Functions and applications of IC 7805 and IC 7912

Functions and applications of IC 7805 and IC 7912

1. What 78xx / 79xx Series Are

78xx / 79xx = three-terminal linear voltage regulator ICs (78xx positive, 79xx negative) that give fixed regulated DC voltage (xx = voltage: 05=5V, 12=12V etc.) with built-in protection.

• IC 7805: +5V regulator (most common, TTL supply).
• IC 7912: -12V regulator (negative, for op-amp dual supply).

Family: 7805,7806,7808,7809,7812,7815,7824 (positive), 7905,7912 etc. (negative). Package TO-220 with 3 pins: Input, Ground, Output.

2. Functions / Features

Feature	Details
Fixed regulation	Maintains $V_{out}$ ±2–4% despite line ($V_{in}$ variation) and load (current) changes, via series pass transistor + error amplifier + bandgap reference inside
Internal protection	Thermal shutdown (~150°C), current limit / short-circuit (~1–2A), safe area
Dropout	Needs $V_{in} ≥ V_{out}+2$–$3$V (e.g., 7805 needs ≥7–8V in) — linear (dissipates $(V_{in}-V_{out})·I$ as heat, needs heat sink for $I>0.5$A)
Ripple rejection	~60 dB (reduces input ripple 1000×)
Current	1A max (1.5A peak) for TO-220 without heat sink ~0.5A; 78L05 (TO-92) 100mA

3. Typical Circuit

Unregulated DC (e.g., 12V after bridge+capacitor) → 7805 Input (pin1) → 7805 GND (pin2) → 7805 Output (pin3) 5V → Load
Capacitors: C_in≈0.33µF at input, C_out≈0.1µF at output for stability (prevent oscillation), larger bulk C for load transient.
For -12V: 7912 input negative (e.g., -15V), output -12V, same capacitors.

4. Applications (exam list)

Application	How
Logic supply	7805 → 5V for TTL, microcontroller, Arduino
Dual supply for op-amp	7805 (+5) + 7912 (-12) / 7915 (-15) for op-amp ±12V
Battery charging / post-regulation	After switching supply, linear for low noise
Reference	5V reference for ADC

5. Worked Example

Need 5V 500mA from 12V unregulated: 7805 dissipates (12-5)·0.5=3.5W ⇒ needs heat sink (junction ~3.5·65°C/W≈227°C without sink ⇒ too hot). Use 9V input: dissipates 2W ⇒ cooler.

Dropout: 7805 with Vin=6V gives Vout maybe 4.5V (not regulated) ⇒ need ≥7V.

6. Exam Points

• State 7805 is +5V and 7912 is -12V three-terminal linear regulator — 3-5 marks, very frequent.
• Sketch input-GND-output pins and typical circuit with capacitors — 5 marks, very frequent (almost certain to ask pin diagram).
• Mention needs $V_{in}≥V_{out}+2V$, thermal shutdown, 1A — 5 marks, frequent.
• Give applications: logic 5V, op-amp dual — 3-5 marks, frequent.