79xx Family 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 simple 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.
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 using a 7805 voltage regulator. The same input can be used with a 79xx Voltage Regulator, to provide the negative part of the supply output, being -5V in this case.
• 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.
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.