LM7912 Negative Voltage Regulator

In all the circuits for our applications, we use both positive and negative voltages are required. Usually, these devices are used with a positive voltage and a ground, instead of negative voltages. Negative voltages are the voltages less than the ground voltage. These can be driven by the power supply by changing the polarity. Negative voltages can also cause damage to the circuit if they are not stable. To make them stable and draw a constant negative output voltage Negative voltage regulators are used. One of such regulators available in the market is LM7912.

What is LM7912?

Usually to supply negative voltages to the circuit such as -3.3V an available support voltage of -5V drawn from the transformer based power supply is used. But these voltages cannot be regulator accurately.

Positive voltage regulators can be used for this purpose but they add noise and disturbance to the output voltage. Moreover, to regulate the output voltage, the circuit will use the resistive divider based on -5V instead on 0V. This leads to poor accuracy of the regulator.

Voltage regulators are required to generate stable output even when the input is fluctuating. Generally, these circuits have a differential amplifier that acts as a comparator between input and output voltages. This is supplied with a reference voltage equal to the output voltage.

The minimum voltage difference in output and reference voltage that can be detected by the differential amplifier and regulated by the regulator is called a Dropout value of the regulator.

The input voltage to be applied for the working of the regulator should greater than the sum of output voltage and dropout value of the regulator. If the input voltage conditions are not applied, the regulator simply works as a resistive circuit where input is equal to output.

LM7912 is a negative voltage regulator used for regulating negative voltages. These are specially designed to regulate negative voltages. These are designed with the lowest noise and highest power supply rejection ratio. The output voltage of LM7912 is -12V.

LM7912 Pin Configuration

The regulator series of LM79xx comes in TO-220 power package. The device contains three terminals. They are Input, Output and Ground. This device gives an output current of 1.5A. LM7912 is available with fixed voltages such as -5V, -12V, -15V.

LM7912 pin configuration
LM7912 pin configuration

The ground terminal is used to provide common ground to both input and output terminals. This pin is connected to the ground. To the input terminal, the unregulated voltage is applied. The regulated output is drawn from the output terminal.

Circuit Diagram of LM7912 and How To Use It

As it is a negative voltage regulator the power source applied or the input applied to the circuit should be negative. So, the polarity of input is reversed in the circuit. LM7912 works with the input voltages in the range of -14V to -27V.

LM7912 circuit diagram
LM7912 circuit diagram

Capacitors are used both at the input and output terminals to filter out the noise in the signals. These circuits also have a protection system for thermal overload which causes due to excessive power dissipation.

To limit the circuit’s maximum current, internal short circuit protection is provided. As the voltage across the pass transistor is increased the output current decreases. To compensate for this loss, output transistor safe area compensation is provided.

Where to Use LM7912?

LM7912 is a negative voltage regulator IC. This circuit is used where there is a requirement of application of negative voltages. Such as, in analog circuits, at sensors etc…

As this IC is available for various fixed voltage configurations such as -5V, -12V, -15V, based on the requirement of the application these are chosen. The output current provided by this IC is 1A, but with a proper heat sink current up to 2.2A can be drawn from it.

Specifications of LM7912

The specifications of LM7912 include the following.

  • LM7912 is a 12V negative voltage regulator.
  • It has a minimum voltage input of -14.5 V.
  • The maximum voltage input of LM7912 is -27 V.
  • It works with a peak output current of 2.2 A.
  • The average output current of LM7912 is 1 A.
  • It has an internal thermal overload and short circuit current limiting protection.
  • LM7912 is available in TO-220 package only.
  • Bypass capacitors are necessary for stable operation.
  • Output bypass capacitor improves the transient response of the regulator.
  • If aluminium electrolytes are used for bypass capacitors, their value should be 10μF or larger.
  • Operating junction temperature range is 00c to +1250c.
  • The storage temperature range is -650C TO +1500C.
  • Lead temperature is 2300C.
  • High Ripple rejection.

Applications of LM7912

The applications of LM7912 include the following.

  • It can be used in analog and digital circuits as power or reference voltage.
  • This IC can be used as a current limiter in various applications.
  • LM7912 has a dual supply.
  • It also has an output polarity-reversal-protection circuit.
  • High stability 1A regulator.
  • As a current source.
  • Light Controller using silicon photocell.
  • High-sensitivity light controller.
  • Dual trimmed supply.

Equivalent Voltage Regulator ICs

The equivalent voltage regulators which have the similar functionality of LM7912 are LM7905, LM7915, and LM7918. Here the value 79 indicates that it is a negative voltage regulator. The next two digits after 79 give the output voltage values provided by the regulators.

LM7905 gives the output of -05V. LM7915 indicates that the regulator produces the output of -15V and LM7918 gives the output of -18V.

Usually, negative references are required using devices such as sensors. Different methods such as using Zener diode to generate negative reference are used. This is an easy solution but, when the power from the transformer is used for references it is bound to have noise and fluctuations in voltages. So, negative voltage regulators are very useful in such situations. Further electrical characteristics can be found in LM7912 datasheet. Have you faced the problem of negative reference voltage for your application? How do you solve it? How LM7912 has helped you?

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