What 7805 Voltage Regulator IC Is and How It works

Voltage sources in a circuit can experience fluctuations, which may prevent them from providing a stable output voltage. A voltage regulator IC is designed to keep the output voltage constant. The 7805 Voltage Regulator, part of the 78xx series of fixed linear voltage regulators, is widely used for this purpose and is a popular voltage regulator integrated circuit (IC).

 

The "xx" in 78xx represents the output voltage it delivers. The 7805 IC provides a +5 volts regulated power supply and includes provisions for adding a heat sink.

 

7805 Voltage Regulator IC Specifications:

 

• Minimum Input Voltage: 7V
• Maximum Input Voltage: 35V
• Current Rating: Ic = 1A
• Maximum Output Voltage (VMax): 5.2V
• Minimum Output Voltage (VMin): 4.8V

 

LM7805 Voltage Regulator Pinout

 

The 7805 Voltage Regulator Pin Diagram is shown below.

Pin	Function	Description
INPUT	Input voltage (7V-35V)	In this pin of the IC, the positive unregulated voltage is applied for regulation.
GROUND	Ground (0V)	This pin is connected to the ground. It serves as a common reference point for both the input and output.
OUTPUT	Regulated output; 5V (4.8V-5.2V)	The regulated 5V output is available at this pin of the IC regulator.

As you may have observed, there is a notable difference between the input voltage and the output voltage of the voltage regulator. This voltage difference is released as heat. The larger the difference between the input and output voltage, the more heat is generated.

 

If the regulator lacks a heat sink to dissipate this heat, it may become damaged and malfunction. Therefore, it is recommended to keep the input voltage no more than 2-3 volts above the output voltage.

 

You have two options: either design your circuit so that the input voltage is limited to 2-3 volts above the regulated output voltage, or use an appropriate heat sink that can effectively dissipate the heat.

 

7805 IC Heating Problem

 

The 7805 voltage regulator is not very efficient and has issues with drop-out voltage. A significant amount of energy is wasted as heat. If you plan to use a heat sink, it's important to calculate its size accurately. The following formula can help determine the appropriate heat sink size for such applications:

 

Heat generated = (input voltage – 5) x output current

 

For a system with an input of 15 volts and an output current of 0.5 amperes:

 

Heat generated = (15 – 5) x 0.5 = 10 x 0.5 = 5W

 

Thus, 5W of energy is wasted as heat, making it necessary to use a suitable heat sink to dissipate this heat. The actual energy used is:

 

Energy used = (5 x 0.5) = 2.5W

 

Therefore, twice the amount of energy that is utilized is wasted. Conversely, if the input is 9V with the same load:

 

Heat generated = (9 – 5) x 0.5 = 2W

 

In this case, 2W of energy is wasted as heat.

 

What should we do then?

 

Keep in mind… the higher the input voltage, the less efficient your IC7805 will be.

 

An estimated efficient input voltage would be around 7.5V.

 

Why do we use Capacitors with 7805?

 

If your voltage regulator is positioned more than 25 cm (10 inches) away from the power supply, capacitors are necessary to filter out residual AC noise. Voltage regulators perform efficiently with a clean DC signal. The bypass capacitors help reduce AC ripple.

 

They essentially short AC noise from the voltage signal, allowing only DC voltage into the regulator. While the two capacitors are not strictly required and can be omitted if line noise is not a concern, they are beneficial for applications like mobile phone chargers or logic circuits where a clean DC line is crucial. Capacitors help maximize voltage regulation, and their values can be adjusted slightly.

 

For more details, you can check "Why Are Filter Capacitors Used in Some Battery Charger Circuits?"

 

Now, let’s examine what makes the IC function.

 

Voltage Regulator 7805 IC Circuit

The core of the 7805 IC is a transistor (Q16) that manages the current between the input and output, thereby regulating the output voltage. The bandgap reference (yellow) ensures voltage stability.

 

It uses the scaled output voltage as input (Q1 and Q6) and generates an error signal (to Q7) to indicate if the voltage is too high or too low.

 

The primary function of the bandgap reference is to provide a stable and accurate reference voltage, even with changes in the chip’s temperature.

 

The error signal from the bandgap reference is amplified by the error amplifier (orange). This amplified signal then controls the output transistor via Q15, completing the negative feedback loop that regulates the output voltage.

 

The startup circuit (green) supplies initial current to the bandgap circuit, preventing it from remaining in an “off” state. The circuit in purple offers protection against overheating (Q13), excessive input voltage (Q19), and excessive output current (Q14).

 

These protective circuits either reduce the output current or shut down the regulator to prevent damage in case of a fault. The voltage divider (blue) scales down the voltage on the output pin for use by the bandgap reference.

 

Scaling the Output

 

The 7805’s scaled output supplies the input voltage (Vin) to the bandgap reference, which in turn provides an error signal based on the output. Unlike a traditional bandgap reference that has its own feedback loop, the 7805’s bandgap circuit integrates the entire chip as the feedback loop.

 

When the output voltage is correct (5V), the voltage divider delivers 3.75V at Vin. Any variation in output voltage affects Q6 and R7, causing the voltage at the base of Q7 to change accordingly.

 

This change is amplified by Q7 and Q8, producing the error output. The error output then adjusts the current through the output transistor, with the negative feedback loop fine-tuning the output voltage until it reaches the correct level.

 

IC 7805 Voltage Regulator Applications

 

The 7805 IC is utilized in various circuits. Major applications include:

 

• Fixed-Output Regulator
• Positive Voltage Regulator in Negative Voltage Configuration
• Adjustable Output Regulator
• Current Regulator
• Adjustable DC Voltage Regulator
• Regulated Dual-Supply
• Output Polarity-Reversal-Protection Circuit
• Reverse Bias Protection Circuit

 

The 7805 voltage regulator is also employed in circuits for inductance meters, phone chargers, portable CD players, infrared remote-control extensions, and UPS power supplies. Additionally, we have designed a Stopwatch Circuit using the IC7805.

 

FAQ

 

What does a 7805 voltage regulator do?

 

A 7805 voltage regulator is an integrated circuit designed to maintain a constant output voltage despite variations in input voltage and load. Specifically, the 7805 is a positive voltage regulator that provides a stable +5V output, making it a popular choice for powering various electronic components and circuits.

 

How much voltage can the 7805 handle?

 

The 7805 voltage regulator is typically used to deliver a fixed +5V output. To ensure proper operation, the input voltage should be higher than the desired output voltage, plus a small overhead voltage for the regulator to function correctly. Generally, the input voltage can range from approximately 7V to 35V, allowing the 7805 to accommodate a wide range of input voltages while maintaining a steady +5V output.

 

What temperature should a 7805 voltage regulator be?

 

The operating temperature of a 7805 voltage regulator is usually specified by the manufacturer. Standard operating temperature ranges are typically from 0°C to 125°C. However, variations may exist, and some models might offer extended temperature ranges for specific applications.

 

What is the difference between LM7805 and 7805?

 

The terms “LM7805” and “7805” both refer to the same type of voltage regulator. The LM prefix indicates that the component is manufactured by National Semiconductor (now part of Texas Instruments), and it is often used for clarity. Therefore, both LM7805 and 7805 represent the same voltage regulator with a fixed +5V output.