Posted on by

Different Voltage Level Generation Using 78XX ICs

Different voltage level generation is required in a complex PCB as at different node different voltage level may be required. Different voltage regulator ICs for different Voltage level can be used but this is not efficient technique. Thus it is necessary to generate all the voltage values from one higher voltage level. In this work, fixed voltage regulator ICs 78XX are used to generate different voltage levels like 3.3 V, 1.8 V etc.

Different Voltage Level Generation Using 78XX ICs
Figure 1: Different Voltage Level Generation Using 7805 IC

Generation of 3.3 V Using 7805 IC

Generation of 3.3 V DC using 7805 IC is shown in Figure 1. Input voltage range for 7805 IC is (+7v to + 35 volt). In this circuit, +9 V dc supply is applied at input through the two pin connector CN1. Capacitor C1 (electrolytic polarized capacitor) is used to filter the input supply if there will be present any kind of ripple voltage. Two fixed resistors R2 and R3 are used to divide the voltage. Output voltage is taken across the resistor R3. Capacitor C2 is placed across R3 for further smoothing. Overall output is taken through CN2.

Components Required

  1. +9 V – 1 A DC power supply.
  2. Screw pin male header (2 pcs).
  3. Two capacitors (C1 = 10 uF/16 V, C2 = 100 uF/25 V).
  4. Two Resistors (R3 = 4.7 K/0.25 W, R2 = 2.2 K/0.25 W).
  5. Voltage regulator IC (L7805CV).

Theoretical Calculation

Input voltage (Vin) =+9V

Range of input= +7 V to +35 V

VO =5 V (From IC)

Vout = VO*(R3/ (R3+R2)) = 5*(4.7/ (4.7+2.2)) V = 3.4 V

As per the theoretical calculation, 3.4 V should be the output. But instead of 3.4 V, output is 3.28 V (approximately 3.3 V). This is due to the tolerance level of the resistors.  Generally, we use 3.3 voltage regulator IC to get 3.3 V constant voltage. But if only 7805 IC is available then this scheme can be used.

Generation of 1.8 V Using 7805 IC

Similarly we can generate 1.8 V using the 7805 IC. The schematic for generation of 1.8 V using 7805 IC is same as shown in Figure 1 but the value of the resistors will vary in this case.

Components Required

  1. +9 V – 1A DC power supply.
  2. Screw pin male header (2 PCS)
  3. Two capacitors (C1 = 10Uf/16v and C2=100uf/25V).
  4. Twp resistors (R2=180 OHM/0.25W and R3 = 100 OHM/0.25W).
  5. Voltage regulator IC (L7805CV).

Theoretical Calculation

The input Vin=+9V;

Range of input= +7volt to +35volt

VO=5 V (From IC)

Vout= VO*(R3/ (R3+R2)) =5*(100/ (100+180)) V = 1.78 V

As per the theoretical calculation, 1.78 V should be the output. But instead of 1.78 V, output is 1.67 V (approximately 1.8 V). This is due to the tolerance level of the resistors.