Power Supplies and Safety

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.