IC 78XX Series includes three terminal positive voltage regulators commonly available in TO 220 package. These are useful in wide range of applications to give precise output voltage. These regulators have internal current limiting, and thermal shutdown circuitry etc. If good heat sinking is provided, these ICs can deliver 1 Amps current. These are generally considered as fixed voltage regulators but their functions can be manipulated to convert them into variable voltage regulators.
The important parameters of 78 XX series are
- Maximum output current 1.5A
- Output voltages – 5, 5.2, 6, 8, 9, 10, 12, 15, 18, 24 volts
- Thermal overload protection
- Short circuit protection
The following circuits give an idea about the input voltage at which the IC gives precise output. Fig 1 and 2 shows the conditions in which the IC7805 is giving precise 5 volt output. 7805 IC requires minimum 6.5 volts to give 5 volt output. To monitor this, a Zener and LED are included at the input. Zener voltage is 4.7 volts and the forward voltage drop of the ordinary diffuse type Red LED is 1.8 volts. When the input voltage is above 6.5 volts (4.7+1.8), Zener conducts and LED turn on. This indicates that IC is getting input voltage above 6.5 volts. When the input voltage reduces below 6.5 volts, Zener cut off and LED turns off.
7805 Circuit 1
Fig 2 shows another LED indication. The brightness of LED reduces as the input voltage reduces below 6.5 volts.