Before measuring: in circuit or out?

Measuring a part is easy; measuring it correctly means following a few rules. A wrong measurement leads you to remove and throw away a good part, or leave a faulty one on the board. Each lesson in this course covers one type of part. First, let's learn the rules that apply to all of them.

📷 Real-world view

1. Is the power off?

For resistance, continuity, diode and capacitance measurements, the multimeter sends a small current through the part from its own battery and looks at the resulting voltage. If there's another power source in the circuit, the measurement is meaningless, the multimeter's fuse can blow or the meter can be damaged.

• Unplug it, and remove any battery or battery pack.
• Discharge large capacitors and confirm they're discharged by measuring on the DC voltage range.
• Voltage measurements are the opposite: they're made with the power on. Always know which kind of measurement you're making.

2. In circuit or out of circuit?

When you measure a part on a board, the multimeter sees not just that part but everything connected in parallel with it. That's why an in-circuit measurement often comes out lower than it should.

3. Comparison: the most powerful repair method

For many parts you won't know the "right" value. But if you have a working board from the same device, or the board has two copies of the same circuit (e.g. the left and right channels of a stereo amplifier), you can measure the same points and compare. The point that differs is near the fault.

4. Know your meter

• Battery: when the multimeter's battery gets weak, readings drift. If a battery symbol appears on the display, replace it.
• Probe resistance: the probes and leads also have about 0.1–0.5 Ω of resistance. Before measuring very small resistances, touch the probes together and subtract that reading from the result. Many meters have a REL (relative) button that does this automatically.
• Diode range: sends about 1 mA through the part and shows the voltage on the display. The unit is volts, not ohms.
• Safety category: for mains measurements use a meter marked at least CAT II 600 V or CAT III.

5. Look first

Before you start measuring, inspect the board under good light, with a magnifier if possible: burn marks, darkened resistors, bulging capacitors, cracked solder joints, liquid residue and corrosion often point straight at the fault. Use your nose too: a burnt smell can tell you where the fault is.

Common mistakes

• Measuring resistance or continuity with the power on.
• Leaving the probe in the A (current) input and measuring voltage: that's a short circuit and blows the multimeter's fuse.
• Assuming every low in-circuit reading means a faulty part.
• Holding both probes and the part with your fingers at the same time: with high resistances your body joins the measurement.

Test yourself