When to use something else
Two tests decide it: is the input at least a volt above the output, and is the drop times the current well under 0.8 W. USB to 3.3 V for sensors and one ESP32 passes both. A battery, a 12 V adapter or a motor fails one of them.
Two tests
- Headroom. The input must be about 1.1 V above the output, a little more at high current. If it is not, nothing about the board can fix it.
- Heat. (Input − output) × current must stay under the 0.8 W or so the chip can shed — well under, if the board is in a box.
A third matters only on batteries: the chip draws about 5 mA doing nothing.
Seven jobs, worked out
Each verdict below is computed from the same arithmetic as the last two pages, not typed in:
The jobs that fail, fail for one of the two reasons, and each reason has its own replacement.
What to use instead
- A big drop (12 V to 3.3 V, or 9 V at any real current): a buck converter, also called a step-down converter. It switches the input on and off very fast and smooths the result, so it wastes a small fraction of what a linear regulator would.
- An input below the output (two AA cells for 3.3 V): a boost converter, which raises a voltage.
- An input on both sides of the output (a lithium cell, 4.2 V falling to 3.0 V, for 3.3 V): a buck-boost converter, or a regulator specified for a few hundred millivolts of dropout if the last part of the charge can go unused.
- An LED strip, a motor or a servo: its own supply. These draw amps, and their current has no business going through a regulator meant for a microcontroller.
What the AMS1117 is for
A few hundred milliamps at most, from a supply one or two volts above the output: 5 V to 3.3 V for sensors, a screen or one ESP32. It is quiet, it needs nothing but the capacitors already on the board, and at that job the heat is nothing to think about.
When it does not work
Use a buck (step-down) converter to bring 12 V down to 5 V, and a 3V3 board for the last step if you want the quiet output of a linear regulator. The buck handles the big drop without the heat; the AMS1117 handles 1.7 V of it comfortably.
A switching converter works by switching, and some of that noise reaches its output. A linear regulator has nothing switching, so it is the quieter supply for an analogue sensor or an ADC reading. For a small drop at a small current the heat is also nothing to worry about.
This chip draws about 5 mA whether the project is asleep or not, and an ESP32 in deep sleep draws about 10 µA. The regulator would be five hundred times the drain of the thing it powers. Pick a regulator specified for microamps of quiescent current.
Two resistors, one of them a trimmer, and 1.25 V held across the other.
How the ADJ board sets its voltage →Edit this page — content/books/ams1117/when-to-use-something-else.mdx
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AMS1117 LDO Regulator Kit, 24 Boards (18 × 3.3 V, 6 × Adjustable)
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