# Power Supply Basics

The purpose of a power supply is to supply power. Power is voltage times current. Therefore, a power supply provides voltage and current. Many of them do a lot of other things as well.

A power supply typically brings in (input) one form of power and provides (output) another form of power. If you live in the United States, it is commonly 120VAC in with a lower DC voltage out.

Power costs money in one way or another. Therefore, when you convert power, you want to minimize how much is lost. At some point though, it will cost more to limit losses even further than you will save.

1. CV (constant voltage): A perfect power supply would hold the set voltage no matter how much current is demanded of it. In real life, power supplies have a maximum current that they can provide before the voltage drops.
• Some power supplies have a fixed voltage. Other’s have either a dial or buttons that you use to adjust the voltage. It they don’t have a display that shows the voltage (and usually other things), then you need to read what the output voltage is if it is fixed, or measure it with a multimeter.
2. CC (constant current): A perfect power supply would keep the set amount of current flowing no matter how much resistance there is. It raises or lowers it’s voltage to whatever is needed to do so. In real life, power supplies have a maximum voltage they can provide before the load will limit the current further.
• Some power supplies don’t limit current and may be damaged if you short them. Some have have a fixed maximum current. Others have a dial or button you can use to adjust the maximum current. If the power supply doesn’t display the current flow, and/or maximum current it will pass, then it might have the value written on the unit, you might have to measure it, or find some other way to observe and/or calculate how much is flowing.
3. Short circuit protection (not always a feature): The power supply usually cuts power when it senses that there is a direct electrical connection between the + and – supply, or with a third middle voltage terminal called ground if there is one.