Get to know the classic three terminal voltage regulators you can use for your projects.
Introduction
A stable power supply is essential for any electronics project. Knowing the right kind can be critical in certain situations, especially if you have a sensitive circuit. You must determine whether you need a linear or switch-mode power supply. The classic three-terminal voltage regulators are the way to go if you need a stable, off-the-shelf linear voltage regulator.
About Linear Voltage Regulators
Previously, you’ve learned about Basic Linear Power Supply Fundamentals. That blog taught you how to set up your very own linear voltage regulator. It discusses all the needed stages, such as the step-down, rectification, filtering, and regulation stages. It is in the regulation stage where you can apply the classic and time-tested three-terminal voltage regulators.
The Classic Three Terminal Voltage Regulators
78XX and 79XX series
National Semiconductor (now Texas Instruments) originally developed the 78XX series of voltage regulators. The 78XX series are robust linear positive voltage regulators with built-in thermal, short-circuit protection and safe output area compensation. Depending on their manufacturer, they come in various fixed-voltage values and can supply currents of up to 1.5A. Here are the fixed voltage variations of the 78XX:
- 7805 for 5V
- 7806 for 6V
- 7808 for 8V
- 7809 for 9V
- 7810 for 10V
- 7812 for 12V
- 7815 for 15V
- 7818 for 18V
- 7820 for 20V
- 7824 for 24V
They are widely used in many linear power supplies and electronic devices. They are reliable, safe, and easy to use, requiring only a few external components (filter and stabilization capacitors).
The 78XX’s negative-voltage counterpart are the 79XXs. Here, the negative-voltage rail is regulated with respect to the ground.
A typical application circuit for the 78XX is seen below:
As for the packaging, the L78XX devices usually come in a TO-220 package for the through-hole version.  DPAK (or TO-252) package is used for the SMD version.
The pin assignments for the 78XX would be:
78LXX or 79LXX Series
If you want a lower-power version of the 78XX series, you can opt for the 78LXX series. These voltage regulators can output a maximum of 100mA. They include the usual built-in thermal and short-circuit protection as their 78XX counterpart. Below are the usual values of the 78LXX:
- 5.0VÂ (78L05)
- 6.2VÂ (78L62)
- 8.2VÂ (78L82)
- 9.0VÂ (78L09)
- 10.0V (78L10)
- 12VÂ (78L12)
- 15VÂ (78L15)
- 18V (78L18)
- 24V (78L24)
Just like the 78XXs, the 78LXX’s negative-voltage counterparts are the 79LXXs. Here, the negative-voltage rail is regulated with respect to the ground.
Below is the usual application circuit for the 78LXX series.
LM340-XX Series
The 78XXs have an equivalent counterpart originally developed by another manufacturer, Texas Instruments. They are the LM340-XXs. These voltage regulators have almost the same characteristics as the 78XXs, only that, it is said that the LM340 tends to be more reliable and robust. With these characteristics, the LM340 is commonly used for industrial applications.
Below are the common voltage variations of the LM340 today:
- 5VÂ (LM340-5.0)
- 12VÂ (LM340-12)
- 15VÂ (LM340-15)
- Â
Note that, previously, they had more voltage variations. The LM340s, unlike the L78XXs don’t have a negative voltage counterpart.
Below is the typical application circuit for the LM340-XX. They only require few external components for filtering and stabilization.
LM317 Variable Voltage Regulator
All is good that you have the fixed form of voltage regulators for the 78XX or LM340-XX series. However, what if you want your voltage (or current) values variable? Or what if you can’t find a specific value among the fixed voltages? Here, you can opt for the LM317 adjustable voltage regulator. The LM317 was originally developed by National Semiconductor and has the same thermal, short-circuit protection, and safe output area compensation feature as the 78XX. This IC can output voltages from 1.25V to 37V and current in excess of 1.5A.
Below is the typical application circuit for the LM317:
According to the datasheet, to get the values of the components right:
- R1 and R2 are required to set the output voltage.
- CADJ is recommended to improve ripple rejection. It prevents amplification of the ripple as the output voltage
is adjusted higher. - Ci is recommended, particularly if the regulator is not in close proximity to the power-supply filter capacitors. A
0.1-µF or 1-µF ceramic or tantalum capacitor provides sufficient bypassing for most applications, especially
when adjustment and output capacitors are used. - CO improves transient response, but is not needed for stability.
- Protection diode D2 is recommended if CADJ is used. The diode provides a low-impedance discharge path to
prevent the capacitor from discharging into the output of the regulator. - Protection diode D1 is recommended if CO is used. The diode provides a low-impedance discharge path to
prevent the capacitor from discharging into the output of the regulator.
LM350 Variable Voltage Regulator
The LM317 has an equivalent counterpart from Texas Instruments, the LM350. This chip boasts of having a current capacity of 3A (more than LM317) and can output voltages in the range of 1.25V to 33V. It still has the standard protection features such as thermal shutdown, short-circuit protect, and safe output area compensation as the LM317.
Below is the typical application circuit used for the LM350:
According to the datasheet:
Where:
- Usually R1 = 240Ω for LM150 and R1 = 120Ω for LM350.
- Full output current not available
at high input-output voltages.
†Optional—improves transient response. Output capacitors in the range of 1 μF to 1000 μF of aluminum or tantalum
electrolytic are commonly used to provide improved output impedance and rejection of transients.
*Needed if device is more than 6 inches from filter capacitors.
Protection diodes are incorporated like in the LM317 to prevent the capacitors from discharging through low current points in the regulator circuit as seen below:
Conclusion
This article presented the classic three terminal voltage regulators. Both the fixed and variable output voltage versions were discussed. Additionally, you’ve learned about their packaging, pinouts, and typical application circuits.
