In short: The 78xx family (7805, 7812) are three-terminal positive linear regulators that deliver a fixed output from a higher, noisier input using a series pass transistor and an internal reference — add a ~0.33 µF input and ~0.1 µF output capacitor, respect the ~2 V dropout, and you have a stable rail in three pins; the 7912 is its negative counterpart.
Few parts have survived as long on the bench as the 78xx series. Introduced by Fairchild in the 1970s and now second-sourced by STMicroelectronics (L7805), Texas Instruments (µA7805) and ON Semiconductor, these three-terminal regulators remain the first linear rail most engineers ever build. They are unglamorous, but understanding exactly why they behave the way they do is a staple of analog interview questions and a foundation for the LDOs and switchers that came after.
What the 78xx actually is
The "78" marks a positive regulator; the trailing digits are the output voltage. So a 7805 gives +5 V, a 7812 gives +12 V, a 7815 gives +15 V. The "79" prefix — the 7912, 7905 — is the mirror family for negative rails, commonly used for the negative supply of op-amps and ADC references. Internally each device is a complete closed-loop system: a bandgap voltage reference (~1.25 V), an error amplifier, a Darlington series pass element, and current-limit plus thermal-shutdown protection. The error amplifier continuously compares a divided sample of the output against the reference and drives the pass transistor to hold the difference near zero.
Key fact: A 78xx is a series linear regulator. The pass transistor burns the voltage difference as heat: Pdiss = (Vin − Vout) × Iload. A 7805 dropping 12 V to 5 V at 500 mA dissipates (12−5)×0.5 = 3.5 W — far more than the 2.5 W delivered to the load. Efficiency here is only ~42%.
Pinout — and the trap in it
On the standard TO-220 package the 78xx reads, pin 1 to 3: INPUT, GROUND, OUTPUT. The negative 79xx does not simply relabel the same pins — its order is GROUND, INPUT, OUTPUT. Dropping a 7912 into a layout designed for a 7812 footprint is a classic lab mistake that lets the smoke out. Always check the datasheet pin table for the exact package you have.
Input and output capacitors — why both matter
Datasheets specify a 0.33 µF input capacitor (Ci) and a 0.1 µF output capacitor (Co). These are not optional decoration:
- Ci is needed when the regulator sits far from the bulk reservoir/rectifier. It suppresses input-line transients and prevents the internal loop from oscillating when the source inductance is high. Place it right at the input pin.
- Co improves transient response to sudden load steps and keeps the loop phase margin healthy. The 78xx tolerates a wide capacitor range (unlike many modern LDOs that demand a specific ESR window), but a small ceramic plus a larger (e.g. 10–100 µF) electrolytic for bulk energy is the standard recipe.
Key fact: Omitting the output capacitor is the single most common reason a bench 7805 breaks into ~100 kHz oscillation. If you see fuzz on a scope at the output, add Co before suspecting the load.
Dropout and the thermal budget
The 78xx needs its input to stay roughly 2 V above the target output to keep the pass Darlington out of saturation. For a 7805 that means Vin must not sag below ~7 V at full load — a real concern on battery or rectified-mains rails where ripple dips the valley voltage. If Vin falls into dropout, the output simply follows the input down and regulation is lost. This high dropout is precisely why LDO regulators were invented for battery work.
| Part | Vout | Typ. dropout | Max Iout | Polarity |
|---|---|---|---|---|
| 7805 | +5 V | ~2 V | 1 A (TO-220) | Positive |
| 7812 | +12 V | ~2 V | 1 A | Positive |
| 7912 | −12 V | ~1.1 V | 1.5 A | Negative |
A real power-supply circuit
The textbook linear bench supply is: transformer → bridge rectifier → large reservoir capacitor (e.g. 2200 µF) → 7805 with its Ci/Co → load. Size the reservoir so the ripple valley never drops the regulator into dropout: for 1 A at 100 Hz ripple, ΔV = I·t/C, so 1 A over 10 ms into 2200 µF gives ~4.5 V of ripple — comfortably inside headroom if the valley still exceeds 7 V. A reverse-biased protection diode from output to input is good practice so a charged Co cannot back-drive the regulator at power-down. For dual-rail op-amp supplies, pair a 7812 and a 7912 from a center-tapped transformer. If you are laying this out in a tool like KiCad or validating a larger mixed-signal board in Cadence Virtuoso or a Siemens/Mentor flow, treat the regulator's ground as the star point.
For the broader theory behind the control loop, see our linear voltage regulator primer, and browse the full CourseTron course catalog for structured analog design tracks. Teams needing production supply design can also engage Avecas analog design services.
Want to go from "it works on a breadboard" to interview-ready analog mastery? Join the CourseTron Analog & Power Fundamentals course — regulators, references, and real supply design with hands-on labs.
FAQ
Why does my 7805 get so hot?
Because it dissipates (Vin−5)×Iload as heat. Lower the input voltage toward ~7–8 V, add a heatsink sized to the junction-to-ambient thermal resistance, or switch to a buck converter for high current.
Can I increase the output current beyond 1 A?
Not from the chip alone. Add an external PNP pass transistor that the 78xx steers, or move to a dedicated higher-current regulator. Paralleling 78xx parts directly is not recommended.
Do I really need both capacitors?
Yes when the regulator is distant from the bulk reservoir. Ci stops input-side oscillation; Co stabilizes the loop and improves transient response. On a tight layout you can sometimes rely on nearby bulk capacitance, but following the datasheet values is the safe default.
What is the difference between 7805 and 7905?
The 7805 regulates a positive +5 V rail; the 7905 regulates a negative −5 V rail and has a different pinout (GND–IN–OUT). They are used together for symmetric supplies.