LM7824: Description, Specs, Applications, Pinout

Introduction to Voltage Regulators and the LM7824

A voltage regulator is an electronic device or circuit that maintains a constant voltage level, providing a stable power supply for other electronic components and circuits. Voltage regulators are essential in many applications, such as power supplies, Battery Chargers, and consumer electronics, to ensure proper functioning and prevent damage caused by voltage fluctuations.

The LM7824 is a popular fixed positive voltage regulator that provides a constant output voltage of 24 volts. It belongs to the 78xx series of linear voltage regulators, which are widely used in various electronic applications due to their simplicity, reliability, and low cost.

Key Features of the LM7824 Voltage Regulator

• Fixed output voltage of 24 volts
• Output current up to 1.5A
• Built-in overheating protection
• Short circuit protection
• Requires minimal external components
• Available in various package types, such as TO-220 and TO-3

How the LM7824 Voltage Regulator Works

The LM7824 is a three-terminal positive voltage regulator that uses a linear regulation technique to maintain a constant output voltage. It consists of an internal voltage reference, error amplifier, pass transistor, and protection circuitry.

Basic Principle of Operation

1. The internal voltage reference generates a stable reference voltage, typically derived from a bandgap reference or a Zener diode.
2. The error amplifier compares the output voltage with the reference voltage and adjusts the current through the pass transistor to maintain the desired output voltage.
3. The pass transistor acts as a variable resistor, controlling the current flow from the input to the output, based on the error amplifier’s output.
4. Protection circuitry, such as thermal shutdown and short circuit protection, ensures safe operation and prevents damage to the regulator and connected components.

Dropout Voltage and Efficiency Considerations

The dropout voltage is the minimum difference between the input and output voltages required for the regulator to maintain regulation. For the LM7824, the typical dropout voltage is around 2 volts. This means that the input voltage must be at least 2 volts higher than the desired output voltage for proper regulation.

Linear voltage regulators, like the LM7824, have lower efficiency compared to switching regulators, particularly when there is a significant difference between the input and output voltages. The power dissipation in the regulator is proportional to the voltage drop across it and the load current. To minimize power dissipation and improve efficiency, it is recommended to keep the input voltage as close to the output voltage as possible.

LM7824 Specifications and Electrical Characteristics

These specifications are typical values and may vary slightly depending on the manufacturer and specific part number. It is essential to refer to the datasheet provided by the manufacturer for detailed information on the LM7824’s electrical characteristics and absolute maximum ratings.

LM7824 Pinout and Package Types

The LM7824 voltage regulator is available in various package types, with the most common being the TO-220 and TO-3 packages. The TO-220 package is a three-lead through-hole package with a heatsink tab, while the TO-3 package is a larger four-lead package designed for higher power dissipation.

TO-220 Package Pinout

TO-3 Package Pinout

It is crucial to ensure proper heatsinking when using the LM7824, especially in high-current applications or when the input-output voltage difference is significant. The heatsink tab or case should be securely connected to a suitable heatsink to dissipate excess heat and prevent thermal shutdown.

Applications of the LM7824 Voltage Regulator

The LM7824 voltage regulator finds applications in various electronic systems that require a stable 24-volt power supply. Some common applications include:

1. Industrial control systems
2. Automotive electronics
3. Telecommunications equipment
4. Sensor and transducer excitation
5. Lighting control systems
6. Audio amplifiers and signal processing
7. Battery chargers and power supplies

Example Application: Simple 24V Power Supply

A basic 24V power supply using the LM7824 can be constructed with minimal external components. The following schematic illustrates a typical configuration:

           +---------+
   Input   |         |   Output
 (27-38V)--+| LM7824 |--+(24V, 1.5A max)
           |         |  |
           +---------+  |
              |  |      |
             === ===   ===
             C1  C2    C3
             1uF 0.1uF 100uF
              |   |     |
              +---+-----+
                 GND

In this example, capacitor C1 (1µF) is used as an input Bypass Capacitor to reduce noise and improve transient response. Capacitor C2 (0.1µF) is an output bypass capacitor that helps to reduce output voltage ripple and improve stability. Capacitor C3 (100µF) is an output Filter capacitor that smooths the output voltage and provides additional stability.

When designing a power supply using the LM7824, it is essential to consider the input voltage range, maximum load current, and power dissipation. Proper heatsinking should be employed to ensure reliable operation and prevent thermal shutdown under high-current or high-temperature conditions.

LM7824 Protection Features and Safe Operating Area

The LM7824 voltage regulator incorporates several protection features to ensure safe operation and prevent damage to the device and connected components. These features include:

1. Thermal Shutdown: If the junction temperature of the LM7824 exceeds a specified limit (typically around 150°C), the internal thermal protection circuitry will shut down the regulator to prevent damage. The regulator will automatically resume operation once the temperature falls below the safe limit.

2. Short Circuit Protection: The LM7824 can withstand a continuous short circuit at the output without suffering damage. The internal current limiting circuitry will limit the output current to a safe value (typically around 1.5A) during a short circuit condition, protecting the regulator and connected components.

3. Safe Operating Area (SOA): The SOA defines the range of output current and voltage drop across the regulator within which the device can operate safely without suffering damage. It is essential to ensure that the regulator is operated within its specified SOA to prevent overheating and potential failure.

Heatsinking Considerations for Safe Operation

Proper heatsinking is crucial for maintaining the LM7824 within its safe operating area, especially when dealing with high load currents or significant input-output voltage differences. The power dissipated by the regulator can be calculated using the following equation:

P = (Vin - Vout) × Iout

Where:
– P is the power dissipated by the regulator (in watts)
– Vin is the input voltage (in volts)
– Vout is the output voltage (in volts)
– Iout is the load current (in amps)

To select an appropriate heatsink, the thermal resistance of the heatsink (in °C/W) should be chosen such that the junction temperature of the regulator remains below the maximum specified value (typically 125°C) under worst-case conditions. The required thermal resistance can be calculated using the following equation:

Rth(heatsink) = (Tj(max) - Ta) / P - Rth(junction-case) - Rth(case-heatsink)

Where:
– Rth(heatsink) is the required thermal resistance of the heatsink (in °C/W)
– Tj(max) is the maximum allowable junction temperature (in °C)
– Ta is the ambient temperature (in °C)
– P is the power dissipated by the regulator (in watts)
– Rth(junction-case) is the thermal resistance from the junction to the case of the regulator (in °C/W)
– Rth(case-heatsink) is the thermal resistance from the case to the heatsink (in °C/W)

By selecting a heatsink with a thermal resistance lower than the calculated value, you can ensure that the LM7824 operates within its safe operating area and maintains reliable performance over its intended operating life.

Frequently Asked Questions (FAQ)

1. What is the maximum input voltage for the LM7824 voltage regulator?
   The maximum input voltage for the LM7824 is 38 volts. Exceeding this value may result in damage to the regulator.

2. Can I adjust the output voltage of the LM7824?
   No, the LM7824 is a fixed voltage regulator with a preset output voltage of 24 volts. If you require an adjustable output voltage, consider using an adjustable voltage regulator like the LM317.

3. How much current can the LM7824 supply?
   The LM7824 can supply a maximum output current of 1.5 amps. However, the actual output current capability depends on the input-output voltage difference and the effectiveness of the heatsinking.

4. Do I need to use external capacitors with the LM7824?
   While the LM7824 can operate without external capacitors, it is highly recommended to use input and output bypass capacitors to improve noise reduction, transient response, and overall stability of the regulator.

5. Can I parallelize multiple LM7824 regulators to increase the output current?
   Parallelizing voltage regulators is not recommended, as it can lead to uneven current sharing and potential damage to the regulators. If you require higher output current, consider using a regulator with a higher current rating or a different topology, such as a switching regulator.

Conclusion

The LM7824 is a reliable and cost-effective fixed positive voltage regulator that provides a constant output voltage of 24 volts. Its simple design, built-in protection features, and wide range of applications make it a popular choice for many electronic systems.

When using the LM7824, it is essential to consider factors such as input voltage range, maximum load current, power dissipation, and proper heatsinking to ensure safe and reliable operation. By understanding the regulator’s specifications, application requirements, and design considerations, engineers and hobbyists can effectively incorporate the LM7824 into their projects and achieve stable and efficient power regulation.