Check it with a meter
Two readings — VIN to GND and OUT to GND — and the LED on the 3V3 board tell you whether a regulator is getting power, making its voltage, overheating or out of headroom, before you suspect the sketch or the part.
Two readings
Set the meter to DC volts. The black probe goes on GND and stays there. The red probe goes on VIN, then on OUT. Measure at the board's own pins, not at the supply: a breadboard row that missed shows up only there.
Measure with the project running. Most regulator faults — a sagging input, an overload, the heat — only exist under load.
Reading the pair
- VIN right, OUT right: the regulator is fine. The fault is further along.
- VIN zero: power is not reaching the board.
- VIN low, OUT about a volt below it: not enough headroom. The input is the problem, not the board.
- VIN right, OUT zero: something on OUT is shorting it, or the chip has shut itself down. Take everything off OUT and measure again.
- OUT coming and going: it is overheating and recovering, over and over.
The LED as a first check
On a 3V3 board the LED is a free third reading. Off with the supply connected means OUT is at zero — no input, a short, or a shutdown. On means OUT has some voltage, which is not the same as the right voltage: swap VIN and OUT and it lights too. The meter settles it.
When it does not work
Something else is feeding OUT — often an ESP32 board on its own USB cable with its 3V3 pin wired to OUT — and pushing back through the diode inside the chip that runs from OUT to IN. VIN then sits a little below OUT. Power the project from one place.
The regulator is working. Measure at the part's own supply pin, to its own ground: a bad breadboard row or a missing ground wire between the board and the part shows up there and nowhere else.
Test it alone: 5 V on VIN, nothing on OUT but the meter. A 3V3 board should read 3.234 to 3.366 V and light its LED; an ADJ board should move smoothly as you turn the trimmer. Anything else, set it aside — the box has plenty.
The specs for both boards, what each supply can run, and the questions people ask first.
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AMS1117 LDO Regulator Kit, 24 Boards (18 × 3.3 V, 6 × Adjustable)
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