79xx Family Negative Voltage Regulators

Sometimes a circuit will require a positive, and negative as well as a ground. This is quite common when Op-Amps are used. A simply way to do this is to use a couple of resistors to form a voltage divider, however, this may not be the best solution, as you will need an input voltage of about double that of the positive voltage that your circuit requires.

Application

A simple and reliable solution to providing a negative output voltage is to use a 79xx voltage regulator.

In our previous article on 78xx series voltage regulators, we discussed how we get a steady 5 volts output from either a 9V or 12V battery. The same input can be used with a 79xx Voltage Regulator, to provide the negative part of the supply output.

Models

Model
VOUT Typical
7905
-5
7908
-8
7909
-9
7910
-10
7912
-12
7915
-15
7918
-18

79xx Family Voltage Regulators.
Items in bold are most commonly used.

Available in different models, can be referred to as 78xx series, 7800 series and sometimes prefixed with 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.

79xx Series Negative Voltage Regulators Pinout
79xx Family Negative Voltage Regulator
Pin Connections in a TO-220 Package

Connections (Refer to Figure 1):

  • Pin 1 (left hand pin) 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 , or GND on your schematic). NOTE: As this is intended for a target circuit with a negative power rail, ground and negative (V-) are separate, and should not be connected together. The heatsink tab (4) is also connected to ground.
  • Pin 2 (the centre pin and/or heatsink tab) connected to +VE or red terminal on a suitable battery acting as the supply voltage (input or VIN), or to the ground terminal on a split rail power supply (one that has +ve, ground and -ve) present.
  • Pin 3 is the output voltage that your target circuit will use as V-.

There is no connection to +ve on the target circuit. A 78xx regulator would be required to provide the positive power rail.

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.

Testing

Firstly, double check connections in your circuit.

The 79xx series of voltage regulators can be tested for correct operation with a voltmeter: Use the volts setting on your multimeter. Tests can usually be carried out in-circuit.

  • Check the supply (input) voltage. Connect the black negative probe from your multimeter to pin 1 (left hand pin - ground) on the voltage regulator and connect the red positive probe from your multimeter to pin 2 on the voltage regulator.
  • 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 1 (left hand pin -ground) on the voltage regulator (as before), and connect the red positive probe from your multimeter to pin three on the voltage regulator. The reading should be close to that of the voltage regulator, i.e. a 7905 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.

Further Reading

If you’re looking for something similar, but with an adjustable voltage, look at our article on the LM317.

For circuits requiring a positive, a different family of voltage regulators is available, the 78xx series. Look at our article on 78xx Voltage Regulators for more information. Be aware they have a different pin configuration!

LM317 Variable Voltage Regulator:

Content coming soon.

78xx Family Voltage Regulators:
http://www.ecobionlabs.com/index.php?page=78xx-family-voltage-regulators

 

 

Sources

Texas Instruments LM78xx Datasheet:
http://www.ti.com/lit/ds/symlink/lm7915.pdf

Fairchild Semiconductor LM78XX Datasheet:
https://www.fairchildsemi.com/datasheets/KA/KA7905.pdf

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