What are the necessary specs to look for in a power adapter? For example, if I have a device that takes 9V–200mA, can I use an adapter that outputs 9V–300mA? Certainly the voltages must match, but what are the consequencs for using lower/higher amperage, since some devices don’t display their amperage? Are there any other vital specs to be aware of?
The amperage rating is a maximum; there is likely an internal fuse that will blow if it goes too much over that rating, to reduce the catastrophe of a short. If your widget requires 200mA at 9V, a power supply rated for 300mA at 9V will be fine. A 300mA widget running on a power supply rated for 200mA may not work so well.
Other than that, check and doublecheck the polarity of the adapter matched the polarity your widget is expecting.
Thank you, good sir.