If you've done much poking around consumer digital electronics, you've probably seen them: Those weird three-pin devices which seem to be connected to the power supply, and thus probably have something to do with power regulation. But maybe you never actually bothered to figure out what they were for, thinking they were silly. Well, voltage regulators, as they are simply called, are actually very simple devices, and they serve an important and useful function in the field of electronics, even though their function has more to do with analog than digital electronics. Essentially, all a voltage regulator does is, obviously, regulate voltage; That is, it limits the voltage that passes through it. Each regulator has a voltage rating; For example, the 7805 IC (these regulators are often considered to be ICs) is a 5-volt voltage regulator. What that means is that no matter how many volts you put into it, it will output only 5 volts. This means that you can connect a 9-volt battery, a 12-volt power supply, or virtually anything else that's over 5 volts, and have the 7805 give you a nice supply of 5 volts out. Handy, eh? There are also 7812 (12-volt) and 7815 (15-volt) three-pin regulators in common use.
The pinout for a three-pin voltage regulator is as follows:
1: Voltage in
3: Voltage out
For example, with a 9-volt battery, you'd connect the positive end to pin 1 and the negative (or ground) end to pin 2. A 7805 would then give you +5 volts on pin 3.
Voltage regulators are simple and useful. There are only two important drawbacks to them: First, the input voltage must be higher than the output voltage. For example, you cannot give a 7805 only 2 or 3 volts and expect it to give you 5 volts in return. Generally, the input voltage must be at least 2 volts higher than the desired output voltage, so a 7805 would require about 7 volts to work properly. The other problem: The excess voltage is dissipated as heat. At low voltages (such as using a 9-volt battery with a 7805), this is not a problem. At higher voltages, however, it becomes a very real problem and you must have some way of controlling the temperature so you don't melt your regulator. This is why most voltage regulators have a metal plate with a hole in it; That plate is intended for attaching a heat sink to.
Do not confuse three-pin voltage regulators with a device known as a TRIAC (short for triode AC switch). It is easy to associate them with each other, since they look similar (both have three pins) and they both regulate power. However, the 78XX type of regulators are used for regulating DC current, while TRIACs are used for AC current. Many thanks to Thomas Maslen for pointing out this to me, so I could fix this page (which previously repeatedly used the term "TRIAC" interchangeably to refer to 7805-type devices).
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