Question

Explain in very details what happen inside the 7805 regulator to convert 7-12 voltage to 5 V. and also show the circuit and calculation

Answer 1

7805 is a three terminal linear voltage regulator IC with a fixed output voltage of 5V which is useful in a wide range of applications.

the circuit is as shown below:

the pin description is as below:

Working:

there is a significant difference between the input voltage and output voltage of voltage regulator. This difference between the input and output voltage is released as heat. The greater the difference between input and output voltage, more the heat is generated. If the regulator does not have a heat sink to dissipate heat, it can get destroyed and malfunction.

7805 is not very efficient and has drop out voltage problems. A lot of energy is wasted in form of heat. if you are going to use a heat sink, better calculate the heat sink size properly.

the below formula helps to determine power dissipation of regulator

heat generated = (input voltage - output voltage)* output current

calculating heat sink size

the heat sink to ambient thermal resistance is , is total junction to ambient thermal resistance

The heart of 7805 IC is a transistor (Q16) that controls the current between input and output, and thus controlling the output voltage. the band gap reference keeps the voltage stable, it takes the scaled output voltage as input (Q1 and Q6) and provides an error signal to Q7 for induction if the voltage is too low or high. the task of band gap is to provide a stable and accurate reference, even as the chip temperature changes.

The error signal is amplified by the error amplifier. this amplified signal controls the output transistor through Q15. this closes the negative feedback loop controlling the output voltage. the start up circuit provides initial current to band gap circuit so that it doesn't get stuck in an off state. the circuit in purple provides protection against overheating (Q13), excessive input voltage (Q19) and excessive output current (Q14). these circuits reduce the output current or shutdown the regulator, protecting it from damage in case of fault. the voltage divider scales down the voltage on output pin for use by band gap reference.

If the output voltage is correct, then the voltage divider provides 3.75V at Vin. Any change in output voltage propagates through Q6 and R7, causing voltage at base of Q7 to rise or fall accordingly. this change is amplified by Q7 and Q8, generating error output. the error output in turn decreases or increases the current through output transistor. the negative feedback loop adjusts the output voltage untill it is correct (5v).

two basic categories of voltage regulation are:

• line regulation
• load regulation

purpose of line regulation is to maintain nearly constant output voltage when input voltage varies purpose of load regulation is to maintain nearly constant output voltage when load varies

line regulation can be calculated using the following formula:

load regulation can be calculated using the following formula:

where Vnl = no load output voltage

Vfl = full load output voltage