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.
When no tolerance or input voltage range is specified, ±5%
should be close enough. ±10% will probably be OK most of the time.
I wouldn't go beyond that, particularly with higher voltage.
Too low voltage
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
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
Too high voltage
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.
Replacing Existing Supply
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.
DC versus AC
DC is pretty much all you will find nowadays, which is why this section is buried down here.
AC "power supplies" aren't really power supplies, just transformers that provide isolation and step down the line voltage. The real power supply is then built into the device.
If the device wants AC, then the voltage and current rating rules still apply. You have to make sure the "supply" is meant for your line voltage.
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.
Regulated versus Unregulated
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
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.
Unregulated supplies are not the norm anymore. 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.
Pretty much any consumer electronics power supply available today will be
regulated. 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 12 volt 1 amp 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 modern supplies to tolerate some input voltage
fluctuations, it's not much harder to make the valid input voltage range
wider and cover 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 from 50 to 60 Hz at least.
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
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.
No supply, even a regulated one, can keep its output voltage exactly at
the rating. Usually due to the way the supply works, there will be some
frequency at which the output oscillates a little, or ripples. With
unregulated supplies, the ripple is a direct function of the input AC.
Basic transformer unregulated supplies fed from 60 Hz AC will generally
ripple at 120 Hz, for example. The ripple of unregulated supplies can be
fairly large. To abuse the 12 volt 1 amp example again, the ripple could
easily be a volt or two at full load (1 A output current). Regulated
supplies are usually switchers and therefore ripple at the switching
frequency. A regulated 12 V 1 A switcher might ripple ±50 mV at
250 kHz, for example. The maximum ripple might not be at maximum output