Voltage Regulators

The voltage regulator parts model linear (series-pass) regulators that turn a higher, possibly noisy input voltage into a clean, stable output. de:volt ships fixed-output regulators (7805, AMS1117-3.3, AMS1117-5.0) and one adjustable regulator (LM317). All four are linear regulators: they drop the difference between input and output across an internal pass element, so the excess turns into heat.

Fixed regulators

These hold a preset output as long as the input has enough headroom above it.

The AMS1117 parts are low-dropout (LDO): they regulate with as little as ~1.1 V of headroom, so the AMS1117-5.0 runs cooler than a 7805 when the input is close to 5 V.

Fixed pinout

All three fixed regulators share the same three-pin model.

Pin order differs by package. On the 7805 (TO-220) the flat face forward gives IN / GND / OUT left to right, and the metal tab is GND. On the AMS1117 (SOT-223) the large tab is OUT. Always check the datasheet pinout for your physical part.

Adjustable regulator (LM317)

The LM317 has no fixed output. Instead it holds a constant 1.25 V reference between its OUT and ADJ pins, and you set the output with two external resistors:

Vout = 1.25 * (1 + R2 / R1)

R1 connects OUT to ADJ; R2 connects ADJ to GND. A common starting point is R1 = 240 Ω with R2 to ground. For example, R1 = 240 Ω and R2 = 720 Ω give Vout = 1.25 * (1 + 720/240) = 5.0 V. The output is adjustable from ~1.25 V (R2 = 0) up to ~37 V.

LM317 pinout

The LM317 pin order is not the same as the 78xx series. On the TO-220 package, flat face forward, it reads ADJ / OUT / IN left to right, and the metal tab is OUT (not GND). The ADJ pin draws ~50 µA of bias current, so R1 should be small enough that the divider current swamps it — the catalog recommends R1 ≥ 120 Ω, and the common 240 Ω gives ~5 mA of divider current, far above the 50 µA bias so it barely shifts Vout.

How the simulation behaves

The engine models each regulator with three operating regimes. Which one is active depends on the input voltage and the load current you draw.

• Constant-voltage (CV). With enough input headroom and a load under the current limit, the output sits at its target (or the divider value for the LM317). This is normal regulation.
• Dropout. If the input falls below Vout + dropout, the regulator can no longer hold the target and the output follows the input down (minus the dropout voltage). For the 7805 the input must stay above ~7 V; for the AMS1117 parts above Vout + ~1.1 V; for the LM317 above Vout + ~2 V.
• Current limit. If the load tries to pull more than the limit (1.0 A for the 7805, 0.8 A for the AMS1117, 1.5 A for the LM317), the regulator caps the current and the output sags.

Because a linear regulator dissipates P = (Vin − Vout) * Iout, a large input-to-output gap at high current means a lot of heat. In real hardware you need a heatsink once dissipation rises past roughly 500 mW.

Input and output capacitors

Linear regulators want a small ceramic capacitor near the input to absorb supply transients and an output capacitor for stability.

• 7805: 0.1 µF ceramic close to IN and OUT.
• AMS1117: these LDOs need bulk capacitance — about 10 µF on both input and output.
• LM317: 0.1 µF on IN and ~1 µF on OUT.

Example circuit

A 9 V battery regulated down to a 5 V logic rail with a 7805:

9V Battery + ─── IN ──[ 7805 ]── OUT ─── +5V rail
                       │
0.1µF ┤              GND ┤ 0.1µF
                       │
9V Battery − ──────────┴────────────────── GND

Adjustable 5 V rail from the LM317 (R1 = 240 Ω, R2 = 720 Ω):

Vin ─── IN ──[ LM317 ]── OUT ──┬─── Vout (5.0 V)
                          │    │
                          │   R1 240 Ω
                          │    │
                          └── ADJ
                               │
                              R2 720 Ω
                               │
                              GND

Tune R2 (or swap it for a potentiometer) to dial Vout from 1.25 V upward.