万用表使用指南

A multimeter is the single most important debugging tool on your bench. It measures voltage, resistance, continuity, and (sometimes) current.

Which Multimeter

Model	Price	Best For
Uni-T UT61E	~$50	Best value, accurate, data logging
Aneng AN8008	~$25	Compact, auto-ranging, good starter
Fluke 117	~$200	Professional, lifetime accuracy
Harbor Freight freebie	~$0	Barely usable — but better than nothing

What to Measure

Continuity (Beep Mode)

Tests if two points are electrically connected. Most-used function.

• Switch to Ω/continuity mode (symbol: →+ or sound wave)
• Touch probes together → should beep
• Touch probes to two points → beep = connected
• Use to: check solder joints, trace PCB traces, verify wires are not broken

DC Voltage

• Switch to V⎓ (DC voltage), pick range above expected voltage
• Red probe to positive, black to ground
• Read display
• Use to: check battery level, verify GPIO output, find short circuits

Current measurement is different

To measure current, you must move the red probe to a separate jack (marked A or mA) and put the meter IN SERIES with the circuit. Most blown multimeter fuses come from trying to measure voltage with probes in the current jack.

Quick Diagnosis Flow

Board not working?
├── Measure VCC → GND. Expected voltage? (3.3V? 5V?)
│   └── No → Power supply issue or short circuit
├── Measure GPIO output. HIGH = VCC? LOW = 0V?
│   └── Wrong → GPIO pin damaged or wrong pinMode
├── Continuity test each solder joint
│   └── No beep → cold joint, reflow
└── Check for shorts between adjacent pins (NO beep expected)
    └── Beep → solder bridge, clean with wick

Common Mistakes

Mistake	Consequence	How to Avoid
Probes in current jack while measuring voltage	Blown fuse (or blown meter)	Always return red probe to V/Ω jack
Wrong range selected	Display shows "1" or "OL" (overload)	Use auto-ranging meter or start high
Measuring resistance with power on	Wrong reading, may damage meter	Power off circuit first
Probes touching each other	Short circuit	Keep probe tips apart