78xx 79xx Series

The 78xx and 79xx series are fixed-output three-terminal linear regulators used in virtually every piece of laboratory and consumer electronics. The LM7812 delivers a rock-steady 12V from a car battery, and the LM7905 provides the negative 5V rail in dual-supply op-amp circuits.

Core Concept

The 78xx series (LM7805, LM7809, LM7812, LM7815) produces fixed positive output voltages. The last two digits of the part number give the output voltage directly: 7805 = +5V, 7812 = +12V. All three-terminal devices share the same pinout: pin 1 is INPUT, pin 2 is GROUND, pin 3 is OUTPUT (for the standard TO-220 package).

The 79xx series (LM7905, LM7912, LM7915) is the negative-voltage counterpart. The internal structure is complementary: it uses PNP pass transistors and a negative reference. The pinout is different from the 78xx family even in the same TO-220 package: pin 1 is GROUND, pin 2 is INPUT, pin 3 is OUTPUT (negative). This difference trips many students during lab work.

Both series require a small external capacitor at the input (0.33 µF) and output (0.1 µF) to prevent oscillation in the error amplifier. Without these capacitors, some combinations of input lead inductance and load capacitance can make the regulator oscillate at high frequency. The output capacitor also improves transient response during sudden load changes.

Key Equations

Output voltage (fixed by part selection): Vout = Vnominal ± tolerance (typically ±4% for standard grade, ±2% for A-grade).

Minimum input voltage: Vin_min = Vout + Vdropout where Vdropout ≈ 2V for 78xx and 79xx series.

Maximum input voltage: Vin_max = 35V for 78xx (5V to 18V output parts)

Power dissipation: Pd = (Vin - |Vout|) * IL

Maximum output current: IL_max = 1.5A (with heatsink) for standard TO-220 package devices.

Given:
  LM7812 regulator (Vout = +12V)
  Vin  = 18V
  IL   = 600 mA
  Find: Power dissipated and junction temperature rise
  Thermal resistance θ_JA (TO-220, no heatsink) = 65°C/W
  Ambient temperature = 30°C

Why this formula:
  Power dissipated heats the junction above ambient by Pd * θ_JA.

Formula:
  Pd = (Vin - Vout) * IL
  Tj = Ta + Pd * θ_JA

Substitution:
  Pd = (18 - 12) * 0.6
     = 6 * 0.6

Calculation:
  Pd = 3.6 W
  Tj = 30 + 3.6 * 65
     = 30 + 234
     = 264°C

Final Answer:
  Pd = 3.6 W
  Tj = 264°C  (EXCEEDS 150°C max rating!)
  A heatsink is mandatory. With θ_JA = 12°C/W (heatsink):
  Tj = 30 + 3.6 * 12 = 30 + 43.2 = 73.2°C (safe)

Exam Tip: GATE questions on 78xx/79xx most often test whether you know the pinout difference between 78xx and 79xx, and whether you can identify when a heatsink is required. Use Tj = Ta + Pd * θ_JA and compare to the 150°C maximum junction temperature. The thermal shutdown inside the IC activates at 150°C, so exceeding this permanently damages the device. Also remember that the 78xx and 79xx are NOT pin-compatible in TO-220.

Key Properties

• The 78xx series provides fixed positive voltages: 5V, 6V, 8V, 9V, 10V, 12V, 15V, 18V, 24V depending on the part number suffix.
• The 79xx series provides the corresponding negative voltages using the same suffix convention (7905 = -5V, 7912 = -12V).
• 78xx pinout (TO-220): 1=IN, 2=GND, 3=OUT. 79xx pinout (TO-220): 1=GND, 2=IN, 3=OUT. These are NOT the same.
• Both series include internal current limiting at approximately 2.2A and thermal shutdown at 150°C junction temperature.
• Input bypass capacitor (0.33 µF) prevents oscillation caused by input lead inductance; output capacitor (0.1 µF) improves transient response.
• The LM7805 delivers up to 1.5A continuously in a TO-220 package with an appropriate heatsink; without a heatsink, safe current is approximately 200 mA at high Vin-Vout differences.
• Output voltage accuracy is typically ±4% for commercial grade (0°C to 70°C) and ±2% for industrial grade (-40°C to 85°C) devices.

Quick Revision

• 78xx = positive regulators; 79xx = negative regulators.
• Part number suffix = output voltage (7805=5V, 7912=-12V).
• 78xx TO-220 pinout: IN-GND-OUT. 79xx TO-220: GND-IN-OUT.
• Minimum dropout: Vin must exceed |Vout| by at least 2V.
• Power dissipation: Pd = (Vin - |Vout|) * IL.
• Thermal shutdown at 150°C junction temperature; current limit at ~2.2A.
• Add 0.33 µF at input and 0.1 µF at output to prevent oscillation.
• Exam trap: Connecting a 79xx regulator with 78xx pinout assumptions reverse-biases the internal pass transistor and destroys the IC immediately.