replacing a wall wart

[haglered]

I read this question in another forum and I thought I would like you guys to give me a perspective on it.

I recently had a "wall wart" power supply go bad on some equipment.  I was
able to find an identical power supply to replace it, but as I was going
through my box of orphaned power supplies, it got me to thinking....
 
These things are typically labeled with volts, amps, and polarity.  For
example, the power supply I replaced was 5 volts, 2 amps, and positive in
the center.  I was once told by a trusted electrician that a replacement
power supply had to be no more than 1 volt off what the equipment calls for
(and some would not agree with that), at least equal in amps, and the same
polarity.  So in my case, I was looking for a power supply that was 4, 5, or
6 volts, and at least 2 amps, with positive in the center.
 
Now, I also know that 5 volts x 2 amps = 10 watts.  So what would happen if
I found a power supply that was 4 volts and 2.5 amps?  Or 3 volts and 3
amps?  Or for an extreme example, 10 volts and 1 amp?
 
I guess what I'm asking is, can I just go by watts, or do I need to pay
separate attention to the volts and amps?  How close do I need to be?

[Hypernova]

It depends on what the equipment is capable of handling, some will indeed just pull more current at a lower voltage. But there is a drop off point where they will just refuse to work. Unless you take it apart and look up the part's datasheet or experiment you just won't know. Although +5% and -10% should be a decent ball park figure.

[Zero999]

I was once told by a trusted electrician that a replacement
power supply had to be no more than 1 volt off what the equipment calls for
(and some would not agree with that),

That depends on the application.

There are two types of wall wart: regulated and unregulated, the former maintain the same voltage regardless of the load and the latter don't, the voltage is higher off load than on load. Generally switched more PSUs are regulated but this isn't always the case with cheaper units.

Most 5V appliances need a stable 5V supply and typically have fairly tight tolerances i.g. 4.5V to 5.5V so a 6V supply might damage it, especially if it's not regulated.

at least equal in amps, and the same
polarity.  So in my case, I was looking for a power supply that was 4, 5, or
6 volts, and at least 2 amps, with positive in the center.

In this case I wouldn't recommend replacing it with anything other than 5V >2A
 

Now, I also know that 5 volts x 2 amps = 10 watts.  So what would happen if
I found a power supply that was 4 volts and 2.5 amps?  Or 3 volts and 3
amps?  Or for an extreme example, 10 volts and 1 amp?

If the voltage is too low, the device might not work properly, if the voltage is too high, it might be damaged.

If the current rating is too low, the adaptor might shut down or could overheat and stop working permanently.

Also don't go too overboard on the current if the wall wart isn't regulated because the voltage could be too high.

I guess what I'm asking is, can I just go by watts, or do I need to pay
separate attention to the volts and amps?  How close do I need to be?

As above make sure the voltage matches as closely as possible and if it's 5V make sure you use a regulated 5V supply. As a generaly rule, you'll be fine if the PSU is regulated and the current rating is the same or higher than required by the device.