Can I use any 12volt AC adaptor for any 12volt DC device

Q1 - Yes
Q2 - Yes

Depending on the device, it may not accept double the current, and if it does, it is a good possibility of it burning out.  If it is a fan for instance, it'll probably run at double the speed it normally would, therefore shortening its' life.

Is it not true if the device is in good working condition, it will only take what amperage it needs.

No! you have to use a dc adaptor for a dc device. An ac power supply at best will make a loud hum on audio equipment or at worst will damage it. Half of the ac wave is negative down the live lead when the device will be expecting a smooth positive signal. A bit like reversing the batteries at the frequency of your mains supply.
Just because the plug fits doesn't mean it's ok. There is no standard yet.

Amps wise, the device will try to draw what it needs and no more if the voltage is correct. so as long as your adapter supplies enough (same or higher) you will be ok. but only dc-dc or ac-ac.

I appreciate your feedback.

Just to make sure I'm clear, I'm only referring to all AC adaptors (such as for laptops, radios routers . .etc)that gets plugged into an AC outlet. It converts AC to DC. (AC input and DC output) It gets plugged into a device that has a DC 12 volt jack. On most adaptors it will not say if the output is DC. I just assumed it is.

I know some AC adaptors are referred to as "Transformers" but I believe they mean the same thing.

Am I referring these correctly? Most Adaptors state that they are AC adaptors. Some folks refer to the same devices as "power supplies"

"... Depending on the device, it may not accept double the current ..."   :-)      The current is determined by the voltage and the impedance of the device you're connecting to.     The rated amperage of your adapter simply shows the maximum current it supports -- it has nothing to do with how much current a device will draw.

You can safely use the adapter for any 12v device as long as two things are true:

(1)   The device doesn't require more than 2 amps;

and

(2)  The polarity of the plug or plug/adapter you're using is correct -- be SURE this is true before connecting the device.

For your specific question ["... can I use this adaptor to replace a failed 12 volt /1A adaptor?"]  the answer is YES.    (Just be sure the polarity of the plug is correct.)

garycase: Thanks

RobinD: It sounds like you are saying the same as "garycase" but I dont understand the "but only dc-dc or ac-ac" as you mentioned above. I dont recall any equipment I mentioned above only dc-dc or ac-ac. I assumed most computer based or related devices (not toasters or heaters) are AC to DC. If thats the case, is the amperage an issue if the adaptor has an amperage rating higher then the device?

I have several AC adapters that have input=120VAC and output 12VAC or 24VAC.  Some are field-adjustable for either output voltage.  So to me, "AC adapter" has AC as an output.

The confusion is understandable.  You mean "AC-to-DC".  The terms "adapter", "converter", "transformer" all have generic definitions and technical ones.  An adapter allows one thing to be used by another, so I use even where technically I should be specifying a transformer, converter, regulator, etc.  I'm not a technical person, so I get confused too.

Agree the description "AC adapter" was incorrect ... but the question clearly defined what was actually meant:  "...  I have a 12 Volt AC adaptor with 12 volts / 2A DC output."

current is supply available  not pushed.. 12v DC at 500ma can be replaced by 12v DC at 1000ma no problem.  going with lower amperage increases the chance of overloading the PS  You should check your tip voltages also some PS's are + tip others are - tip  cut tips and reverse iif necessary.  

You mentioned AC  and DC  AC is not DC and you cannot swap the 2

if you have to swap them use a regulator and then you can go from AC 2 DC

Apparently I assumed too much. I had no idea that this would be so involved.

This is an area where most folks take this for “granted”. All the 12volt AC adaptors I was referring to did not all say the output was DC. I just assumed all of them were. I have seen some adaptors say "Output DC"

For example: I have one in my hand that says: This adapter is for a set of "Dell Speakers". There are no polarity indications. Or maybe the polarity indicator is the (- . +  or + . - ) diagram displayed. If the adapter says AC adapter, are we to assume its AC output? If the adaptor indicates OUTPUT DC then we need to make sure the device plugged into is - in fact a DC jack

Class 2 Power Supply
INPUT: 110-120VAC 60Hz 14W
OUTPUT: 12VDC 0.5A

I have this other model called "switching adaptor"
INPUT: 110-240VAC 50/60Hz 1.0A  (doesn’t say anything about wattage)
OUTPUT: 12V - 2.0A   - >+(doesn’t say anything about DC)
this one has 3 prongs (additional ground) plug that you plug into an AC outlet

I have this other  adaptor for a Linksys Wireless access Point called:
AC adapter
INPUT: 110-240VAC 50/60Hz 0.5A
OUTPUT: +12V   0.5A 6W  + . –

Then I see another adapter labeled as this
Netgear PWR-002-004 AC Adapter Power Supply 12VDC 1.2A

This is an interesting topic and very few folks really know about. I majored in Electronics 20 years ago but can’t recall all the differences. When you go online to research a replacement power supply/AC Adapter, they don’t always provide all the power configurations except for “volts and amperage” . You just have to make sure you look at both adapters and make sure they're in the same class.

>make sure you look at both adapters

That's the first clue...the input side.  If it looks like it plugs into the wall, then 99.99% chance it's AC input.

All of the labels you've listed above make it clear that it's AC input / DC output.  Watts are just Amps x Volts.  So the first power supply you listed (120VAC, 14W) would be 14/120 = 0.12A draw on a 120VAC circuit.

Think of a 1500W hair dryer that sometimes trips the circuit breaker.  1500W/120V =  12.5A.  Enough to pop a 15A circuit that's got something else on it, or a breaker that's getting old.

"... I majored in Electronics 20 years ago ..."    ==>  I assume you at least remember the basics from "Mr. Ohm"  :-)      i.e. wattage = current x volts        So when an adapter "... doesn’t say anything about wattage..."  but says the output is "12V - 2.0A"   it clearly DOES say everything you need to know about the wattage (12v x 2A = 24 watts).

Note:  A "switching adapter" means it's a switching power supply with a DC output.    If it had an A/C output it would simply be a transformer (and would almost certainly indicate that it was an A/C output).    ... if you ever have any doubts, just measure the output voltage with a simple multimeter (you can buy a reasonable one at Home Depot for ~ $20 ... http://www.homedepot.com/h_d1/N-5yc1vZ1xh4/R-100147713/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053 

I'm not concerned about the watts. I remember how to get the wattage as mentioned above. I also understand the amperage concerns when replacing one adapter with an other

I'm still not 100% clear as to "How to tell if one is DC output vs AC output"

aleghart: stated all my mentioned AC adapters (I say AC because I plug them into an AC outlet) are AC in and DC out. Even the ones that dont say DC output. What tells you that they are all DC output ? (as I assumed all were)

I apologize it I'm running this to the ground. I jsut want to be perfectly clear becasue I think this is a important issue that should not be taken for granted

If it says "switching" adapter it's DC;   if it shows a DC output voltage it's DC.    I've never seen one with an AC output that wasn't marked with the appropriate AC output voltage.

But, as I noted above, if you have ANY reservations, just measure the output with a multimeter -- that's the surest way to not take it for granted  (and has the added benefit of both confirming that the adapter is working okay AND verifying the polarity for DC outputs).

If the adapter doesn't have both input and output ratings, I won't use it.  If the label's gone, I don't use it.  I could stick it with a meter and read 12.0VDC...but what if it was a 14.8VDC.  And on multi-pin outputs (like for a large laptop or small-form-factor computer), you have more than voltage and polarity.

See attached picture.  Both are from the same manufacturer.  Both are Class 2 transformers.

Just follow the label, and all should be well.
Class2-transformers.jpg

You'll notice the AC-DC adapter show polarity.  The AC-AC adapter does not.

AC-AC adapters are used for door controllers, locks, CCTV cameras, fans, heaters, etc.  Less loss over distance on small-gauge wire compared to DC.  I've not measured and gathered statistics.  I just match up the power supply to the equipment spec.

In place of wall warts I also have central power supply.  Battery backup, fuses, then run over low-voltage wiring to the equipment.  In those cases, the power supply will have a full manual as well as labels to adhere inside the cabinet.

yes the little circle thingy with the + and - tell you the tip.. which is the inside point.. Use a voltmeter to verify.

watts and amps.. both are power ratings.. there is a formula for converting them from one to the other...

Amps = Watts/Volts

switching is more efficient than non switching it means it flips on and off very fast to attain the proper voltage.. kinda like R/C servos do.  it uses PWM  pulse width modulation to set the power.. thus you dont lose power thru heat dissipation like you would with a non switching supply.. thus switching is better almost all the time.

USE a voltmeter.. if its AC or DC it will be readily apparent and it will also tell you the true voltage.. which is almost always higher than what is specified on the wall wart.....

>the true voltage.. which is almost always higher than what is specified on the wall wart.....

Yes, the loss in DC voltage over a distance can be a problem.  Solution is to make the voltage higher at the source so it drops down to what you need....and don't connect 6" away, or you'll be overvoltage.   OR...use AC.

aleghart:
your examples are perfect but I'm back to where "not all adapters show the output rating" to include AC or DC. From all the information posted here, I'm assuming that "the little circle thingy with the + and -" is common with all "DC output" adapters.

Please see my examples
So I used a volt-meter to on the "Bestec" AC adapter and it showed 11.92 DC volts.

My only concern was - when I go to match up a replacement ac/dc adapter, I am aware of the voltage and wattage requirements as mentioned above. My only concern are ot those AC adapters that dont tell you if the output is AC or DC.
Bestec-adapter.JPG
D-link.JPG
emachine.JPG

You can see why I assumed all AC adapters not labeled as DC output and have the + and - indicators have all been DC output.

I'm totally onboard with all AC adapters or the adapters that arent labeled as "AC adapters" that do show AC or DC output.

All of the adapters in your picture have DC outputs.    As noted above, any time they show polarity, they're DC, as AC doesn't have any polarity associated with it.

>AC doesn't have any polarity associated with it.

Generally.  AC "polarity" refers to hot versus neutral wires.  Important to maintain this when required by the equipment for safety.  But, all my small wall-wart AC supplies lead to things like cameras, relays, buzzers, that kind of stuff where it doesn't matter which leg is hot.  I wouldn't say it never matters, because I don't know that that's true.

I suppose you're right.  I don't think any of my wall-wart devices care (individually).

For multiple units, however, the "polarity" is kept constant between hot/netural.  Cameras have fewer problems when on the same phase and wired identically.  All fed off the same transformer.  All the installer literature notes the hot & neutral legs, and warns to observe polarity.  Probably has a lot to do with eliminating roll/sync issues between camera feeds.

Yes, if you're using multiple devices it's best if they're all on the same phase :-)

Unbelievable great information. I feel like I just finished a semester in Electronics. Thank you all for all the input.

Unbelievable great information. I feel like I just finished a semester in Electronics. Thank you all for all the input.

You are welcome.. and thanks for the points.