AC adaptor for 12VDC -does milliamp rating matter ?

Question

I went to buy an adaptor at Radio Shack and was told that the specified 300mA rating "didn't matter". What happens if I use an adaptor that is rated too high or too low ? Will it damage the device ? Also is there a table to show what adaptor could be used to eliminate batteries (for example two AA batteries in series would need a 3VDC but how many mA ? Or a 9VDC needs how many mA ? etc.) I can't get the concept nailed down in my mind - in an analogy to a water pipe is voltage or amperage the "diameter" of the pipe and which would be the PSI pressure of the water flow ? Amperage? Or is that an unworkable analogy ?

Answer 1

An adapter can supply up to it's rated current. It doesn't matter that you may want to use less. In the sense that as long as the adapter rating is higher than required, then saying it doesn't matter is correct. Of course, it may matter if the cost rises to uneconomic scales. Or size.

Voltage is a different matter. Too high a voltage can damage the connected item. Too low can also have unforseen effects which may lead to damage. So the voltage must be correct. However, it is not necessary to be too pendantic about voltage as all electronic devices will work over a range of voltages. This is usually given as percentage range and might be say +/-15%.

There is no such table for batteries as the cell voltage depends on the chemistry. You can calculate though if you remember that carbon zinc cells give 1.5v, re-chargeable NiCd and NiMh give 1.2v each cell and Li-ion give 3.7v each cell.

In analogy, the diameter (and length) of the pipe would be similar to resistance, Pressure (PSI) would be similar to Voltage and amount of water flowing would be similar to Current.