Overview

The LM7824 (and TO-220 packaged L7824CV / LM7824CT) is a 3-terminal fixed positive linear voltage regulator IC manufactured by STMicroelectronics, Texas Instruments, and ON Semiconductor. It supplies a regulated +24.0V+24.0\text{V} DC power rail from unregulated DC input voltages ranging from 27.0V27.0\text{V} up to 40.0V40.0\text{V}.

As the highest fixed voltage member of the classic 78xx linear series, the LM7824 is frequently employed in industrial control panels, 24V relay driver boards, PLC interfaces, 24V DC solenoid drivers, and audio power amplifier stages. It features internal thermal overload shutdown, short-circuit current limiting, and safe-area protection.

Quick reference

Regulator TypeFixed Positive Linear Voltage Regulator
PackageTO-220 (L7824CV) / TO-263 / DPAK / TO-3
Pinout (TO-220 Front)Pin 1: Input (IN), Pin 2: Ground (GND), Pin 3: Output (OUT)
Fixed Output Voltage+24.0 V+24.0\text{ V} DC (±4%\pm 4\% tolerance across temperature)
Input Voltage Range (VINV_{IN})27.0 V27.0\text{ V} min to 40.0 V40.0\text{ V} max
Dropout Voltage2.0 V2.0\text{ V} typ (Requires VIN≥27.0VV_{IN} \ge 27.0\text{V} for full regulation)
Continuous Output Current (IOUTI_{OUT})>1.5 A> 1.5\text{ A} (with adequate heatsink)
Ripple Rejection Ratio50 dB…67 dB50\text{ dB} \dots 67\text{ dB} at f=120 Hzf = 120\text{ Hz}

Pinout (TO-220 Package)

Looking at the front labeled face of the TO-220 package with metal tab at top and leads pointing down:

text
        ┌──────────────┐
        │ [LM7824 Tab] │  (Metal Tab connected internally to Pin 2 GND)
        ├──────────────┤
        │    LM7824    │  (Front Face)
        └─┬────┬────┬──┘
          1    2    3
         IN   GND  OUT
PinNameDescription
1INUnregulated DC input voltage pin (+27.0V to +40.0V DC)
2GND / TABGround reference (0 V, connected internally to heatsink tab)
3OUTFixed regulated +24.0V DC output pin

Specifications

ParameterSymbolMinTypMaxUnitConditions
Regulated Output VoltageVOUTV_{OUT}23.024.025.0V27.0V≤VIN≤38V,5mA≤IO≤1.0A27.0\text{V} \le V_{IN} \le 38\text{V}, 5\text{mA} \le I_O \le 1.0\text{A}
Maximum Input VoltageVIN(max)V_{IN(max)}——40.0VAbsolute Maximum Rating
Dropout VoltageVdV_d—2.02.5VIOUT=1.0A,TJ=25∘CI_{OUT} = 1.0\text{A}, T_J = 25^\circ\text{C}
Peak Output CurrentIOSI_{OS}1.52.2—AShort circuit peak current
Quiescent CurrentIdI_d—4.68.0mATJ=25∘CT_J = 25^\circ\text{C}
Quiescent Current ChangeΔId\Delta I_d——0.5mA5mA≤IOUT≤1.0A5\text{mA} \le I_{OUT} \le 1.0\text{A}
Output Noise VoltageVNV_N—170—µV10 Hz≤f≤100 kHz10\text{ Hz} \le f \le 100\text{ kHz}

Standard Circuit & Capacitors

text
       +V_IN Unregulated Input (27.0V - 40V DC)
          │
       [Pin 1: IN]
        LM7824
       [Pin 3: OUT] ──────────────┬─────────────── +24.0V Regulated DC Output
          │                       │
       [Pin 2: GND]          [ C2 = 0.1µF Ceramic ]
          │                       │
   [ C1 = 0.33µF Ceramic ]       GND
          │
         GND
  • Input Capacitor (C1=0.33 μFC_1 = 0.33\ \mu\text{F}): Required to stabilize the input and eliminate parasitic line inductances when positioned away from the filter capacitor.
  • Output Capacitor (C2=0.1 μFC_2 = 0.1\ \mu\text{F}): Ensures high-frequency stability and reduces transient dips during sudden load step changes.

Common mistakes

  • Exceeding the 40V absolute maximum input voltage: While standard LM78xx parts (5V–18V) are rated for 35V max, the LM7824 is rated for 40V. However, unregulated transformers can produce open-circuit DC voltages exceeding 40V during high AC line conditions, causing catastrophic dielectric breakdown.
  • Under-sizing heatsinks: Because linear regulators dissipate (VIN−VOUT)×ILOAD(V_{IN} - V_{OUT}) \times I_{LOAD}, stepping down 36V to 24V at 1A produces 12W12\text{W} of heat. Without a generous heatsink (≤4∘C/W\le 4^\circ\text{C/W}), the IC will reach thermal shutdown (∼150∘C\sim 150^\circ\text{C}) rapidly.
  • Input voltage dip below 27V: 24V output requires at least 27.0V27.0\text{V} input under all AC mains ripple conditions. If the input trough dips below 26.5V26.5\text{V}, ripple will feed directly through to the output.

Notes

  • Complementary Negative Rail: The LM7924 provides the −24V-24\text{V} companion rail for symmetrical ±24V\pm 24\text{V} high-voltage audio or industrial driver circuits.

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