Three-terminal voltage regulators—like the 78xx (positive) and 79xx (negative) series—remain popular due to their simplicity, low cost, and ease of use. While less advanced than modern LDOs, they’re still a smart choice in many applications when selected wisely.
1. Common Application Scenarios
Industrial Controls & Motor Drives: Provide stable 5V/12V bias for PLCs, relays, or optocouplers from a 24V rail. The ample input headroom suits high-dropout regulators like the TO-220-packaged LM7805.
Education & Prototyping: No external components needed—ideal for students building basic power supplies.
Consumer Appliance Aux Supplies: Power MCUs or sensors in devices like microwave ovens or toothbrush chargers, often using compact SOT-223 packages.
Dual-Supply Analog Circuits: Pair 7812 (+12V) and 7912 (–12V) to power op-amps or audio amplifiers requiring symmetrical rails.
2. Key Selection Criteria
Output Voltage & Tolerance: Fixed outputs (e.g., 7805 = 5V ±2%); for other voltages, choose matching part numbers or use adjustable types like LM317 (with resistors).
Output Current: Standard 78xx = 1A; 78Mxx = 500mA; 78Lxx = 100mA—select based on load to save cost and space.
Dropout Voltage: Typically ≥2V. A 7805 needs ≥7V input for 5V output. For low headroom (e.g., 6V→5V), use an LDO instead.
Package: TO-220 for high power (heatsinkable); SOT-223/TO-252 for compact designs; TO-92 for ultra-low-power.
3. Thermal & Protection Considerations
Efficiency is low: η = Vout / Vin. Power dissipation P = (Vin – Vout) × Iout must be managed. If P > 1W:
Use packages with thermal tabs;
Add a heatsink or large copper pour;
Ensure ambient temperature keeps junction temp <125°C.
Also, add an input reverse-polarity diode and an output clamp diode (to protect against capacitor discharge)—simple steps that greatly improve reliability.
4. When to Avoid
Battery-powered devices (quiescent current >5 mA);
Low dropout conditions (<2V);
Noise-sensitive RF or precision ADC circuits;
Systems needing enable control or power-good signals.
In Summary:
Three-terminal regulators aren’t obsolete—they’re fit-for-purpose. In cost-driven, non-portable systems with sufficient input headroom, they offer unmatched simplicity and value. Know their limits, and you’ll keep this classic solution working reliably in modern designs.