Wall Wart Power Supply Question

[(unknown)]

Assuming I have the same voltage, correct polarity, and sufficient current, can I generally replace any class 2 power supply with any other?  This will be used for common household electronics.  In particular, when researching power supplies, I see specifications for ripple, noise, setup, rise, and hold up time, and I'm not sure if these things are important to take into consideration when swapping power supplies.  I have access to these specs for the new supply, but not the old one so I can't make any direct comparisons.  I'd also like to know if it matters that the power supply types may be different (switching vs non-switching).

Thanks for any responses.

[ghurd]

Where did the 'class 2' specification come from?

Class 2 needs to be grounded (I think), and not many wall-warts are grounded.

Switching may not be measurable without a load.  

Non-switching (unregulated) may be considerably over-voltage with an undersized load.

It can matter, but usually the wall-wart is what loses the battle in my projects.  I went through a half dozen wall warts in half an hour a couple months ago.

I don't have much trouble with the items.

Cell phones could be an exception.  I have not had problems, but their inputs look pretty specific lately.

G-

[dnix71]

What will this be powering? It does matter. I bought some 12v fans by "O2Cool" at Walgreens for half price, wondering why they were so cheap. I intended to use them off my battery bank but when the a/c at work quit one day I took two with me.

The wall warts that came with them lasted less than 1/2 hour before burning out. There was nothing wrong with the wall warts, but they were seriously undersized and got hot and fried real quick.

If you are going to run sensitive electronics like a printer or cell phone, you need to be careful of the quality of the output, too. Your phone is expensive, a decent dc adapter isn't. The unloaded voltage may be fairly high on a cheap adapter. A power supply intended for a printer will have much better regulation. I save HP printer power supplies when the printers get dumped, because they have a nice stable voltage and usually some short circuit protection, too.

[(unknown)]

Okay, here's a little more detail:  I want to replace some of the old, inefficient power supplies that I have with newer, switching ones that are about 5x more efficient.  I'm considering replacing those on my cordless phone / answering machine, DSL modem, network switch, and more.  The new power supply will be switching, the old ones probably aren't.  "Class 2" is written on the old power supplies, and is on the spec sheet of the new ones.  The specs on the new power supply says "0 to 1A" at the specified voltage and tolerance, so I assume the unloaded voltage won't spike.

On one hand, I'm pretty sure the quality of the new power supply exceeds that of the old one.  On the other hand, I don't want to fry (or shorten the life of) my electronics in this process by making uneducated assumptions.  I know about voltage, current, and polarity, but am not very familiar with ripple, transients, or the effects of a switching power supply on electronics that current use the non-switching variety.

[cardamon]

I have never had a problem.  Everything I have tried funky things with hasn't minded.  I did some experiments with my answering machine and found that it can take a wide range of voltage.  It calls for 9vac and i figured it was rectified immediately anyway so I gave it DC and found it works on 6-12 vdc.  Ive given it up to 15 and it doesnt damage it but it acts funky if given over 12.  Also my external hard drive I give 12 volts right off my batts, so that gets up around 15v and doesnt seem to mind.

[Flux]

I agree with the others, some appliances can tolerate virtually anything and anything near the nominal voltage will do. These usually come with very cheap unregulated supplies. If you use one of these supplies on a sensitive device it will probably kill it. Many of the cheap unregulated things nominally claimed to be 12v can give up to 18v off load.

Similarly this new switching era is not all it seems to be, yes you do save a bit of power but most of the loss in conventional supplies is magnetising current in the transformer and you don't pay for that. Some of the cheap switching things have dreadful ripple on the output and may make audio devices squeal, the RF noise from the switching is also significant and may affect some devices.

As I see it this is mainly aimed at lowering cost and in the end you get what you pay for.

If you have some decent high quality wall warts they will probably power anything within their rating satisfactorily and safely but watch it with the cheap ones.

I have a digital camera and I suspected it was critical on supply so I spent a small fortune on the makers wall wart, this turned out to be an absolute load of crap and was in reality dangerous as the case came apart the second time I pulled it out of a socket. What a load of crap inside, I seriously doubt that it would ever meet its claimed 5kV isolation from creepage distances alone. This is supposed to be a class 3 device, I just think the CE approval on it was just printed on, I can't imagine that the device would pass the tests.

Flux

[ghurd]

My VTech brand cordless phone has a simple "9V" wall wart that reads 17V open, nearly double!

G-

[joestue]

The class II means double insulated

http://en.wikipedia.org/wiki/Double_insulated#Class_II

Hold up times don't matter, load step regulation doesn't either because you are using a real switching power supply. Iron core wall warts have terrible voltage regulation due to their small size (for instance, their load step response is around 2ms rise time and 50% overshoot). (Small size by definition causes low efficiency, to increase voltage regulation, they saturate the transformer.)

If the output voltage matches what the device requires, and the supply is floating, there will be no issues other than EMI. If said device fails due to EMI issues, then a ferrite bead should take care of it or you will have to add bypass caps etc.. etc.. etc.

Thing to watch for is if the device requires a floating supply, a phone for instance, or other cheap poorly designed product.

A lot of consumer electronics are now running off 3.3 volts, and have a buck regulator on the board, most all Linksys and other cheap routers are in this category, they will run on anything from 4 volts to 15, 20 is pushing it.

[(unknown)]

Joestue, you said "Thing to watch for is if the device requires a floating supply, a phone for instance, or other cheap poorly designed product."  Just so that I understand, did you mean an unregulated power supply where the voltage goes up when there is no load? I think of a floating power supply as one with no reference to ground (only 2 wires), and all the applications I have fall under this category.  Sorry if I'm misunderstanding, I'm kinda new to this.

Cardamon, thanks for the info on the various things you've tried.  You certainly are more adventurous than I am, lol.  Even if they worked, have you ever noticed if your devices had a shorter lifespan as a result of not using the OEM-supplied power supply?

Flux, when you mention "dreadful ripple," can you define this quantitatively?  The replacement power supply I'm looking at has a spec of 120mVp-p for ripple and noise, but I have absolutely no idea what this means.  Would this be classified as a decent rating?

[ghurd]

A Class II Appliance is double insulated.

A Class II Power Supply,

"A short installation guideline for Class 2 circuits follows:

• Class 2 requires dry indoor use
  
  
• Only for non hazardous location areas
  
  
• Circuits shall be grounded
  
  
• Two or more Class 2 circuits are permitted within the same cable, enclosure or raceway
  
  
• Separate Class 2 circuits from other circuits"

  
  
  

From

http://www.solahd.com/support/Pdfs/NECCLASS2DCPSTechNote.pdf

Confounding matters is UL1585, a DC power supply can not be considered as a

transformer.

From wiki- "A Class III appliance is designed to be supplied from a SELV (Separated or Safety Extra-Low Voltage) power source. The voltage from a SELV supply is low enough that under normal conditions a person can safely come into contact with it without risk of electrical shock. The extra safety features built into Class I and Class II appliances are therefore not required."

So none of it applies to these 5~9VDC supplies anyway, as far as I can tell.

Warning label on most Christmas lights "Caution / Warning: For Indoor or Outdoor use Only".

G-

[Flux]

These things are rectified to dc and smoothed with capacitors and ideally it would be smooth dc like a battery. Without the smoothing transformer based units will have a ripple at 50/60 Hz. The smoothing and series linear regulators brings the ripple to near nothing.

The switchmode units have ripple at the switching frequency and again it costs money to filter this out. 120mv ripple on a 12v supply doesn't seem much but really it is not very good and as many switch in the audio range that may cause lots of noise on audio equipment. For many applications it may not matter in the least but if you want a unit to deal with everything then that one may not be adequate.

Flux

[ghurd]

Ripple is one of those 'try it and see if it works for you' quantities, IMHO.  IMHO depends on the cost of the item.

I did 12VDC into 12VAC, 12VDC, 9VDC, 9VAC power ports, and some 6 and 8V AC or DC too.

The vast majority worked fine.  The items that did Not work fine didn't really work at all.  Not many were ruined (blew up), and I can not recall one, specifically, that blew up, though I know some did blow up.

Floating means the voltage is not related to ground, or much of anything else.

'I am $20 on the positive side' could mean I have $100 more, or I am $100 less in debt.

And a 12V floating system can shock the sailor vocabulary out of you.

Like I found with an answering machine, on the phone system, connected to the floating 12V system, here,

http://www.fieldlines.com/story/2009/10/28/7530/9571

Big Fun!

G-

[wooferhound]

Almost all 12vdc wallwarts that I have measured were up around 17vdc open circuit. Although I opened one up last week that wasn't working and found a blown 7812 12v voltage regulator inside. All the 9vdc wallwarts I have measured were almost 14v open circuit.

I have been working a lot with wallwarts lately, using them to make LED lighting. I have found that most all of them are different and I would never calculate an LED dropping resister without first checking the actual output from a Wallwart.

I also recently had a 6vdc Wallwart I went ahead and made a small nightlight out of it and found out that it was 6v halfwave pulsating DC when I first tested it. I found a different transformer to use cause I don't want flashing LEDs around the house.

[ghurd]

This is a 900mhz VTech #9111 phone with the molded into the plastic power supply specs as a "classII 9VDC 300ma UL 8L55 power supply Only".

The power supply says class II (with 2-prongs),117VAC 60HZ 6.5W input, 9VDC 300ma output, LEI-4, UL 12J2 US listed E82456 US model #280903003C0.

It is not very heavy, but seems like it is much heavier switcher.

The open output today is 17.55VDC taken with a true RMS Sperry model 350A meter.

Interestingly enough, the non-molded numbers (the branded in with stamps numbers) on the prong side say "0018", which is only 0.45 from to 18.

I had several of this model phone and don't know what the other wall-warts said, and they may have became intermingled, but "the same" models were run from MSW to many other DC voltages, with no catastrophic failures.

This one is Puke Purple, the other one was Gag Green.

There were 2 Gag Green ones, and at least another nasty color too.

The phones are nice, but it would not bother me if one blew up!

G-

[joestue]

What I meant by floating, your 9 vac adapter may be 48 volts below ground in the case of a phone.

If the class II specification indeed demands that the output be grounded, then you don't have to worry about that.

1% ripple is a little high, but its a vast improvement from 20%.