Before this:Picking a board
Power supplies
Key takeaways More “broken” SBCs are victims of undervoltage than of anything else. A board needs its supply to hold a steady voltage (typically 5 V) while delivering enough current (amps) for the board plus every USB device hanging off it — and a thin or long cable can drop the voltage below spec even when the charger is rated correctly. Symptoms are maddeningly random: crashes, USB resets, SD corruption, throttling. The fixes are boring and absolute: the official or name-brand supply, a short thick cable, and a powered hub when peripherals add up.
Power is the least glamorous purchase in the box and the most common cause of “my board is flaky.” This lesson gives you just enough electricity to buy correctly and to recognise undervoltage instantly when you meet it — because in Unit 6 an undervoltage event means dropped radio samples.
Volts and amps, in two paragraphs
Voltage is the electrical pressure a supply maintains; your board is designed for a specific one (5 V on most SBCs) and malfunctions when it sags below a threshold (roughly 4.63 V on a Pi). Current, in amps, is how much flow the load draws; a supply’s amp rating is the maximum it can deliver while holding its voltage. The board draws what it draws — a bigger-amp supply is never harmful, it’s headroom.
The failure mode: the total draw approaches the supply’s limit, or resistance in a poor cable eats the margin, and the voltage sags. Digital logic misbehaves in random ways at low voltage — which is why undervoltage symptoms look like anything except a power problem.
Why is the cable half the story?
Every centimetre of wire has resistance, and at the currents an SBC draws (2–5 A at just 5 V), a thin charging cable drops a meaningful fraction of a volt between charger and board. A charger rated 5 V/3 A through a bad cable delivers 4.5 V at the connector — under spec before the board even boots. Long cables and worn connectors make it worse.
Rule of thumb: buy the official supply for your board (its cable is captive and correctly sized), or a name-brand supply explicitly rated for that board. Never power an appliance from a phone charger, a laptop USB port, or “some cable from the drawer.”
How do you recognise undervoltage?
Learn the signature now and save hours later:
- Random crashes and reboots, especially under load — the CPU’s draw spikes, the voltage dips, the system falls over.
- USB devices disconnecting and reconnecting — an SDR that “vanishes” mid-decode is classic (USB SDR gotchas returns to this).
- SD-card corruption — writes interrupted by brownouts.
- A lightning-bolt icon on a Pi’s display, and warnings in the logs.
The board records it. On a Pi, ask the firmware directly:
$ vcgencmd get_throttled
throttled=0x50005
Any non-zero value decodes to a set of flags; bit 0 (0x1) means undervoltage right
now, and bit 16 (0x10000) means it has occurred since boot. The kernel log tells
the same story:
$ dmesg | grep -i voltage
[ 1234.567890] hwmon hwmon1: Undervoltage detected!
If you see these, stop debugging software. It’s the power.
How do you budget for peripherals?
The supply powers the whole system: board, storage, and every unpowered USB device. Sketch a budget:
| Load | Rough draw at 5 V |
|---|---|
| Board, idle | 0.5–1 A |
| Board, CPU fully loaded | 1.5–2.5 A (more on faster boards) |
| RTL-SDR dongle | ~0.3 A |
| USB SSD | 0.5–1 A, with spin-up spikes |
| Fan | ~0.1 A |
A loaded board plus SDR plus SSD can brush against a 3 A supply’s ceiling. Two ways out: a supply with more headroom (a 5 A-class supply on boards that accept it), or moving hungry peripherals onto a powered USB hub — a hub with its own supply, so peripherals stop drawing through the board at all. USB & powered hubs covers when and how.
What about the appliance angle — outages and placement?
Two habits for an always-on build. First, expect power cuts: the appliance must boot back into its job unattended, which is exactly what Unit 3’s systemd services guarantee, and your filesystem should survive the cut (journalled Linux filesystems do; quality storage from last lesson helps). A small UPS is a nice-to-have, not a requirement. Second, placement: cheap switching supplies can radiate RF noise — for a radio appliance, a quality supply and a bit of distance between it and your antenna feedline are part of the RF hygiene story that USB SDR gotchas completes.
Quick check: a board crashes randomly under load and its USB SDR keeps disconnecting. What's the first suspect?
Recap
- Undervoltage is the classic SBC mystery failure: the supply (or cable) lets the 5 V rail sag, and everything downstream misbehaves randomly.
- The cable is half the story — thin or long cables drop voltage even behind a correctly rated charger. Buy the official or name-brand supply.
- Recognise the signature: random crashes, USB resets, SD corruption — and
confirm with
vcgencmd get_throttled/dmesgrather than guessing. - Budget amps for the whole system, board plus peripherals; move hungry devices to a powered hub when the sum gets close.
- An appliance must recover from power cuts unattended — services at boot plus sound storage make an outage a non-event.
Next up: Cases & cooling.