Too many volts for a 3.3 V part
A 3.3 V part on a 5 V supply needs something between them that holds 3.3 V still while the part works. Two resistors manage it until the part draws current. A regulator keeps managing it.
The supply you have is the wrong voltage
Most 3.3 V parts must not see more than about 3.6 V. Most supplies are higher: 5 V from USB, 9 V from a battery, 12 V from an adapter. Something has to take the difference away, and keep taking it away as the part changes what it draws. An ESP32 goes from about 45 mA to 240 mA the moment its radio transmits.
Why not two resistors
A 1 kΩ and a 2 kΩ resistor split 5 V into 3.33 V. Try it with a part attached:
With nothing connected the divider is right. With anything connected it is wrong, because the part's current flows through the top resistor too and drops more of the 5 V there. A divider makes a voltage to be measured, never one to run from.
What a regulator does instead
It measures its own output and corrects it, continuously. The part can draw 5 mA or 500 mA and still get 3.3 V, as long as the supply is high enough and the chip stays cool enough. Those two conditions are the rest of this book.
The AMS1117 is a linear regulator, and a low-dropout one — LDO for short. Linear means it gets rid of the extra voltage by turning it into heat. Low dropout means it needs only about a volt more in than it puts out, which was low when the design was new and is middling now.
When it does not work
Many parts survive over-voltage for a while, which is why it looks fine. An ESP32's absolute maximum is 3.6 V; above that, current flows into the chip through protection diodes never meant to carry it. The failure comes later and looks random.
Often not. The 3V3 pin on an ESP32 board is the output of a regulator like this one. You need a separate board when the main board runs at 5 V, when the 3.3 V part draws more than that pin can spare, or when there is no 3.3 V board in the project at all.
Because its limit is set by the chip inside it. Some modules carry a regulator of their own and accept 5 V, and their label or listing says so. A bare 3.3 V chip does not.
A transistor, two resistors and a 1.25 V reference, and where the extra volts go.
How a regulator holds still →Edit this page — content/books/ams1117/too-many-volts.mdx
Questions about this product
See what other owners have asked, and read their solutions.
AMS1117 LDO Regulator Kit, 24 Boards (18 × 3.3 V, 6 × Adjustable)
Loading discussions…
Discuss this article
Ask about this page. The answer stays here, on the page it belongs to, for whoever hits the same wall next.