In short: Feed an Uno 7–12 V on the barrel jack or VIN (6–20 V is the absolute range), because the on-board linear regulator turns every extra volt into heat at (Vin − 5 V) × I, and budget roughly 500 mA from USB or about 800 mA from the jack before the regulator or fuse becomes the limit.
Three doors into one 5 V rail
The Uno has three ways in: the USB-B connector, the 2.1 mm barrel jack and the VIN header pin. All three end up on the same 5 V rail that feeds the ATmega328P, the USB bridge and your shields. What differs is how much conditioning happens on the way. The rules below are written for the Uno R3; clones may substitute parts, so always check your board's schematic. If you want the chip-level picture first, our Arduino Uno specifications and features guide covers the MCU side.
- USB: 5 V arrives straight from the host through a resettable polyfuse rated around 500 mA, matching the USB 2.0 unit-load limit for a configured device.
- Barrel jack: centre-positive, passes through a series protection diode, then into the 5 V linear regulator (an NCP1117-class LDO in SOT-223).
- VIN pin: joins the same net as the jack output. It lets you power the board from a battery pack or another board, but it sits beyond the jack diode, so a reversed battery gets no protection.
Why 7–12 V, and what the 6–20 V limits mean
A linear regulator needs headroom, called dropout. The Uno's 5 V part needs about 1.1 V at light load, and the series diode eats roughly another half volt, so 6 V in only just keeps the rail alive. Below that, the board browns out as soon as a motor or radio draws a spike. Arduino therefore recommends 7–12 V.
The upper limit is thermal, not electrical. The regulator and its input capacitor tolerate 15–20 V, but the hotter the input, the more power the little SOT-223 package must shed. Many bench-power engineers treat 9 V as the sweet spot: enough headroom, little waste.
Key fact: "Absolute maximum" is a damage threshold, not a design target. Running at 20 V leaves zero margin; running at 9 V leaves you room for a hot enclosure.
Regulator dissipation: the number that decides everything
A linear regulator is a variable resistor in series with the load. It passes the load current and burns the difference between input and output voltage:
P = (Vin − Vout) × Iload, and the junction rises by P × θJA.
The SOT-223 package on a small copper pad has a θJA of very roughly 100–160 °C/W. Run the numbers for a board with a few sensors and an LCD drawing 150 mA total:
- At 12 V in: (12 − 5) × 0.15 = 1.05 W, a rise of 100–170 °C. The thermal limit trips and the rail sags.
- At 9 V in: 4 × 0.15 = 0.6 W, a rise of 60–95 °C. Warm, but survivable.
- At 7 V in: 2 × 0.15 = 0.3 W, a rise of 30–50 °C. Comfortable.
This is why a 12 V wall adapter can run a bare Uno happily and then shut it down the moment you add a servo shield. Power electronics designers use the same arithmetic when sizing LDOs on SoCs, and tools such as Cadence Celsius or Siemens Simcenter Flotherm turn the same equation into full board-level thermal maps.
Watching the energy go
The animation below shows the same idea in motion. Each moving dot is a packet of charge. The load receives the same current, but the dots lose energy as they cross the regulator, and that lost energy is the glow.
Building a safe current budget
Budget current at the correct node. Mixing them up is the classic interview error.
| Node | Practical limit | Why |
|---|---|---|
| USB total | ~500 mA | Polyfuse and host port; includes the board's own ~50 mA |
| Jack/VIN, 9 V in | ~400–500 mA | Thermal limit of the LDO, not its 1 A rating |
| 3V3 pin | 150 mA | Small separate 3.3 V LDO |
| Each I/O pin | 20 mA (40 mA abs. max) | Pin driver |
| All I/O combined | ~200 mA | ATmega328P VCC/GND pin limits |
A worked example: a Uno driving eight LEDs at 10 mA, one SG90 servo (stall near 650 mA) and a 16×2 LCD (about 25 mA with backlight). The LEDs and LCD add roughly 105 mA, plus 50 mA for the board itself. Fine on its own, but the servo's stall current exceeds USB's limit and the LDO's thermal budget. Give the servo its own 5 V supply and tie the grounds together.
Key fact: High-current loads (motors, servos, LED strips, relays) never belong on the Uno's 5 V pin. Use a separate buck converter or supply, and share ground only.
Practical rules that save boards
- Prefer a 7.5–9 V supply, or a small buck module set to 7 V, if you need more than ~200 mA.
- Never power from USB and a separate 5 V source on the 5 V pin at the same time; the 5 V pin bypasses the selector and can back-feed your computer.
- Add a 100 µF bulk capacitor near inductive loads to stop brown-out resets; watch for a rising reset count as a symptom.
- Measure, do not guess: put a USB power meter or a bench supply with current display on the input and compare against your budget.
- Battery packs: four AA cells sag from about 6.4 V to 4.8 V under load, so use six cells or a boost/buck regulator for dependable operation.
These habits generalise. Every embedded power tree, from an 8-bit board to a multi-rail SoC, comes down to dropout, dissipation and distribution. If you want to see how professional teams design and sign off power delivery, Avecas design services work on exactly this class of problem, and our follow-up on Arduino Uno pinout explained maps these limits onto each pin.
Frequently asked questions
Can I power an Uno with 12 V?
Yes, it is within the recommended range, but the regulator wastes 7 V times your current. Under about 100 mA it is fine; with shields or sensors it may overheat and shut down.
Is it safe to connect USB and the barrel jack together?
Yes. The on-board source selector automatically picks the jack or VIN rail when it is present, and USB is not back-fed through it.
How much current can the 5 V pin supply?
About 500 mA over USB, and typically 400–500 mA from a 9 V jack supply before heat becomes the limit. The 3V3 pin only gives 150 mA.
Why does my Uno reset when a motor starts?
The motor's start-up surge pulls the rail below the brown-out threshold. Give the motor a separate supply, add bulk capacitance and flyback diodes.
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