Open this lesson in your favourite AI. It'll walk you through the why, explain the demo, and quiz you on the try-it list.
Most circuits run on DC. Most outlets give AC. Most beginner accidents involve overconfidence with mains voltage. Knowing the safety hierarchy — battery > USB > wall-wart > bench supply > mains — and which to use when is essential. A 12V drill battery can weld metal. A 5V USB can't kill you but can fry a $40 chip.
Power source hierarchy for hobby work.
Use these three in order. Each builds on the one before.
Explain in one paragraph the safety hierarchy of power sources from coin cell to mains AC.
Walk me through using a bench power supply safely with a new circuit.
Given a circuit that draws unexpected current, what 3 diagnostic checks would you make before scaling up power?
SAFE FOR HOBBY (DC, low voltage, low current):
1. COIN CELL (CR2032): 3V, ~50mAh.
Use: tiny sensors, watches, prototype wearables.
Risk: low. Worst case: a chip burns.
2. AA/AAA: 1.5V each (alkaline) or 1.2V (NiMH).
Use: basic LED, simple microcontroller projects.
Risk: low.
3. 9V battery: 9V, ~500mAh.
Use: legacy radios, simple projects needing more headroom.
Risk: low but capable of small sparks if shorted.
4. Li-Po pouch cells: 3.7V nominal, variable mAh.
Use: wearables, drones, anything that wants tight form factor.
Risk: MEDIUM. Punctured cells can vent flame. Charge with a
proper Li-Po charger, never bare leads.
5. USB power (Type-A or Type-C): 5V nominal.
Use: anything that needs steady 5V at <500mA (Type-A) or up to
3A (Type-C).
Risk: low. Computers have over-current shutdown on USB ports.
6. BENCH POWER SUPPLY: adjustable 0-30V, 0-5A.
Use: bench experimentation. Current limit protects.
Risk: low at low voltages. Set current limit BEFORE turning on.
GETTING DANGEROUS (start being careful):
7. 12V car/UPS lead-acid: short-circuit current ~hundreds of amps.
Can weld a wire. Can burn skin.
Use: hobby motor projects, intermittently.
Risk: HIGH if shorted.
8. 24V industrial: similar to 12V but more headroom.
ACTUALLY DANGEROUS (don't until trained):
9. MAINS AC (110V or 230V):
LETHAL with brief contact.
Capable of arc-flash that destroys eyesight.
Don't work on this until you know what you're doing.
10. HV/RF: thousands of volts. Definitely don't.
THE BENCH PROTOCOL:
Before connecting power:
- Visually inspect the circuit.
- Set bench supply voltage to the EXPECTED supply (5V if 5V circuit).
- Set CURRENT LIMIT to slightly above expected current (e.g., 200mA
for a circuit that should draw 100mA).
- Turn on. Observe.
If current goes immediately to limit:
- SHORT CIRCUIT. Turn off. Investigate.
If voltage drops:
- UNDERPOWERED. Need higher current or different supply.
If circuit smokes:
- TURN OFF IMMEDIATELY. Don't add more.
PROTECTING THE CIRCUIT:
- In-line fuse on the supply: 0.5A polyfuse + appropriate rating.
- Decoupling caps on every IC's power pins (covered later).
- Reverse-polarity protection: schottky diode in series, or P-channel
FET for low-drop.
- TVS diodes on input lines that may see ESD.
WALL-WARTS (WALL-MOUNT POWER SUPPLIES):
Look at the label:
- Input: 100-240V AC, 50/60Hz.
- Output: 5V DC, 2A (for example).
The brick converts mains to safe DC.
The DC output is what you wire to your circuit — handle that.
Never open the brick. Never modify mains-side wiring.
Beware:
- Cheap unregulated wall-warts. Their voltage drops under load.
- "5V" might be 4.5V at 1A. Use a regulated supply.
THIS LESSON'S FOCUS:
Pick a power source that matches your project. Set current limit
before connecting. If the circuit smokes, turn off — don't add more
voltage hoping it'll resolve.