Variable voltage regulator using 7805 and 741

Usually, when we want to implement a variable voltage regulator, we use components like the LM317 voltage regulator. Since we don’t always have these components, we have to use other options. If we have a 7805 fixed voltage regulator and a 741 op amp, we can achieve the variable voltage regulator we want.

With this new variable voltage regulator design, we can get output voltages ranging from 7 to 30 volts and a current of up to 1.5 amps.

How the Variable voltage regulator works?

The LM7805 monolithic voltage regulator has 5VDC at its output if pin # 2 is grounded, but this is not the case.

Variable voltage regulator using 7805 and 741 op amp.

Pin # 2 of the 7805 regulator has a voltage that varies according to the voltage value obtained from the 10K potentiometer (P). This voltage is applied to pin # 2 of the 7805 regulator through the op amp configured as a voltage follower.

Any increase in voltage on the potentiometer arrow implies an increase in the voltage at pin # 2 of the voltage regulator and, therefore, an equal increase in the output (Vout).

LM7805 Voltage Regulator Pinout.

For the voltage range mentioned above, the unregulated voltage applied to the Vin input must be at least 32 volts, because the voltage drops 2 volts between the input and the output of this regulator.

741 op amp pinout

Capacitor C1 is necessary 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 capacitor C2 is necessary to improve the stability and transient response of the circuit.

Component List for Variable Voltage Regulator

• 1 LM7805 (5 volts fixed) monolithic voltage regulator (U1)
• 1 LM741 op amp (U2)
• 1 10K potentiometer (P)
• 1 0.33 uF (microfarads) capacitor (C1)
• 1 0.1uF capacitor (C2)
• 1 heatsink for the lm7805 integrated circuit

Note: The unregulated voltage source (transformer, diode bridge and capacitor), that need to be connected to Vin is not included on the circuit diagram.