Want to utilize that "DC power" jack on your walkman, but it didn't come with a plug?
Need a power supply for that special project but not sure which one to use?
Here's my little guide for picking the best power supply for your needs.
Step 1: AC and DC Power
AC power comes from the power company, and is accessible through the outlets in your house.
DC power comes from batteries.
Sometimes we need to convert the AC power from the wall to DC power in order to run a device which normally might run on batteries. The tool for this job is an AC/DC adapter or transformer which is often referred to as a "wallwart" as it sticks out of the wall in an unsightly manner.
Most appliances that require an adapter like this have a label near the power jack which details about the voltage and polarity required.
Step 2: Understanding Electricity Basics
To help explain electricity I will use a metaphor which is mostly analogous to the behavior of electricity. That Metophor is the behavior of water in a system of pipes and tanks. Electricity is made of charges, free electrons, which in most metals are able to jump from one atom to another with little problem. In our metaphor we will consider the volume of water to be as a quantity of charge, trading liters for coulombs as it were.
Electrical Resistance is analogous to water flowing in a hose. If you turn the water on a little bit, the water begins to flow out of the hose at some slow rate, maybe 500mL/sec. If you put a kink in the hose, the water begins to flow more slowly maybe 100mL/sec. The pressure at the source, spigot, remains the same, but the rate is decreased as the resistance increases.
Electrically we call the rate of flowing charges Current, and we measure it in Amps which are defined as Coulombs/Second.
We call pressure Voltage, and measure it in Volts.
Resistance is called the same and measured in Ohms.
Just as with water, if we increase the resistance in a circuit and leave everything else the same, the current will decrease.
Now back to the hose. If we let go of the hose so it is flowing at 500mL, and then turn up the spigot to full blast, the rate of flow will increase, maybe to 1L/sec.
In this case electricity also behaves similarly. As the voltage is increased, if resistance stays the same, current increases.
This is why the most important thing to note about your wall wart will be voltage. IF THE VOLTAGE RATING DOESN'T MATCH EXACTLY, THE ADAPTER IS NOT SUITABLE FOR THE JOB. The result will be either the device not powering on for lack of power or the device frying in spectacular ways as large amounts of current it was never designed for burn away its delicate circuitry.
Step 3: Understanding Electricity Basics Part 2
Your wall wart will have an amperage rating, and your device should have a current requirement as well. If they don't you can try a fairly strong one and check it repeatedly for heat build up, but I strongly suggest finding out what the actual requirement is from the manufacturers specs or the manual if it's not printed right on the device itself.
Lets say your device says that it need 4.5V 1A DC. If you plug a 4.5V 600mA power adapter into the device, it may work. BUT DON'T BE FOOLED! You have created a pyrotechnic time bomb waiting for it's chance to melt open and burn your house and family and all you hold dear!
NEVER NEVER NEVER NEVER USE AN UNDER RATED ADAPTER!
The point is have the # of mA higher or at least equal to the devices needs.