I often face this question. A friend gives me a lot of used modems and routers, but they usually don't come with a power adapter.
Currently, I'm trying to use a router that is specified to work with a 12VDC/0.5A, but the only power supply I have that fits it is 5VDC/0.85A. And of course I could look for an appropriate adapter, but since I got a lot of 5V adapters, I'd like to try.
Thanks
In general it is acceptable to use an adapter that can provide more current than the device needs - the device will only use what it needs.
The voltage must be correct though.
In your example a 12V 1Amp or 2Amp adapter would be OK.
Make sure that the supply is AC or DC as shown on the device.
If DC make sure that the polarity is correct.
If you don't get these things correct the device may be destroyed.
When I face this kind of problem, I'd rummage through my box of "keepable" wall warts for the right voltage and a current compliance that is higher.
Then I'd test it a little bit with various loads to make sure I know it can handle the load (lots of big resistors here -- in this case I'd look for a \$22\:\Omega\$ resistor rated for \$10\:\textrm{W}\$ or better.)
Assuming it passes that test, then I'd rummage in a few small drawers with solderable plugs and see if I have one that fits the router or modem power input snugly to the outer diameter (but not too snugly) and also grabs the inner pin correctly, as well. If I have one, I'd use it. If not, I'd go buy a re-supply of them and wait for arrival.
That in hand, with solder and wire etc., I'd just cut off the old wall wart end and solder on a new one designed to fit. (Verifying that the polarity is correct, obviously.) And then I'd label that adapter for the unit.
Finally, I'd check things out and make sure things are functioning correctly.
It's really not that much work, or that hard to do.
And no, a \$5\:\textrm{V}\$ DC supply isn't going to do anything good with a device expecting a DC \$12\:\textrm{V}\$ supply.
12V input device will not work with 5V adapter normally. You need to check minimum input voltage required by the device as well to take the call of using a different adapter, not just power.
