Power Supplies

A power supply converts AC from the wall (or battery voltage) into the stable DC that electronics needs. Every circuit has one, whether it's the 5V regulator on an Arduino or a multi-rail ATX supply in a PC. Understanding how they work explains why some supplies are heavy and some are tiny, why switching supplies are noisy, and why your audio gear sounds better with a linear supply.

Linear Power Supplies

Classic approach: transformer steps down the AC voltage, bridge rectifier converts it to pulsating DC, filter cap smooths it out, linear regulator stabilises it.

AC in → Transformer → Bridge rectifier → Filter cap → Linear regulator → DC out
120VAC → 12VAC (step down) → 16.8V DC (rough) → 16.5V DC (smooth) → 12V DC (stable)

The linear regulator works by dissipating the excess voltage as heat. If your input is 16V and your output is 5V with 1A of load, the regulator is burning 11W as heat. The efficiency is (Vout/Vin) × 100% = about 31% in this case. Terrible for efficiency, great for noise — the output is clean and the regulation is tight.

Advantages: simple, low noise, fast transient response, no switching interference. Disadvantages: heavy (transformer), hot, inefficient for large voltage step-downs.

Switching Power Supplies (SMPS)

Rectify the AC directly to high-voltage DC, then use a high-frequency switching circuit (20kHz–MHz range) to convert it. The core switching topologies:

Buck converter — steps voltage down. Input voltage higher than output. The inductor and output cap smooth the switched waveform.
Boost converter — steps voltage up. Input voltage lower than output. Common in battery-powered devices where battery voltage drops below the required operating voltage.
Buck-boost — can step up or down. Used in battery applications where the battery voltage ranges above and below the required output.
Flyback — isolated topology, the workhorse of offline supplies (wall warts, PC supplies). Uses a coupled inductor (transformer) to provide isolation between input and output.

Efficiency: 80–95% typical, versus 30–60% for linear at large step-downs. Smaller and lighter because the switching transformer operates at high frequency (and therefore much smaller than a 60Hz mains transformer). The downside is switching noise — the chopping creates EMI that gets into the output. Good filtering, layout, and sometimes LC output filters suppress this, but a switcher will always be noisier than a linear supply.

The 78xx/79xx Linear Regulator Family

The workhorses for small linear supplies:

7805: +5V output
7812: +12V output
7815: +15V output
7905: −5V output (negative rail)

Datasheet minimum specs:
- Input at least 2V above output (dropout voltage)
- Minimum 100nF ceramic on both input and output
- Add heatsink if power dissipation > ~500mW

Basic 5V supply circuit:
Input (7–35V DC) → [7805] → 5V output
           |                    |
          100nF               100nF
           |                    |
          GND                  GND

LDO Regulators

Low-dropout regulators work with less headroom between input and output — useful when you're regulating from a battery that's nearly depleted. The classic 7805 needs about 2V of headroom; an LDO might need only 100–300mV. Common LDOs: LP2950, MCP1700, LM3940. LDOs are PMOS or bipolar PNP based internally, versus the NPN Darlington of the classic 78xx — this allows the lower dropout but typically means lower maximum current.

Ripple and Noise

Output of a bridge rectifier has ripple at twice the mains frequency (120Hz in North America). The filter cap reduces this but doesn't eliminate it — the ripple voltage is:

V_ripple ≈ I_load / (f × C)

Example: 1A load, 120Hz, 1000µF cap:
V_ripple ≈ 1 / (120 × 0.001) ≈ 8.3V peak-to-peak

With 2200µF: V_ripple ≈ 3.8V
After 7812 regulator: ~5mV ripple (regulator has ~70dB PSRR)

The ripple rejection of the linear regulator (PSRR — power supply rejection ratio) is what cleans up the remaining ripple. This is why a linear regulator's output is clean even if the input is rough.