Jan 092016
 

Variable voltage regulator using 7805 and 741

Usually, when we want to implement a variable voltage regulator, we use components like LM317 voltage regulator. As we not always have these components, we have to use other options. If we have a 7805 fixed voltage regulator and a 741 operational amplifier, we can achieve our goal. With this new variable voltage regulator design, we can obtain output voltages ranging from 7 to 30 volts.

How the Variable voltage regulator using 7805 and 741 works?

Variable voltage regulator using 7805 and 741

The monolithic voltage regulator LM7805 deliver 5VDC, if the # 2 IC terminal is grounded, but this is not the case.  The 7805 # 2 terminal has a voltage that varies according to the voltage value obtained at the 10K potentiometer shaft (P). This voltage is applied to the # 2 terminal of the regulator through the operational amplifier configured as a voltage follower.

Any voltage increase at the arrow of the potentiometer, implies an increase in the voltage at the # 2 terminal of the voltage regulator and thus an equal increase at the output Vout. For the voltage range mentioned above, the unregulated voltage applied to the Vin input must be at least 32 volts, because the voltage drop between the input and the output on this regulator is 2 volts.

The C1 capacitor is required if the voltage regulator is far from the capacitor that is part of the “unregulated voltage source” (transformer, diode bridge and capacitor, which are connected to Vin) and the capacitor C2 is needed to improve the stability and transient response of the circuit.

List of components of the variable voltage regulator

  • 1  LM7805 monolithic voltage regulator (U1)
  • 1  LM741 operational amplifier (U2)
  • 1  10K potentiometer (P)
  • 1  0.33 uF capacitor (C1)
  • 1  0.1 uF capacitor (C2)
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