The most basic type is an NPN It is wired as a switch and saturates when the input rises above about 0.7v. Because of the low volt drop across the transistor, (Vce) very little power is dissipated.
Most low power types can drive up to 200mA or so with gains (hFE) of about 200-300.
|
|
If more power is required then another high power transistor can be added (TIP31C) to form a Darlington Pair where the second transistor takes most of the power and the first provides most of the gain.
Alternatively a Darlington package can be used instead - such as TIP122 or TIP142 for up to 5Amps!
|
|
For currents up to 500mA, a Darlinton driver array can be used where up to 8 Darlington pair drivers are housed in an 18-pin DIL IC package. These are ok if you are using medium power displays, but the drawback is that you must not exceed the maximum IC package dissipation. Also if one driver malfunctions, you have to replace the whole IC. |