Increase Output Voltage on 78XX Regulators

Dec 302020

Increase output voltage on 78XX regulators

Why does the output voltage increase when we change the voltage of the “adj” pin?

Remember that when we connect a group of resistors in series, each one causes a voltage drop that is proportional to the value of the resistor. Assuming the resistors are equal, the voltage drop across each resistor will be the same.

Voltage regulator IC are like a power supply stabilized by a zener diode, and this diode is placed between the “Adj” pin of the IC and ground.

How to increase output voltage on 78XX regulators?

To understand why the output voltage of an integrated circuit of this type increases, when we place a diode or a resistor at the “Adj” input, we start from a practical assumption. Let’s see the following circuit.

In the image above (left side) we see a power supply stabilized by a zener diode. To understand the assumption, we have considered a circuit in which there is a common line (0 V) and two positive lines of + 10 V and +25 V.

If we place the 5 V zener diode between the + 25 V and 0 V lines, as we see in the image, the outputs we get are + 5V and + 10V. The equivalent circuit with a voltage regulator is seen in the image above (right side).

If we now place the 5 V zener diode between the 10 V and 0 V lines, we get + 25 V and + 5 V at the output. The equivalent circuit with a voltage regulator is seen in the image below.

The diagram of this circuit and its equivalent with an integrated regulator can be seen in the following image.

Assuming that the diode is placed between the 25 V and 10 V lines, the diode will stabilize the voltage between these two lines at 5 V.

Then the outputs that we will obtain will be +10 V on the one hand, since we have not touched this line and +15 V that is obtained by the sum of the 10 V of the previous line and the 5 V that the zener has stabilized between the two positive lines.

So we have created a dummy 10V line (in the example above); then the output will be 10V + 5V = 15V.

Assuming instead of a resistor we put a 1.5 V zener diode between the IC “Adj” pin and ground (0V), the output will be + 5V + 1.5V = + 6.5V. 5 V is the output of the voltage regulator and 1.5 V is the voltage across the diode.