78xx Family Voltage Regulators
When you need to supply a specific voltage to your circuit from a higher voltage input, the 78xx family of voltage regulators are champion, still commonly used today in a considerable number of electronic devices due to their reliability, robustness, and simplicity.
Application: Used for delivering a reasonably consistent voltage from a power supply that may have some minor variation. A typical application (one that we regularly use) would be to power a microcontroller circuit requiring 5 volts, from either a 12V or 9V battery.
Batteries lose their power through use (the voltage drops), and the voltage regulator helps maintain a constant voltage. With microcontrollers, the supply voltage may be used as a reference for an analogue input, and without the regulator, the reference would be changing resulting in different readings from the original.
Items in bold are most commonly used.
Available in different models, can be referred to as 78xx series, 7800 series and sometimes prefixed with L, LM or MC, depending on the manufacturer.
You will always need a higher supply voltage than the output voltage of the regulator (ideally 2-3 volts above the output voltage, but can be considerably more), and should keep in mind that a large difference between the supply and output voltage, or heavy power consumption from the circuit you are powering will cause more heating and less efficiency of the voltage regulator.
The TO-220 package features a hole on the grounded heatsink tab, allowing it to be bolted to a larger heatsink if additional cooling is required.
The pin configuration of 0.1 inches is ideal for breadboards, making it perfect for your prototyping projects. The pins can be carefully bent backwards with strong tweezers or pliers, allowing it to lay flat against the breadboard or circuit board.
TO-220 Package 78xx Family Voltage Regulator Pin Connections
Figure 2 Straight and bent configuration being used on a breadboard
It is quite common to see the heatsink tab soldered to circuit boards as a grounding point (and for some heat dissipation) with the centre ground pin trimmed off or not connected on the DPAK-3 surface mount package version. Further variants of the DPAK-3 are available with a hole for mounting an additional heat sink, or in a smaller format where space is premium. TO-3 Metal-can packages can also be found, used for higher current applications where additional heatsinking may not be possible.
Figure 3 4DPAK-3 78xx Family Voltage
Figure 4 A DPAK-3 package surface mounted on this PikDem 2 Plus demonstration board is used to reduce a 9V battery supply to a 5V supply for the microcontroller
Connections (Refer to Fig 1 or Fig 3):
· Pin 1 (left hand pin) connected to +VE or red terminal on a suitable battery acting as the supply voltage (input or VIN).
· Pin 2 (the centre pin and/or heatsink tab) is connected to ground both on your supply voltage (-VE or black terminal on the battery), and also to the ground of your target circuit (Shown as , V- or GND on your schematic).
· Pin 3 is the output voltage that your target circuit will use as V+.
Branded units are more reliable and have thicker heat sinks, whereas unbranded units are considerably much cheaper, but may be more prone to failure if pushed too hard.
The voltage regulator will work without any additional circuitry, however, manufacturers recommend adding a couple of capacitors (shown below) to help smooth the output, especially if the target circuit is drawing a lot of current. CI (C in) shown as 0.33µF* (on datasheets) connected between Pins 1 and 2, and CO (C out) shown as 0.1µF** connected between pins 2 and 3.
Figure 5 78xx Voltage Regulator Circuit
· Constant Current: With some additional components, the 78xx series can be made to regulate the current output at the given voltage.
For example, a 1.0 A current source would require R to be a 5Ω, 10 W resistor and the output voltage result would be the input voltage less 7V.
· Adjustable Voltage Output: The 78xx series can handle variable voltage with the help of a 741 Op Amp and 10K potentiometer. The output voltage in this instance would be higher than the input voltage, as an example, with the 7805 voltage regulator, you would have the ability to provide an input of 2V, and obtain an output of 7V to 20V.
7805 Adjustable Output Regulator
· Further examples can be found on manufacturer’s datasheets, and are typically for more advanced applications, and will be covered in more advanced articles.
Firstly, double check connections in your circuit. The supply ground and output ground are shared common to pin 2 (or the heatsink tab 4).
The 78xx series of voltage regulators can be tested for correct operation with a voltmeter: Use the volts setting on your multimeter.
- Check the supply (Input) voltage. The black ground probe should be connected to the centre pin (pin 2) and connect the read positive probe to pin 1 (left hand pin) on the voltage regulator. The reading should be at least 2 volts more than the voltage regulator is rated for, and no more than 35V.
If the supply voltage is out of the specified range, check the supply.
- Check the output voltage. Connect the black negative probe from the multimeter to pin 2 (centre pin ground) or the heatsink tab (4) as before, and connect the red positive probe to pin 3 (right hand pin) on the voltage regulator. The reading should be close to that of the voltage regulator’s rated output value, i.e. a 7805 regulator should measure around 5V.
If the measured output voltage is significantly different, the voltage regulator may be faulty, and should be replaced.
Failure could occur from over-current (branded voltage regulators are rated at approx. 1A, non-branded types may not be able to handle this), or over-heating where a large difference in supply and output voltage or high current applications is present.