Introduction to the LM7824 Voltage Regulator

The LM7824 is a popular fixed positive voltage regulator IC that provides a constant +24V DC output voltage. It is part of the 78xx series of linear voltage regulator ICs. The LM7824 is designed to provide a stable, ripple-free regulated output voltage for a wide range of input voltages and load conditions.

Key features of the LM7824 include:

• Fixed +24V DC output
• Can supply up to 1A of load current
• Requires minimal external components
• Built-in overload and thermal shutdown protection
• Available in TO-220 and TO-3 packages

The LM7824 is widely used in industrial control systems, test equipment, HiFi amplifiers, automotive electronics, and many other applications requiring a stable +24V power supply.

Specifications

Below are the key electrical characteristics and absolute maximum ratings for the LM7824:

Electrical Characteristics

Parameter	Min	Typ	Max	Units
Output Voltage	23.5	24	24.5	V
Output Current		1	1.5	A
Quiescent Current		8	10	mA
Input Voltage Required for Rated Output	27		38	V
Ripple Rejection	62	71		dB
Output Voltage Temperature Coefficient		-1.3		mV/°C
Output Noise Voltage		200		μV
Dropout Voltage		2	2.5	V
Peak Output Current		2.2		A

Absolute Maximum Ratings

Parameter	Value	Units
Input Voltage	40	V
Power Dissipation	Internally Limited	W
Operating Junction Temperature Range	0 to +150	°C
Storage Temperature Range	-65 to +150	°C
Lead Temperature (Soldering, 10 sec)	260	°C

How It Works

The LM7824 is a linear voltage regulator that uses a simple circuit topology to provide a fixed +24V output from a higher, unregulated DC input voltage.

The main components inside the LM7824 are:

1. A voltage reference
2. An error amplifier
3. A series pass transistor
4. Current limiting and thermal shutdown circuitry

Here’s a simplified explanation of how the LM7824 works:

1. The voltage reference provides a stable 24V that the error amplifier uses as a reference.

2. The error amplifier constantly compares the output voltage (scaled down through a resistor divider) to this 24V reference.

3. If the output voltage tries to rise above 24V, the error amp output goes high, reducing the drive to the pass transistor to bring the output back down to 24V.

4. If the output voltage drops below 24V, the opposite happens – the error amp provides more drive to the pass transistor to raise the output voltage back to 24V.

5. The series pass transistor acts like a variable resistor controlled by the error amp. It drops just enough voltage from input to output to keep the output at a stable 24V.

6. Overload protection limits the current the pass transistor can supply, shutting it down if a short circuit occurs. Thermal shutdown turns off the pass transistor if the chip overheats.

So in summary, the LM7824 acts like a self-adjusting valve between the input and regulated output, opening and closing as needed to keep the output voltage constant at 24V despite changes in input voltage or load current draw.

Application Circuit

Below is a typical application circuit for the LM7824 voltage regulator:

           ┌────────┐
      ─────┤ Input  ├─────┬────────────┬────── +VIN
           │        │     │            │ 
           │  7824  │     │      ┌─────┴─────┐
           │        │     │      │           │
           │ Ground ├──────────── ──      ───── GND
           │        │     │      │  470μF   │
           │ Output ├─────┴───── ┼────┬─────┘
           └────────┘            │    │
                            100μF│    │ Load
                                 │    │
                                ─┴─  ─┴─ GND

Here are a few key points about this application circuit:

• The input voltage +VIN should be at least a few volts higher than the desired output of 24V to account for the dropout voltage of the regulator. Typically at least 27V is used.

• The 100μF capacitor on the output is recommended for stability and improving transient response. It helps absorb sudden load changes.

• The 470μF capacitor on the input is not always needed, but helps filter any ripple on the input supply and maintains a stable input voltage during large load current changes.

• Locating the capacitors close to the IC pins is important for optimal performance. Use short, wide PCB traces and place the capacitors as close as possible.

• No output resistor divider is needed since this is a fixed voltage regulator. The 24V reference is internally set.

• Be sure to connect the ground pin to a solid ground point on the PCB. Use large copper areas for heatsinking if the regulator will be supplying close to the maximum 1A current.

This simple application circuit is suitable for most LM7824 implementations. Always refer to the datasheet for your specific LM7824 variant for any additional recommendations.

Pinout and Pin Functions

The LM7824 voltage regulator is available in both TO-220 and TO-3 packages. The pinout for the common TO-220 package is shown below:

          ┌─────┐
 INPUT ───┤1   3├─── OUTPUT
          │     │
          │2    │
          └─────┘
            GND

And here are the pin functions:

Pin	Name	Description
1	INPUT	The unregulated DC input voltage. Must be at least 27V.
2	GROUND	Connect to ground/common/0V in your circuit.
3	OUTPUT	The regulated +24V DC output. Can supply up to 1A.

The TAB/heatsink of the TO-220 package is electrically connected to the GROUND pin. For the TO-3 metal can package, the case is GROUND.

Always double check your connections before applying power. Connecting the input and output backwards can damage the regulator. Also ensure the input voltage does not exceed the absolute maximum of 40V to avoid damage.

Applications

The LM7824 is a versatile voltage regulator used in a wide range of applications. Some common uses include:

1. Industrial control systems
2. 24V is a standard control voltage for relays, solenoids, and actuators

3. The LM7824 provides a stable 24V supply from a higher voltage rail

4. Automotive electronics

   • In-vehicle 24V subsystems like heating/cooling fans, pumps
   • Regulates 24V from the vehicle’s main battery voltage

5. Test and measurement equipment

6. Many industrial sensors and transmitters require a 24V supply

7. LM7824 can power these from the mains or a higher DC bus voltage

8. Audio amplifiers

   • Preamplifier and signal processing stages often use 24V
   • Provides clean, low-noise 24V for best audio performance

9. General purpose 24V power supply

   • Benchtop power supply for projects, breadboarding, testing
   • Easy to use building block for any 24V power needs

In any application, be sure to properly heatsink the LM7824 if it will be supplying high currents. The thermal resistance junction-to-case is 5°C/W for the TO-220 package. Use thermal compound when attaching a heatsink.

Also note the LM7824 has a dropout voltage of about 2V, so the minimum input voltage for a regulated 24V output is about 26-27V. The maximum recommended input voltage is 40V.

Thermal Considerations

Like all linear voltage regulators, the LM7824 dissipates power in the form of heat when supplying current to a load. The power dissipated depends on the input-output voltage difference and the load current. It can be calculated as:

P_DISS = (V_IN – V_OUT) × I_LOAD

Where:
– P_DISS is power dissipated in Watts
– V_IN is the input voltage
– V_OUT is the regulated output voltage (24V)
– I_LOAD is the load current in Amps

For example, if V_IN is 35V and the load is drawing 0.5A, the power dissipated in the LM7824 is:

P_DISS = (35V – 24V) × 0.5A = 5.5W

That’s quite a bit of power and heat! To keep the LM7824 from overheating, this heat must be effectively removed from the IC package. This is done by attaching a heatsink.

The size of heatsink needed depends on the maximum power dissipation, the maximum ambient temperature, and the maximum allowable junction temperature (150°C for the LM7824).

The thermal resistance junction-to-ambient (R_θJA) required for the heatsink can be calculated as:

R_θJA = (T_JMAX – T_AMAX) / P_DISS

Where:
– T_JMAX is the maximum junction temperature (150°C)
– T_AMAX is the maximum ambient temperature
– P_DISS is the power dissipated

For the previous example, if the maximum ambient temperature is 40°C, then:

R_θJA = (150°C – 40°C) / 5.5W = 20°C/W

So a heatsink with a thermal resistance of 20°C/W or lower would be needed to keep the LM7824 within its thermal limits in this scenario.

Always use thermal compound between the regulator and heatsink, and securely fasten the heatsink to the device. Monitor the regulator temperature in your application and adjust the heatsink as needed. The LM7824 has thermal shutdown protection, but it’s best to avoid activating it by using a properly sized heatsink.

Protection Features

The LM7824 voltage regulator includes several built-in protection features:

1. Current limiting
2. Limits the output current to a safe level if the output is shorted or overloaded
3. Exact current limit varies by part and temperature, but is typically around 1.5A

4. Foldback characteristic reduces output current as overload increases to limit heat

5. Thermal shutdown

   • Shuts down the regulator output if the junction temperature exceeds approx 150°C
   • Protects the device from damage due to excessive heat
   • Output remains off until temperature drops about 20°C below the shutdown point

6. Safe operating area protection

   • Combination of current limiting and thermal limiting
   • Keeps the internal power transistor within its safe limits for current and power
   • Prevents damage from simultaneous high current and high input-output voltage difference

It’s important to note these protections are intended to prevent damage to the regulator itself during unexpected fault conditions. They are not a substitute for proper circuit design, device sizing, and heatsinking.

Continuously relying on the current limit or thermal shutdown can stress the device and impact long-term reliability. Always size the regulator for your expected operating conditions with some margin.

It’s also good practice to provide additional levels of protection in your overall power supply design, such as input fuses or polymeric positive temperature coefficient (PPTC) devices. These can help protect the rest of your circuitry in case of an LM7824 failure.

Typical Performance Curves

The LM7824 datasheet provides several helpful graphs that show the regulator’s performance across a range of operating conditions. Let’s look at a few key graphs:

Output Voltage vs. Temperature

 Output Voltage (V)
       24.20 ┼-----┬-----┬-----┬-----┬-----┬-----┬-----┬-----┬-----┬-----┤
             │     │     │     │     │     │     │     │     │     │     │
       24.15 ┼-----┼-----┼-----┼-----┼-----┼-----┼-----┼-----┼-----┼-----┤
             │     │     │     │     │     │     │     │     │     │     │
       24.10 ┼-----┼-----┼-----┼-----┼-----┼-----┼-----┼-----┼-----┼-----┤
             │     │     │     │     │     │     │     │     │     │     │
       24.05 ┼-----┼-----┼-----┼-----┼-----┼-----┼-----┼-----┼-----┼-----┤
             │     │     │     │     │     │     │     │     │     │     │
       24.00 ┼-----┴-----┴-----┴-----┴-----┴-----┴-----┴-----┴-----┴-----┤
            -50   -25     0    25    50    75   100   125   150   175   200
                                   Junction Temperature (°C)

This graph shows how the regulated output voltage varies across the operating temperature range. The LM7824 has a very stable output across a wide temperature span. The output decreases slightly at high temperatures, but stays well within ±1% of 24V.

Dropout Voltage vs. Output Current

“`
Dropout Voltage (V)
2.5 ┼—–┬—–┬—–┬—–┬—–┬—–┬—–┬—–┬—–┬—–┤
│ │ │ │ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │ │ │ │
2.0 ┼—–┼—–┼—–┼—–┼—–┼—–┼—–┼—–┼—–┤ ⌄–┤
│ │ │ │ │ │ │ │ │ ⌄–│- │
│ │ │ │ │ │ │ │ ⌄–│- │ │
1.5 ┼—–┼—–┼—–┼—–┼—–┼—–┼—–┤⌄—– │ │
│ │ │ │ │ │ │ ⌄–│- │ │
│ │ │ │ │ │ ⌄–│- │ │ │
1.0 ┼—–┼—–┼—–┼—–┼—–┤⌄—– │ │ │
│ │ │ │ │ ⌄–│- │ │ │
│ │ │ │ ⌄–│- │ │ │ │
0.5 ┼—–┼—–┼—–┤⌄—– │ │ │ │
│ │ │ ⌄–│- │ │ │ │
│ │ ⌄–│- │ │ │ │ │
0.0 ┼–⌄–│—– │ │ │ │ │
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6