am/fm radio.  They are generating RF interference.

Will a AC adapter printed with a FCC label be more quiet?

Thanks!


« Last Edit: April 11, 2021, 04:19:47 pm by bobcat2000 »
 

Offline madires

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Re: How to pick a quiet ac adapter
« Reply #1 on: April 11, 2021, 04:38:53 pm »
You can test that by simply taking a sharpie and writing "FCC" on the wallwarts. ;D Even the cheapest stuff is often marked "CE", "FCC" and so on. So you can't really rely on those markings unfortunately. I'd go for PSUs from known manufacturers, e.g. MeanWell.
 

Online BrokenYugo

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Re: How to pick a quiet ac adapter
« Reply #2 on: April 11, 2021, 07:01:36 pm »
Look for the heavy ones with a mains frequency iron core transformer.
 

Offline bobcat2000

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Re: How to pick a quiet ac adapter
« Reply #3 on: April 11, 2021, 10:52:46 pm »
Look for the heavy ones with a mains frequency iron core transformer.

How can you tell what is inside?  They are all black enclosed in the modding.  Are you saying the heavier the better?  Like going to buy some meat from the market?
 

Online ledtester

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Re: How to pick a quiet ac adapter
« Reply #4 on: April 12, 2021, 12:02:54 am »
 

Offline bobcat2000

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Re: How to pick a quiet ac adapter
« Reply #5 on: April 12, 2021, 01:59:34 am »
Thanks!

So, the keyword is to get a quiet AC adapter is to get a "Transformer".  One of those old school tofu looking cubic transformer that won't transform into a fighting robot.

Correct?
 

Online ledtester

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Re: How to pick a quiet ac adapter
« Reply #6 on: April 12, 2021, 02:16:25 am »
Yup.

Linear power supplies also use transformers which make them a lot heavier but you get a lot less noise on the output.

Ham radio operators, for instance, like to use linear power supplies because of their low noise.

For a look inside a typical linear power supply, skim through this video:

https://youtu.be/dXtSTpBXItI

You'll see a big huge transformer, big caps and a large heat sink.
 

Offline bobcat2000

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Re: How to pick a quiet ac adapter
« Reply #7 on: April 12, 2021, 02:33:08 am »
Those power supplies that look like a brick is switching.  They turn on and off the electricity very fast.  That's why they are making radio noise.  Correct?
 

Online ledtester

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Re: How to pick a quiet ac adapter
« Reply #8 on: April 12, 2021, 02:51:46 am »
Yes. Their switching frequency is typically in the range of 50 kHz to 2 MHz, so you get that plus all of the harmonics.
 

Offline tooki

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Re: How to pick a quiet ac adapter
« Reply #9 on: April 12, 2021, 05:08:28 am »
But bear in mind that energy efficiency rules have all but killed off the transformer-based wall wart. (They waste more electricity when under low load or no load.) So it’s nearly impossible to buy them new.

Additionally, and this is a big one: not all transformer based wall warts are fully regulated. Some have a proper voltage regulator in them and output a regulated voltage. Some use minimal regulation that keeps it distantly where it need to be. And others do nothing more then rectify it and smooth it with some capacitors. This means the unloaded voltage can go significantly higher than the rated voltage, causing damage in a device not designed for that.


Ultimately, it’s entirely possible to design a switching power supply that isn’t noisy: reputable manufacturers do it all the time. EMI noise isn’t the only reason to stick to reputable manufacturers, though: performance and safety are two more huge ones. El-cheapo ones often aren’t capable of anywhere near their rated power, and often are hair-raisingly dangerous in their construction.
 
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Offline james_s

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Re: How to pick a quiet ac adapter
« Reply #10 on: April 12, 2021, 05:29:36 am »
How can you tell what is inside?  They are all black enclosed in the modding.  Are you saying the heavier the better?  Like going to buy some meat from the market?

The iron transformer type are much bulkier and heavier than the switching type, a 12V 3A iron transformer wall wart will weigh over a pound and be a handful to hold, most of that type are 1A or less and still several times heavier than comparably rated switchmode types.

AFAIK the iron transformer wall warts were outlawed due to energy efficiency requirements but there are still a lot of them around. They do tend to be considerably more reliable though, I'm using one for my broadband router after having multiple switchmode PSUs fail. The somewhat higher energy consumption is worth it for the reliability.
 

Offline madires

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Re: How to pick a quiet ac adapter
« Reply #11 on: April 12, 2021, 09:54:37 am »
Yep, small transformers have a poor efficiency. The small ones typically used in wall warts are around 50-60%. A 100VA is about 80% while large ones, like in power substations, can have 99%. However, small cheap switching mode wall warts aren't really more efficient, but smaller, cheaper to produce, and have less weight. Smaller and less weight also means less expensive shipping.
 

Offline tooki

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Re: How to pick a quiet ac adapter
« Reply #12 on: April 12, 2021, 10:28:37 am »
Look for the heavy ones with a mains frequency iron core transformer.

How can you tell what is inside?  They are all black enclosed in the modding.  Are you saying the heavier the better?  Like going to buy some meat from the market?
Oh yeah, one other clue is that transformer wall warts are almost always single-voltage on the input side (e.g. 120V or 230V). In contrast, a switch mode supply is usually world voltage (e.g. 90-250V). Exceptions exist in both kinds, so it’s not a guarantee, but is usually a good clue.

(Switchable input voltage is not unusual for devices with built-in transformer based power supplies, for example bench multimeters. But it’s really rare in external AC adapters.)
 

Offline tooki

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Re: How to pick a quiet ac adapter
« Reply #13 on: April 12, 2021, 10:38:31 am »
Yep, small transformers have a poor efficiency. The small ones typically used in wall warts are around 50-60%. A 100VA is about 80% while large ones, like in power substations, can have 99%. However, small cheap switching mode wall warts aren't really more efficient, but smaller, cheaper to produce, and have less weight. Smaller and less weight also means less expensive shipping.
AFAIK, the big advantage in little ones is much lower waste when under no load or very low load. (That’s why transformer wall warts are de-facto banned for most purposes, as the energy laws require wall warts to have practically zero power draw when unloaded. That’s achievable with switch mode, but not with transformers.)
 

Offline madires

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Re: How to pick a quiet ac adapter
« Reply #14 on: April 12, 2021, 11:06:37 am »
That's right! Stand-by power usage is lower for switching mode wall warts in general. But switchable power strips save even more. ;) The differences in efficiency are quite interesting. I have replaced all wall warts (nearly all switching mode) in my SOHO data center with a central high efficiency SMPSU and also have measured the power consumption before and afterwards. The power consumption for the former wall wart powered devices dropped to 50%.

 

Offline bobcat2000

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Re: How to pick a quiet ac adapter
« Reply #15 on: April 12, 2021, 06:20:35 pm »
Right, those transformer ac adapters are no good.  I have a bunch of those.  They never give out the correct voltage.

So, what are the "reputable manufacturers"?  This is a myth like the Big Foot.  Buying electronic is like playing Russian Roulette nowadays.



 

Offline tooki

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Re: How to pick a quiet ac adapter
« Reply #16 on: April 12, 2021, 07:18:35 pm »
No, it’s not a myth. If you buy only name brand from authorized distributors, you’re fine.

For USB chargers, I stick to Apple and Anker. (And the ones included with Logitech gear.) DO NOT buy Apple, Samsung chargers on Amazon, eBay, etc, as they’re likely to be fake. But if you get one at e.g. Best Buy, BH Photo, etc. it’ll be genuine.

For most other things, Mean Well is a great value.
« Last Edit: April 12, 2021, 07:20:19 pm by tooki »
 

Online BrokenYugo

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Re: How to pick a quiet ac adapter
« Reply #17 on: April 12, 2021, 08:05:37 pm »
Right, those transformer ac adapters are no good.  I have a bunch of those.  They never give out the correct voltage.


They do under load, or, where more precise regulation is required, with a linear regulator after them. Everything is a compromise, the easy path to a low noise power supply, batteries aside, is a linear one.
 

Offline Zero999

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Re: How to pick a quiet ac adapter
« Reply #18 on: April 12, 2021, 09:14:53 pm »
Additionally, and this is a big one: not all transformer based wall warts are fully regulated. Some have a proper voltage regulator in them and output a regulated voltage. Some use minimal regulation that keeps it distantly where it need to be. And others do nothing more then rectify it and smooth it with some capacitors. This means the unloaded voltage can go significantly higher than the rated voltage, causing damage in a device not designed for that.
Some cheap switched mode power supplies are also unregulated. I echo your comment about only buying genuine brand units, which will also give good voltage regulation, as well as for safety reasons.
 

Offline james_s

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Re: How to pick a quiet ac adapter
« Reply #19 on: April 12, 2021, 09:31:19 pm »
Yep, small transformers have a poor efficiency. The small ones typically used in wall warts are around 50-60%. A 100VA is about 80% while large ones, like in power substations, can have 99%. However, small cheap switching mode wall warts aren't really more efficient, but smaller, cheaper to produce, and have less weight. Smaller and less weight also means less expensive shipping.

If you look at the specs for good quality ones they're typically around 80-85% efficient, that's a significant improvement over 50-60% of iron transformer types. I'm not sure outlawing them was the right approach though, the transformer type are much more reliable and last much longer, and iron and copper are both easily recycled. This all offsets some of the inefficiency.
 

Offline james_s

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Re: How to pick a quiet ac adapter
« Reply #20 on: April 12, 2021, 09:35:32 pm »
Right, those transformer ac adapters are no good.  I have a bunch of those.  They never give out the correct voltage.

So, what are the "reputable manufacturers"?  This is a myth like the Big Foot.  Buying electronic is like playing Russian Roulette nowadays.

Sure they do, they give the correct rated voltage when placed under the specified load with the specified input voltage, outside of that area the voltage will not be what it says on the nameplate, that is not a defect, that is how they are designed. The vast majority of them are not regulated and make no claims of being regulated, they are used in applications where regulation is not needed. Even with the switchmode adapters regulation is often not needed. The broadband routers I mentioned for example, the vast majority of them run on 3.3V internally but come with a 5V or even 12V adapter. They have internal switching regulators to generate whatever voltages are needed, they will work just fine over a fairly wide range of input voltage.
 

Offline perieanuo

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Re: How to pick a quiet ac adapter
« Reply #21 on: April 13, 2021, 01:25:42 pm »
when i'm in a hurry for smps with low rf noise, i pick usually some router smps power supply (12V/2.5-3A) and it just do the job (last week i did this for dvb-t and fm amplifiers and it worked, another smps suppliy was noisy)
or just the transformer ones, you already got the idea, the 'heavy' bulky ones
i think the future is smps, old fellas will tell you smps is toooo noisy for this and for that, that's not true, there are sensitive stuff powered with smps, just good smps supplies :)
 

Online TimNJ

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Re: How to pick a quiet ac adapter
« Reply #22 on: April 13, 2021, 11:27:46 pm »
Right, those transformer ac adapters are no good.  I have a bunch of those.  They never give out the correct voltage.

So, what are the "reputable manufacturers"?  This is a myth like the Big Foot.  Buying electronic is like playing Russian Roulette nowadays.

Delta, XP Power, Artesyn are probably a little more upper-end. If you are genuinely concerned by noise, I feel you might have a better shot starting off in a little higher price bracket.

Mean Well seems to have a bad rep around here. But, Mean Well usually has the competition beat in terms of price by quite a lot. And, since people are probably buying the cheapest power supply that meets their needs (on paper), there's probably a lot more people buying Mean Well compared to from other suppliers. So, maybe disproportionate bias against Mean Well is due to larger pool of adopters.

That said, I've known Mean Well to cut it pretty close in the EMC department.
 

Offline james_s

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Re: How to pick a quiet ac adapter
« Reply #23 on: April 13, 2021, 11:39:05 pm »
Right, those transformer ac adapters are no good.  I have a bunch of those.  They never give out the correct voltage.

So, what are the "reputable manufacturers"?  This is a myth like the Big Foot.  Buying electronic is like playing Russian Roulette nowadays.

Delta, XP Power, Artesyn are probably a little more upper-end. If you are genuinely concerned by noise, I feel you might have a better shot starting off in a little higher price bracket.

Mean Well seems to have a bad rep around here. But, Mean Well usually has the competition beat in terms of price by quite a lot. And, since people are probably buying the cheapest power supply that meets their needs (on paper), there's probably a lot more people buying Mean Well compared to from other suppliers. So, maybe disproportionate bias against Mean Well is due to larger pool of adopters.

That said, I've known Mean Well to cut it pretty close in the EMC department.

Nobody is going to argue them to be top notch high end gear, but they are reasonably good and I think offer good value. As far as low cost power supplies go Mean Well is some of the better stuff IMO.
 

Online TimNJ

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Re: How to pick a quiet ac adapter
« Reply #24 on: April 14, 2021, 03:32:24 pm »
Right, those transformer ac adapters are no good.  I have a bunch of those.  They never give out the correct voltage.

So, what are the "reputable manufacturers"?  This is a myth like the Big Foot.  Buying electronic is like playing Russian Roulette nowadays.

Delta, XP Power, Artesyn are probably a little more upper-end. If you are genuinely concerned by noise, I feel you might have a better shot starting off in a little higher price bracket.

Mean Well seems to have a bad rep around here. But, Mean Well usually has the competition beat in terms of price by quite a lot. And, since people are probably buying the cheapest power supply that meets their needs (on paper), there's probably a lot more people buying Mean Well compared to from other suppliers. So, maybe disproportionate bias against Mean Well is due to larger pool of adopters.

That said, I've known Mean Well to cut it pretty close in the EMC department.

Nobody is going to argue them to be top notch high end gear, but they are reasonably good and I think offer good value. As far as low cost power supplies go Mean Well is some of the better stuff IMO.

No doubt that Delta, et. al. are not "top-of-the-line", but after inspecting/tearing down many of their products, it's clear that they are highly competent manufacturers. (They have no choice at their size and annual volume!) And, Mean Well is also just as competent, but their priority is always on providing high value, and the design methodologies seem to reflect that.
 


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