LM7824 Description and Features
The LM7824 is a three-terminal positive voltage regulator in the 78xx series from manufacturers such as Texas Instruments, ON Semiconductor, STMicroelectronics and others. It is designed to provide a fixed +24V DC regulated output voltage for a wide range of input voltages and load currents.
Key features of the LM7824 include:
- Fixed +24V DC output voltage
- Output current up to 1A
- Input voltage range: 27-40V
- Built-in overheating protection
- Short-circuit protection
- Output transistor safe operating area protection
- No external components required
The LM7824 utilizes a linear voltage regulator architecture, meaning it linearly reduces a higher, unregulated input voltage down to the fixed +24V output voltage. It does this by continually adjusting its effective series resistance based on the load current drawn, to maintain a constant output voltage.
While switching regulators can be more efficient, the simple linear architecture of the LM7824 lends itself well to low-cost, compact, and reliable designs for moderate loads up to 1A. The built-in protection features also make it robust to common fault conditions.
LM7824 Specifications
Below is a table summarizing the key specifications of the LM7824 voltage regulator:
| Parameter | Value |
|---|---|
| Output Voltage | +24V |
| Output Voltage Tolerance | ±4% |
| Input Voltage Range | 27 – 40V |
| Output Current (Continuous) | 1A |
| Output Current (Peak) | 1.5A |
| Quiescent Current | 8mA |
| Ripple Rejection | 80dB |
| Output Voltage Drift | -1mV/°C |
| Operating Temperature Range | 0°C to +125°C |
The LM7824 provides a regulated +24V output voltage within ±4% tolerance. It requires an input voltage at least 3V higher than the output, up to an absolute maximum of 40V. At the rated 1A maximum continuous load current, the input voltage will need to be at least 27V to maintain regulation.
The typical quiescent current, or current consumed by the LM7824 itself, is a low 8mA. Line and load regulation are 50-100mV and 25-50mV respectively, meaning the output voltage changes very little with input voltage and load current variations. Ripple rejection is a high 80dB, suppressing input voltage ripple.
LM7824 Application Circuit
Using the LM7824 is very straightforward – it requires only 3 connections and no external components in most cases:
Simply connect the input voltage (27-40V) to the INPUT pin, the +24V regulated output to the OUTPUT pin, and the common ground to the GROUND pin.
While not strictly required, using an input bypass capacitor (CIN) of 0.33µF or more and an output capacitor (COUT) of 0.1µF or more is recommended when the regulator is located far from the power supply Filter capacitors. These capacitors provide stability and noise reduction.
The COUT output capacitor in particular helps improve transient response and reduces output voltage ripple. For applications sensitive to noise and ripple, a larger value can be used. Tantalum or aluminum electrolytic capacitors are recommended for both.
When not using a heat sink, it’s important not to exceed the LM7824’s power dissipation limits. The power dissipated in the regulator is simply:
PD = (VIN – VOUT) × ILOAD
So with a 30V input, 24V output, and 0.5A load current, the dissipated power would be (30V – 24V) × 0.5A = 3W. Refer to the datasheet for the maximum allowable power dissipation at different ambient temperatures. A heat sink can be added to pin 4 to increase the power handling capability.
LM7824 Applications
Some common applications well-suited for the LM7824 voltage regulator include:
- General +24V power supplies for industrial controls, sensors, solenoids, relays, etc.
- Powering 24V automotive accessories from unregulated 24V vehicle power
- Reducing +36V telecom voltages down to regulated +24V for equipment
- Powering 24V LED lighting systems
- Providing 24V regulated power in wind/solar systems with 24V battery banks
- Acting as a pre-regulator for +24V powered devices to reduce power dissipation
In general, the LM7824 is a good choice anytime a regulated +24V is needed from a higher unregulated voltage at load currents up to 1A. Its simplicity, reliability and low cost make it a popular building block in many power systems.
LM7824 Package and Pinout
The LM7824 regulator is commonly available in the industry-standard TO-220 and TO-3 through-hole packages:
The TO-220 package is a popular choice for its compact size and easy mounting on a heat sink. The TO-3 package allows for higher power dissipation.
Both share the same basic 3-pin configuration:
| Pin | Name | Description |
|---|---|---|
| 1 | INPUT | Unregulated DC input |
| 2 | GROUND | Common ground |
| 3 | OUTPUT | Regulated +24V DC output |
The fourth pin/tab is internally connected to the ground pin and used for heatsinking.
Thermal Considerations
As a linear regulator, the LM7824 does have to dissipate substantial heat when operated with high input voltages and load currents. The allowable power dissipation is largely determined by the ambient temperature and thermal resistance from junction to ambient.
The junction-to-ambient thermal resistance (θJA) for the TO-220 package is around 65°C/W without a heatsink. If operating at an ambient temperature of 50°C, this means 25°C of temperature rise is available before reaching the maximum 125°C junction temperature. So the maximum power dissipation in this case would be:
PDmax = ΔTmax / θJA = 25°C / 65°C/W ≈ 0.4W
To determine if a heatsink is needed, first calculate the actual power that will be dissipated in the regulator based on the input voltage, output voltage, and load current:
PD = (VIN – VOUT) × ILOAD
If this exceeds the maximum power dissipation at the expected ambient temperature, then a heatsink is required to lower the θJA. Refer to heatsink datasheets and the LM7824 datasheet for the appropriate calculations.
In most moderate power applications, the LM7824 may not need a heatsink. But it’s important to check the math and ensure the regulator will operate within its safe limits.
LM7824 FAQ
What happens if the input voltage exceeds 40V?
The LM7824 has an absolute maximum input voltage rating of 40V. Applying higher voltages risks damaging the device. Use additional pre-regulation or protection circuits if higher input voltages are possible.
Can the output voltage be adjusted?
No, the LM7824 is a fixed +24V output regulator. Its output voltage cannot be adjusted. For an adjustable output, consider an LM317 regulator.
How much input capacitance is needed?
In most cases, 0.33µF or greater suffices for the input bypass capacitor. More may be needed if the input voltage is noisy or the regulator is located far from the input filter capacitors.
Is the LM7824 suitable for powering digital circuits?
The LM7824 provides a clean, regulated 24V output suitable for analog and digital circuitry. However, most digital logic runs at lower voltages like 5V or 3.3V. An additional regulator like an LM7805 can be used to further step the voltage down for digital chips.
What is the typical efficiency of the LM7824?
The efficiency depends on the input/output voltage differential and the load current. It is calculated as:
η = POUT / PIN = (ILOAD × VOUT) / (IIN × VIN)
So with VIN = 30V, VOUT = 24V, IOUT = 0.5A, and assuming IQ = 8mA:
η = (0.5A × 24V) / (0.508A × 30V) ≈ 78.7%
Typical efficiency ranges from 50-80% depending on operating conditions. A switching regulator can provide higher efficiency but with increased complexity and cost.
Conclusion
The LM7824 is a versatile and easy-to-use +24V linear voltage regulator well-suited for many applications up to 1A load current. By providing a detailed look at its specifications, features, applications, and usage considerations, this article has equipped you with the knowledge to put the LM7824 to use in your own projects. Always refer to the official datasheet for complete details when selecting components.
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