Power supplies are available in a wide range of voltage and current ratings. If I have a device that has specific voltage and current ratings, how do those relate to the power ratings I need to specify? What if I don't know the device's specs, but am replacing a previous power supply with particular ratings?
Is it OK to go lower voltage, or should it always be higher? What about current? I don't want a 10 A supply to damage my 1 A device.
If a device says it needs a particular voltage, then you have to assume it needs that voltage. Both lower and higher could be bad.
At best, with lower voltage the device will not operate correctly in a obvious way. However, some devices might appear to operate correctly, then fail in unexpected ways under just the right circumstances. When you violate required specs, you don't know what might happen. Some devices can even be damaged by too low a voltage for extended periods of time. If the device has a motor, for example, then the motor might not be able to develop enough torque to turn, so it just sits there getting hot. Some devices might draw more current to compensate for the lower voltage, but the higher than intended current can damage something. Most of the time, lower voltage will just make a device not work, but damage can't be ruled out unless you know something about the device.
Higher than specified voltage is definitely bad. Electrical components all have voltages above which they fail. Components rated for higher voltage generally cost more or have less desirable characteristics, so picking the right voltage tolerance for the components in the device probably got significant design attention. Applying too much voltage violates the design assumptions. Some level of too much voltage will damage something, but you don't know where that level is. Take what a device says on its nameplate seriously and don't give it more voltage than that.
Current is a bit different. A constant-voltage supply doesn't determine the current: the load, which in this case is the device, does. If Johnny wants to eat two apples, he's only going to eat two whether you put 2, 3, 5, or 20 apples on the table. A device that wants 2 A of current works the same way. It will draw 2 A whether the power supply can only provide the 2 A, or whether it could have supplied 3, 5, or 20 A. The current rating of a supply is what it can deliver, not what it will always force thru the load somehow. In that sense, unlike with voltage, the current rating of a power supply must be at least what the device wants but there is no harm in it being higher. A 9 volt 5 amp supply is a superset of a 9 volt 2 amp supply, for example.
If you are replacing a previous power supply and don't know the device's requirements, then consider that power supply's rating to be the device's requirements. For example, if a unlabeled device was powered from a 9 V and 1 A supply, you can replace it with a 9 V and 1 or more amp supply.
The above gives the basics of how to pick a power supply for some device. In most cases that is all you need to know to go to a store or on line and buy a power supply. If you're still a bit hazy on what exactly voltage and current are, it's probably better to quit now. This section goes into more power supply details that generally don't matter at the consumer level, and it assumes some basic understanding of electronics.
Very basic DC power supplies, called unregulated, just step down the input AC (generally the DC you want is at a much lower voltage than the wall power you plug the supply into), rectify it to produce DC, add a output cap to reduce ripple, and call it a day. Years ago, many power supplies were like that. They were little more than a transformer, four diodes making a full wave bridge (takes the absolute value of voltage electronically), and the filter cap. In these kinds of supplies, the output voltage is dictated by the turns ratio of the transformer. This is fixed, so instead of making a fixed output voltage their output is mostly proportional to the input AC voltage. For example, such a "12 V" DC supply might make 12 V at 110 VAC in, but then would make over 13 V at 120 VAC in.
Another issue with unregulated supplies is that the output voltage not only is a function of the input voltage, but will also fluctuate with how much current is being drawn from the supply. A unregulated "12 volt 1 amp" supply is probably designed to provide the rated 12 V at full output current and the lowest valid AC input voltage, like 110 V. It could be over 13 V at 110 V in at no load (0 amps out) alone, and then higher yet at higher input voltage. Such a supply could easily put out 15 V, for example, under some conditions. Devices that needed the "12 V" were designed to handle that, so that was fine.
Modern power supplies don't work that way anymore. Pretty much anything you can buy as consumer electronics will be a regulated power supply. You can still get unregulated supplies from more specialized electronics suppliers aimed at manufacturers, professionals, or at least hobbyists that should know the difference. For example, Jameco has wide selection of power supplies. Their wall warts are specifically divided into regulated and unregulated types. However, unless you go poking around where the average consumer shouldn't be, you won't likely run into unregulated supplies. Try asking for a unregulated wall wart at a consumer store that sells other stuff too, and they probably won't even know what you're talking about.
A regulated supply actively controls its output voltage. These contain additional circuitry that can tweak the output voltage up and down. This is done continuously to compensate for input voltage variations and variations in the current the load is drawing. A regulated 1 amp 12 volt power supply, for example, is going to put out pretty close to 12 V over its full AC input voltage range and as long as you don't draw more than 1 A from it.
Since there is circuitry in the supply to tolerate some input voltage fluctuations, it's not much harder to make the valid input voltage range wider and cover any valid wall power found anywhere in the world. More and more supplies are being made like that, and are called universal input. This generally means they can run from 90-240 V AC, and that can be 50 or 60 Hz.
Some power supplies, generally older switchers, have a minimum load requirement. This is usually 10% of full rated output current. For example, a 12 volt 2 amp supply with a minimum load requirement of 10% isn't guaranteed to work right unless you load it with at least 200 mA. This restriction is something you're only going to find in OEM models, meaning the supply is designed and sold to be embedded into someone else's equipment where the right kind of engineer will consider this issue carefully. I won't go into this more since this isn't going to come up on a consumer power supply.
All supplies have some maximum current they can provide and still stick to the remaining specs. For a "12 volt 1 amp" supply, that means all is fine as long as you don't try to draw more than the rated 1 A.
There are various things a supply can do if you try to exceed the 1 A rating. It could simply blow a fuse. Specialty OEM supplies that are stripped down for cost could catch fire or vanish into a greasy cloud of black smoke. However, nowadays, the most likely response is that the supply will drop its output voltage to whatever is necessary to not exceed the output current. This is called current limiting. Often the current limit is set a little higher than the rating to provide some margin. The "12 V 1 A" supply might limit the current to 1.1 A, for example.
A device that is trying to draw the excessive current probably won't function correctly, but everything should stay safe, not catch fire, and recover nicely once the excessive load is removed.
The three main parameters for a power supply are
If your device or broken wall wart says 9 V DC, get a 9 V DC replacement. The right voltage and type of voltage are important: a too high voltage may damage your device, a too low voltage too, though that's less common. But at a too low voltage it may not work properly.
Get a power supply at minimum the rated current. If the device says 500 mA, get a power supply that can deliver at least that. A 100 mA wall wart may overheat and set your house on fire if not properly protected. A 1000 mA is safe, even if you only need 500 mA.
Olin explained with fruit, I'll try to explain with another flow: water. If my faucet can fill a bucket in a minute that's its maximum flow, or current. That's the 1000 mA the wall wart specifies. Yet I can open the faucet partly to fill my glass, and then I'm the 500 mA device. The faucet still can supply 10 liter per minute, but will supply less if I ask for less. I can ask for anything as long as it isn't more than the 10 liter per minute. If I want only a few drops to fill a thimble I'll be a 1 mA device. From a 1000 mA supply.
If you are replacing a "wall wart" with a salvaged one then there are a couple more things to be aware of in addition to what the other commenters have already said.
AC/DC: Power adapters can output either alternating current or direct current. You must make sure it matches what the device expects. Most adapters and devices are DC. You can tell by looking at the symbols: a "~" symbol means AC, a "=" with a dashed line means DC.
Polarity: If you are using an adapter with the round barrel jacks, then be aware that sometimes the centre is positive and the outside is negative, and sometimes it is the opposite. This must match what the device expects, or it could damage the device. If it doesn't match then the fix is simple, just cut the jack off and swap the wires around. There will be a symbol on the adapter showing which is positive and negative. Usually it's "centre positive".
A related question came up recently:
I'd like to power my new LG M2450D monitor with a basicXL BXL-NBT-U02 universal AC-DC power adapter. This PSU is rated 90W max and supports 15-24V output voltage. The original monitor PSU (PA-1650-68) was fixed at 19V and 3.32A.
Now, doing the math: 90W / 19V = 4.74A
Of course this is an ideal value, what is "safety margin" i should assume for the real max amperage?
To be conservative in the face of ambiguous specifications like these, you should assume that the maximum power rating of the universal adapter occurs at the maximum output voltage, and that all lower output voltages are limited to the same current.
In this case, 90W/24V = 3.75A, so you should be good to go with the output set at 19V with this amount of current.
Power supplies are available in a wide range of voltage and current ratings. If I have a device that has specific voltage and current ratings ... is it OK to go lower voltage, or should it always be higher? What about current? I don't want a 10 A supply to damage my 1 A device.
See Olin's superb answer for a more detailed treatment.
My aim here is to directly address the two most common core question that many beginners come with. They are answered in Olin's response but (based on my significant experience assisting beginners) I consider they are well enough buried among other detail that they would be missed by many newcomers.
Voltage:
The specified voltage of device and the voltage of supply
should always be close to identical.
What constitutes "close" varies with devices, but, as a guide
A supply whose voltage is within -10% / +5% of device voltage rating will usually work and usually not cause damage. In a very few cases it may not work, but this would be unusual.
For example, provided the current rating is adequate (see below)
A 19V device will usually operate from an 18V supply.
A 6V device may operate from a 5V supply - but as that's nearly 20% below spec, in some cases this difference may be too high
A 16V device may well NOT operate from a 12V supply - in some cases it will.
Damage to supply or device is unlikely but may occur.
BUT trying to operate a 12V rated device from a 16V supply (25% high) is risky and may cause damage to device or supply.
Current:
A device that draws a specified current can be operated from a supply able to supply
the same or higher current.
eg consider a 12V, 2A device and a 12V 20A power supply.
12V is the "electrical pressure".
20A is the electrical current that the supply CAN provide at that pressure.
2A is the current that the load WILL take at that pressure.
You then have 20-2 = 18A for use elsewhere.
One thing that doesn't appear to be in the provided answers is about fusing and the possibility of a fire hazard. For instance, if you are using a supply that can deliver 1 amp maximum and you change it to a supply that can deliver (say) 10 amps maximum then for sure, it will still adequately power your "device" but, this might now become a fire hazard.
In the original set-up, your "device" may not have needed a fuse when working with the 1 amp (maximum) supply. If your device failed short-circuit, there would be little reason to suspect it might melt or burn when powered with the original 1 amp power supply under short-circuit conditions.
However, now that it's connected to (say) a 10 amp supply, if your device went short-circuit, it might very well become a fire hazard. To rectify this situation you would need to add a 1 amp fuse. I guess, for protection against fire, a replacement with anything more than the original rated current needs to be considered as a possible fire risk.
