7805 vs LM317 vs AMS1117: three linear regulators, three different jobs

The 7805 and the LM317 come in the same TO-220 body, and people swap one for the other all the time. The pins don't match: the 7805 runs input, ground, output from the left; the LM317 runs adjust, output, input. The pinout drawings at the top of this page show all three parts.

Drop an LM317 into a 7805 footprint and your input voltage lands on its ADJ pin, the output pin sits on your ground track, and the input pin feeds the circuit that expected 5 V.

The tab matters too. A 7805 can bolt straight onto a grounded metal case or heatsink. An LM317 or AMS1117 tab carries the output voltage, so on a grounded heatsink it needs an insulating pad and an insulated screw bushing, or it shorts the output.

The AMS1117 and the LM317 share the same order (adjust or ground, output, input), which is why many boards take either one in the adjustable version.

The thermal resistance figures are measured on standard test boards. A part standing in free air on a thin board runs hotter, so treat the temperatures in the examples below as the best case. The AMS1117 is made by several companies under the same number, and their sheets don't all agree; the AMS1117 page compares two of them.

• 5 V from 9 V or 12 V, with no adjustment: the 7805. There are no resistors to set, the tab is ground, and TI quotes 4.8 to 5.2 V at room temperature.
• Any other voltage, or one you want to trim: the LM317. Two resistors set it, and the LM317 calculator picks them.
• 3.3 V from a 5 V rail (USB, Arduino, a 5 V adapter): the AMS1117-3.3. The 7805 can't make 3.3 V, and the LM317 wants 3 V of headroom where you only have 1.7 V.
• A big drop at more than a few hundred milliamps: none of them. Work out the watts first (example 1). Above roughly 2 to 3 W, a switching buck module wastes far less, and the LM2596 page covers one.

1. 12 V in, 5 V out at 500 mA

The regulator drops 12 − 5 = 7 V at 0.5 A, so it turns 3.5 W into heat.

• 7805 (TO-220, 19 °C/W): 3.5 W × 19 °C/W = 66.5 °C above the air. At 25 °C ambient the junction sits near 92 °C. That is under TI's 125 °C limit, but far too hot to touch, so add a small heatsink.
• LM317 (TO-220): 3.5 W × 23.5 °C/W = 82 °C rise on TI's KCS build, about 107 °C at the junction. On the KCT build (37.9 °C/W) the figure is 133 °C rise, well past the limit. It needs a heatsink.
• AMS1117-5.0 (SOT-223): SLKOR allows 0.75 W at most in SOT-223, and 3.5 W is 4.7 times that. It will heat up until its thermal shutdown cuts in (150 °C typical, per the datasheet). Don't use it here.

Double the load to 1 A and the 7805 has 7 W to lose: 7 × 19 = 133 °C rise with no heatsink. At that point a buck module is the better answer. The 7805 page sizes the heatsink for this case, and the regulator thermal design lesson explains the method.

2. 5 V USB to 3.3 V at 300 mA

The headroom is 5 − 3.3 = 1.7 V.

• 7805: no 3.3 V version in the µA78xx family.
• LM317: TI recommends at least 3 V between input and output. 1.7 V is not enough.
• AMS1117-3.3: dropout is 1.4 V max at 1 A, so 1.7 V works with 0.3 V to spare. It burns 1.7 V × 0.3 A = 0.51 W. At 135 °C/W that is a 69 °C rise, about 94 °C at the junction in 25 °C air. That is under the 0.75 W and 125 °C limits, but hot, so give the tab some copper on the board.

Keep the input steady. A USB port that sags to 4.6 V leaves only 1.3 V of headroom. That is the typical dropout at 1 A and below the 1.4 V maximum. The dropout is smaller at 300 mA, but the sheet only guarantees the 1 A figure, so at that point a 3.3 V rail can start to follow the input down.

3. 9 V battery to 5 V

A 9 V battery drops as it drains, so the question is how long each regulator holds 5 V.

• 7805: TI's recommended minimum input is 7 V. Below that the output is no longer guaranteed, even though a fresh battery starts well above it.
• AMS1117-5.0: needs 5 + 1.4 = 6.4 V at most (the 1 A figure; the dropout shrinks at lighter loads). It keeps regulating for longer as the battery runs down.

Count the standby current too. With no load, the 7805 draws 4.2 mA typical and the AMS1117 3 mA typical (10 mA max). On a small battery that alone matters, and a regulator built for low quiescent current does much better.

4. LM317 set to 5 V

With TI's usual R1 = 240 Ω, the formula is VOUT = 1.25 × (1 + R2/R1) + IADJ × R2, with IADJ at 50 µA typical. Exactly 5.00 V needs R2 = 713 Ω.

• R2 = 720 Ω gives 1.25 × 4 + 0.036 = 5.04 V.
• 720 Ω is not a standard value. The nearest E24 value, 750 Ω, gives 5.19 V. A 680 Ω and a 39 Ω in series (719 Ω) give 5.03 V.
• The reference itself can be anywhere from 1.2 V to 1.3 V (TI, full temperature range), so the output can land 4% either side whatever resistors you pick: 4.83 V to 5.23 V with the 719 Ω pair. If you need exactly 5.00 V, use a trimmer for part of R2, or use a 7805.

The LM317 calculator does this for any voltage and snaps to standard values. The resistor combination finder finds series and parallel pairs like 680 + 39.

• Work out the watts first: (VIN − VOUT) × current. Over about 1 W in SOT-223 or 2 W in a bare TO-220, plan for copper area or a heatsink.
• Fit the AMS1117's output capacitor. SLKOR asks for at least 10 µF tantalum or 50 µF aluminium electrolytic, with an ESR of 0.5 Ω or less. Without it the part can oscillate, and the output looks fine on a meter while it rings on a scope.
• Give the LM317 a load. TI's minimum load current is up to 10 mA at the highest input-output difference. The 240 Ω R1 already draws about 5.2 mA (1.25 V / 240 Ω), which is why that value is so common.
• Check the input limit on the AMS1117 sheet for the part you buy. SLKOR allows 18 V, but AMS1117s from different makers don't all share that figure, and a "12 V" adapter can read well above 12 V with no load.
• Insulate the tab on the LM317 and AMS1117 if it touches anything grounded.

Using a linear regulator for a big drop at real current. A 7805 fed from 24 V and driving 500 mA wastes 9.5 W, nearly four times what the load gets (2.5 W). With no heatsink, 9.5 W × 19 °C/W works out to a 181 °C rise, far past the 125 °C limit. Use a buck converter for the big drop, and a linear regulator after it if you want a quieter rail. The switching regulator lesson shows why a buck wastes so much less.

For a 5 V rail from a 9 V to 12 V adapter at a few hundred milliamps, we'd use a 7805 with a small clip-on heatsink, because the ground tab makes mounting easy. For 3.3 V off USB, an AMS1117-3.3 with a 10 µF tantalum on the output. For a bench supply or any odd voltage, an LM317 with the resistors from the calculator. Past about 3 W of heat, we'd skip all three and use a buck module.

The linear regulators lesson covers how all three work inside. Each part has its own page with the full ratings and a bench check: 7805, LM317 and AMS1117. For other parts compared the same way, see 1N4007 vs 1N4148 vs 1N5819 and LM358 vs 741 vs TL072.