Multimeter Lab 3D

A multimeter is one tool that does the job of many meters. It measures voltage (V), current (A) and resistance (Ω), and most digital meters also test diodes, continuity and capacitance.

• Check the dial BEFORE the probes touch anything. The wrong mode can damage the meter or hurt you.
• Hold the probes behind the finger guards. Never touch the metal tips while measuring.
• Never measure resistance, diode or continuity on a circuit that is powered. Switch off and discharge capacitors first.
• For phones and boards: remove the battery before Ω, diode or continuity tests.
• When you don't know the voltage, start on the highest range (analog) and go down.
• After measuring current, move the red lead back to the VΩ jack. Otherwise the next voltage test makes a short circuit.
• Use a meter and probes rated for the job (CAT III 600 V or better for mains). Replace cracked probes.
• Turn the dial to OFF when you finish (analog: OFF or a high ACV range) to save the battery.

Mains (220–240 V) can kill. Students should measure mains only with an instructor present, using one hand and keeping the other hand away from metal.

• V = volt (voltage, 'electrical pressure'). A = ampere (current, the flow). Ω = ohm (resistance). F = farad (capacitance).
• OL means 'over limit': the value is too big for the range, or the circuit is open (no connection).
• A minus sign (−) on a DMM means the probes are the other way round. The number is still correct.
• Parts are often marked like 4k7 = 4.7 kΩ, 2R2 = 2.2 Ω, 4n7 = 4.7 nF. The letter shows where the decimal point is.

DC voltage (V⎓): batteries, power supplies, phone rails

1. Black lead in COM, red lead in VΩ. Dial on V⎓ (analog: a DCV range above the expected voltage).
2. Connect in PARALLEL: touch the two points across the part (red to the + side, black to − or ground).
3. Read the value. On a DMM a minus sign means reversed probes.

AC voltage (V~): mains outlets, transformer outputs

1. Dial on V~ (analog: ACV 250 for 220–240 V).
2. Touch the two points. Polarity doesn't matter for AC.
3. Never switch the dial while the probes are on mains.

Resistance (Ω)

1. Power OFF. Take the part out (or lift one leg) for a true reading.
2. Dial on Ω (analog: pick the range that puts the needle near the middle, then zero it).
3. Touch the two leads. OL means open or too high for the range.

Continuity (·))) beep): wires, fuses, switches, PCB tracks

1. Power OFF. Dial on →|·))).
2. Touch both ends. A beep means connected (below about 50 Ω).
3. No beep / OL means the path is broken.

Diode test (→|): diodes, LEDs, transistors, phone lines

1. Power OFF. Dial on →|.
2. Red on the anode, black on the cathode → shows the forward voltage (Si about 0.5–0.7 V, LED about 1.8–3 V).
3. Swap the probes → OL. A beep both ways means shorted.

Capacitance (⊣⊢)

1. Discharge the capacitor and remove it from the circuit.
2. Dial on ⊣⊢. Red on +, black on −.
3. Wait for the value to settle and compare it with the marking.

Current (A): only in SERIES

1. Switch the circuit off. Break the circuit at one point.
2. Move the red lead to the mA or 10A jack (use 10A when unsure). Dial on A.
3. Connect the meter IN SERIES so all the current flows through it, then switch on.
4. Afterwards move the red lead back to VΩ!

Never put the meter on the A jack across a battery or outlet. That is a dead short and blows the meter's fuse (or worse).

1. Choose the range: for volts, the lowest range ABOVE the expected value. For ohms, the range that puts the needle near the middle.
2. On Ω: touch the probes together and turn 0Ω ADJ until the needle sits exactly on 0. Do it again after every range change.
3. Look straight at the needle: line it up with its reflection in the mirror strip (this removes parallax error).
4. Read the scale that matches the range, then multiply or divide if needed.

The Ω scale runs from right (0) to left (∞) and is squeezed on the left. Readings near the middle are the most accurate.

On Ω the analog meter's BLACK probe is connected to the + of its battery. Remember this when testing diodes, LEDs, transistors and electrolytic capacitors.

Resistor colour code

Example: yellow-violet-red-gold = 4, 7, ×100 = 4 700 Ω = 4.7 kΩ ±5%. Memory aid: Black Brown Red Orange Yellow Green Blue Violet Grey White = 0…9.

Capacitor codes

• 3-digit code in pF: first two digits + number of zeros. 104 = 10 + 0000 = 100 000 pF = 100 nF = 0.1 µF. 103 = 10 nF. 472 = 4.7 nF.
• Electrolytics print the value directly: 100 µF 25 V. Never use one above its voltage.
• The stripe with '−' marks the negative leg. The longer leg is +.

SMD resistor codes

• 3 digits: 103 = 10 + 000 = 10 kΩ. 4R7 = 4.7 Ω (R = decimal point). 000 or 0 = 0 Ω jumper.
• 4 digits: 1002 = 100 + 00 = 10 kΩ. 4701 = 4.7 kΩ.
• Tiny phone resistors have no marking: measure them out of circuit or check the schematic.

Polarity and pin markings

• Diode: the band is the cathode (K).
• LED: long leg = anode (+), short leg / flat side = cathode (−).
• BC547 / BC557: flat face towards you, legs down → C B E.
• Phone battery connector: usually + , NTC/BSI, − (check the marking or schematic).

Most phone faults (no power, not charging, no sound, no vibration) can be narrowed down with a multimeter before you replace parts.

Before you measure

• Remove or unplug the battery before Ω, diode or continuity tests. Measuring a powered board can damage it and gives wrong readings.
• Wear an anti-static (ESD) wrist strap and work on an ESD mat.
• Use sharp probe tips (or needle tips). Don't bridge two tiny pads with one probe: that can short them.
• Ground (GND) is any shield can, screw hole or the battery − pad.

Step 1: battery and battery connector

1. Look for swelling. A swollen battery is bad, so replace it.
2. V⎓ across + and −: 3.8–4.4 V is charged, below about 3.4 V is low.
3. Ω from NTC/BSI to −: about 10 kΩ (sometimes 47 kΩ). OL can cause 'not charging'.
4. On the board connector, check that the battery + and GND pads are not shorted (diode mode, no beep).

Step 2: charging problems

1. Check the charger and cable with a USB meter first (cheap and quick).
2. Battery out, diode mode, RED on GND, BLACK on VBUS, D+, D− (and CC on USB-C). Write down each value.
3. Compare with the same model's good board. A beep on VBUS = short (often a capacitor or the charging IC). OL = broken line or port.

Step 3: the diode-mode 'red on ground' method

Every line on a phone board shows a typical diode value to ground (often 0.2–0.8 V), coming from the protection diodes inside the chips. Put RED on GND and BLACK on a test point or capacitor. A reading far from a good board's value points to the faulty area.

Make your own reference book: measure good boards of common models and write down the values for VBUS, VBAT, VPH_PWR and the 1.8 V rail.

Step 4: finding a short on a power rail

1. Diode mode on the rail gives 0.000 and a beep: the rail is shorted to ground.
2. Every capacitor on that rail beeps too, so the meter alone can't tell which one is bad.
3. With your instructor: inject a low voltage with a current-limited power supply (e.g. 1 V, 1 A) and find the part that heats up (thermal camera, freeze spray or rosin smoke).
4. Or lift the capacitors one at a time and measure again until the short disappears.

Step 5: speaker, earpiece, vibrator and buttons

• Loudspeaker: about 6–9 Ω (8 Ω type). Earpiece: about 16 or 32 Ω. OL = open coil.
• Vibration motor: about 15–45 Ω. It should buzz on a 1.5 V AA cell.
• Buttons: continuity mode: OL when released, beep when pressed.
• SMD coils and fuses: continuity: they must beep. OL = open.

• Measuring voltage with the red lead still in the 10A jack: this makes a short circuit and blows the fuse.
• Measuring resistance with the power on: wrong readings and a damaged meter.
• Measuring a resistor in circuit: parallel parts make it read LOW.
• Holding both probe tips with your fingers: your body adds resistance in parallel.
• Forgetting that the analog meter's black probe is + on Ω.
• Not zeroing the analog meter after changing the Ω range.
• Reading the wrong scale on an analog meter (for example 0–250 on the 10 V range).
• Testing a capacitor without discharging it first.
• Leaving the meter ON: the battery runs flat.