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SMPS v Linear PS

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Musicmanager

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Morning Guys

I've got a bit foxed !
I'm currently putting together a parts list for a future project which will end up with a Christmas present for the Grandkids. The scheme calls for a 12vdc 250mA wallwart but stresses that it must be a linear not switch mode power supply.

I've got a large box full of wallwarts, even some which are usable for correct voltage and either enough or more than enough power but how do I tell which are SM and which are linear. I understand that a couple of them actually say 'Switch Mode' on the label but most have little marking. I know that size of the unit is good clue .. but I'd hope for more than that. There is even one marked 'LPS' which might be an obvious clue, but with my luck it's more likely to stand for 'Like's Plenty of Salt' or something else even more obscure .. .. ..

Any guidance please ..

Thanks
S
 
If you take a simple 12v 1 amp linear wallwart ( PSU ) it will weigh a couple of pounds.. Whereas the Switch mode 12v 1a will be the size on a normal plug and weigh much lighter.. This is because linear PSU's contain a huge transformer with an iron core as they are stuck with 50 hz... Switch mode have tiny transformers as they operate at 52Khz +...

So in essence... If you can't pick it up it's linear!!
 
A linear wall wart usually looks a lot like a cube, because the main thing inside is the cube shaped transformer. SMPS will have more rectangular shape. So for example this http://g01.a.alicdn.com/kf/HTB1n64f...2A-amp-regulated-Wall-Wart-Charger-5-5-mm.jpg is likely a SMPS, while this http://www.winfeng7.com/bbs/data/attachment/forum/201201/24/105206fajft2zpftqxeltx.jpg is most likely linear. Also SMPS will have much smaller volume for the same power than a linear.

Also there is another aspect that Ian remembered me, if you place a magnet on the case of the wall wart in question and you have it powered, if it is a linear one you should feel the magnet vibrate at 50/60 Hz.
 
A suitable load resistor and measure Vdc and Vdc . If Vac >=1% it is SMPS. if <1% it's linear or a good filtered SMPS.
If Vdc>>12V with no load. then no regulation.

The other way is compare gm/W (power density) SMPS are much lighter.
 
Also there is another aspect that Ian remembered me, if you place a magnet on the case of the wall wart in question and you have it powered, if it is a linear one you should feel the magnet vibrate at 50/60 Hz.
As long as it isn't powering something???
 
As long as you don´t touch the mains with said magnet. I don´t think that the fact that it is powering something should be of any consequence.
 
As long as you don´t touch the mains with said magnet. I don´t think that the fact that it is powering something should be of any consequence.
It may do to the "Something"...
 
When you measure the DC resistance between the plug blades of a transformer wallwart, the resistance will be quite low. A switch mode will be much higher, if not appear open circuit.

At least that was the case with the few I tested here. There may be other input circuits to switchmode, but since they don't have a transformer input, I suspect the plug resistance is still higher.

John
 
Is that transonductance per watt?
nice try.

Unregulated DC bricks and those with Linear regulators just regulate rectify AC line f and thus prefer high mu Steel core.

SMPS regulators prefer ferrite for low eddy current loss, a few higher end SMPS may have linear Reg post SMPS. Unlikely here.

The only meaningful metric here is regulated noise ripple if a linear regulated output is specified, usually meaning <= 0.1%.


I think you meant transconductance. gm


There are many grades of Silicon Cold Rolled Grain Oriented Steel Laminate and for lowest cost, low grades are used for consumer cores typical rated as M-6 to M22 where higher M numbers indicate higher loss per kg. Although historically it was rated in USA that M-22 was 2.2 Watts loss per pound, this scale no longer is relevant as variations in Mg content and Si content affected permeability Bmax and loss/weight ratios. Also efficiency of transformers drops from 99% when costs of core are reduced meaning Watts/kg or Watts per gram rises.

1500 kG or 1.5T as the preferred unit of measure now as Bmax is possible but costs a bit more than lower rated Bmax. Steel cores.
 
Thanks Guys ...

So in essence... If you can't pick it up it's linear!!

I think I'll stick with this one ... it sounds a lot safer to me .. I'd kinda like to be around at Christmas to see the kids play with my creation ! :)

S
 
Chances of you having a 12Vdc linear cheap regulator are slim for a wallwort as these are higher cost/Watt and much lower efficiency than SMPS.

So try a SMPS then add a filter. or if you have a 14~16Vdc add a linear 12V LDO
 
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At least that was the case with the few I tested here. There may be other input circuits to switchmode, but since they don't have a transformer input, I suspect the plug resistance is still higher.

You are correct, a linear one will read fairly low across the mains input - a switch-mode one will read high (at least in the hundreds of k probably).

Just tried a couple, linear read 550 ohms, SM read tens of megs and was still going up.
 
What about by just putting a load at the output and see what kind of noise the scope shows?
 
If this has a mic and audio input, just plan on adding a clean regulator using very low LDO adjustable types < 0.2V dropout. or use MOSFET and OpAmp with ref V or anything or define your Vcc/Vdd supply and noise requirement...(hint)


Why guess?
 
Using seat-of-the-pants observations, the product of size and weight versus output power should be a pretty clear indicator f linear vs. switching. Also, you can measure the unloaded output voltage. The vast majority of linear wall warts are unregulated, and 50% overvoltage unloaded is possible. OTOH, the vast majority of switching ww's are at least loosely regulated within 5% or 10%.

Separate from that, what is the system and why does it require a linear ww? That might be mystical thinking on the part of the original designer, someone who had a bad experience with a switching ww but didn't/couldn't figure out why.

ak
 
Hi Guys

The 'Thing' is a Theremin, more especially an Opto-Theremin

The Scheme was published in the EPE magazine in Sept/October issues and I know from previous experience the fun there is to be had with such a device. All the parts, perhaps with the exception of the RF coils are readily available and are used to populate two pre-made PCB's supplied by the magazine.

You need reminding that I'm very much a novice, especially with RF matters, but I imagine the reason for a linear power supply is to do with unwanted noise; also looking at the PCB's and the cases, there isn't room for adding additional V. Regs & filters into the circuit.

It's not a problem in that I have a 12vdc 1.5 amp linear in my box which is very heavy and will be just the job, I just needed help with identification !

Thanks for all the help .. ..

S
 
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how do I tell which are SM and which are linear.
I have a tiny AM radio based on MK484 chip. I use it as audio oscilloscope. Keep near the radio to turned ON wallwarts, if radio starts to make sound like "humm....." (more bass) then that is 'Linear' and if the radio starts to make sound like "cheeeee......" (more treble) then that is SMPS, sure. :)
 
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