A multimeter is an essential tool for electronics work that measures voltage, resistance, current, and continuity in one instrument. Almost every check starts with a multimeter: the continuity test to look for shorts after soldering, checking power rail voltage, and measuring part values. This article covers the main functions of a digital multimeter, the specs and safety ratings to look at when buying, the most commonly used measurement methods, and things to be careful about.
Basic functions
| Function | Symbol | Use |
|---|---|---|
| DC voltage | V⎓ | Batteries, power rails, IC pin voltages |
| AC voltage | V~ | Adapter output, AC signals (measuring a wall outlet is not recommended for beginners) |
| Resistance | Ω | Resistor values, wiring resistance |
| Continuity | Beep symbol | Checking for shorts and breaks |
| Diode | Triangle + bar symbol | Checking diode, LED, and transistor junctions |
| Current | A⎓ (mA, A) | Current consumption (use the dedicated terminal) |
| Capacitance (some) | ⊣⊢ | Capacitor value |
| Frequency (some) | Hz | Simple frequency |
Specs to look at when buying
- Auto-ranging: it sets the range automatically, which is convenient for beginners. Manual ranging has the advantage of a faster response.
- Display digits (counts): 2,000 counts or more is enough for general work, and 6,000 counts or more lets you see finer detail.
- Accuracy: DC voltage accuracy of around ±0.5% is enough.
- Input impedance: 10 MΩ is typical and has almost no effect on the circuit during measurement.
- True RMS: gives the right value when the AC is not a sine wave (PWM, switching supplies). It is not essential for DC-centered electronics hobby work.
- Fast continuity buzzer and backlight: very handy for checks after soldering.
- Current fuse: check that the current terminal is protected by a fuse.
- Probe quality: thin, sharp tips make it easy to touch perfboard pads. Detachable alligator clips are even more convenient.
Safety ratings (CAT)
Multimeters have a CAT rating (CAT II, III, IV) and a voltage (600 V, 1000 V) marked, indicating the measurement environment. The higher the number, the safer the design against large overvoltage accidents. If you only do low-voltage electronics like 5 V and 12 V, CAT II 600 V or higher is enough, but if you need to measure household mains you should use a CAT III or higher certified product, and beginners should not measure it themselves but leave it to a professional.
Basic measurement methods
Measuring DC voltage
- Plug the black probe into COM and the red probe into the V (voltage) terminal.
- Set the dial to V⎓.
- Touch the two points to measure in parallel. With the red probe on the + side and the black probe on GND, you get a positive reading.
- If you do not know the range, start from the largest range or use auto-ranging.
Continuity test
- Set the dial to continuity mode.
- First touch the two probes together to confirm it beeps.
- With power off, touch the two points to check, and if it beeps they are connected (or shorted).
For the order of checks, see It Is Finished but It Does Not Work: A Step-by-Step Checklist.
Measuring resistance
Measure with power off. When a part is soldered into a circuit, other parts affect the reading in parallel and the value reads low. To measure accurately, lift one leg. If you hold a probe with your fingers, your body's resistance comes in parallel and makes large resistances inaccurate.
Checking diodes and LEDs
In diode mode, put the red probe on the anode and the black probe on the cathode and the forward voltage (0.5 to 0.7 V, higher for an LED) appears; the other way round, you get OL (infinity). An LED may light faintly.
Measuring current
Measure current by breaking the circuit and connecting in series.
- Move the red probe to the mA or A terminal.
- Set the dial to A⎓.
- Turn the power off, cut the line to measure, and connect the multimeter in series in the gap.
- Turn the power on and read it.
Checks before measuring
- With a new multimeter, measure a battery (such as a 1.5 V cell) and check that the value is right.
- Check the probe leads for breaks by touching the two probes together in continuity mode.
- When the battery is low, readings can be wrong.
Common mistakes
| Mistake | Result | Prevention |
|---|---|---|
| Measuring voltage in current mode | Short, blown fuse | Check the voltage terminal and dial |
| Measuring resistance with power on | Wrong value or a damaged meter | Measure with power off |
| Measuring resistance in-circuit | Parallel paths make it read low | Lift one leg |
| Poor probe contact | The reading wobbles | Press perpendicular to the pad |
| Measuring a large voltage on a low range | Overload indication | Auto-range or start from a large range |
Summary
The basics are to measure voltage in parallel and current in series, and to do the continuity test with power off. For beginners, auto-ranging, a fuse-protected current terminal, and a fast continuity buzzer are enough. To see the shape of signals too, continue to Oscilloscope Basics, and for how to supply power safely, see Choosing a Bench Power Supply.