Anyone that has played with these little workhorses of our world and wondered how they work, read on. The standard three-terminal regulators like the 7805 (5v) are what most people see and use without thinking too much about them. All voltage regulators work about the same. They read a reference voltage between Volts In and Ground and/or Volts Out and they use this reference to set the output voltage. When a load is applied, this reference keeps the output voltage steady.

You can play games with these old regulators by putting a resistor between the Ground Pin and Ground. This can make a standard 7805 give any voltage between 5 volts and your supply voltage. This shows how much regulators are the same.

About supply voltage to a regulator: You need to have at least the regulated voltage plus 2/3 more for basic operation of a regulator. That is, 5 plus about 4 equals about 9 volts, the minimum to get a steady regulated output from a 7805. You also need the current capability of your supply to equal at least twice as much current as your circuit needs. Why? Too little input current equals poor regulation. The regulator will lose its reference voltage.

A good starting point is about twice the voltage and the current that your circuit needs. So for a 5-volt regulator, a 12-volt, 1.5-amp supply is fine. Twelve volts is on the low side for good stable regulation, but having 1.5 amps available makes it a good power supply for a great many things. Each LED uses about 5-10 milliamps. Each servo uses about 20 milliamps. Each motor, 20 to 50 milliamps; and so on. (One amp is 1000 milliamps.)

A power LED is a great way to monitor your power supply. Use a standard-sized LED, in series with a 460Ω to 1000Ω (1kΩ) resistor, connected across power and ground. It will dim and flicker a little each time something like a servo or motor starts up, and that is normal. If the LED stays dim, then you may have something wrong. Not having enough power will do this. Regulators will just shut down or act funny if they do not have enough input voltage for stable operation.