A diode lets current flow one way and blocks it the other. That one property makes it easy to test: a healthy diode shows a voltage drop in one direction and an open circuit in the other. On a laptop board you will meet several kinds — ordinary silicon diodes, Schottky diodes in power paths, Zener and TVS diodes for protection, ESD arrays on connectors and LEDs on the edges — and each gives a slightly different reading. This guide explains what each one should show and how to avoid misreading a good part as bad.
How diode mode works
In diode mode the meter pushes a small, fixed current through the probes and displays the voltage needed to make it flow. With the red probe on the anode and the black on the cathode, a silicon diode conducts and the meter shows its forward drop. Swap the probes and the diode blocks; the meter reaches its maximum test voltage and shows OL.
The cathode is the end marked with a bar on the part and on the schematic symbol. On tiny packages the bar may be a faint line or a notch, so confirm it in the boardview or on the footprint.
A full explanation of diode mode, including how it is used on whole rails rather than single parts, is in Diode Mode Measurements Explained.
Typical readings by diode type
| Diode type | Forward reading (red on anode) | Reverse reading | Notes |
|---|---|---|---|
| Silicon rectifier or switching diode | Roughly 0.5–0.7 V | OL | The classic reading |
| Schottky diode | Roughly 0.15–0.4 V | OL, sometimes a high value | Lower drop; may leak slightly in reverse |
| Zener diode | Like a silicon diode | OL if its Zener voltage is above the meter's test voltage | Breakdown voltage needs a separate test |
| Unidirectional TVS diode | Like a silicon diode | OL | Behaves like a Zener |
| Bidirectional TVS diode | OL | OL | Both directions blocked at meter voltage |
| ESD array line to ground | A diode-like drop, often 0.4–0.8 V | OL or high | Compare similar lines |
| LED | Higher drop, about 1.5–3 V depending on colour, or OL | OL | May glow faintly |
The exact numbers depend on the part, the temperature and the meter's test current. What matters most is the pattern: one direction conducts, the other blocks.
Testing a diode out of circuit
This is the reference test, and the only one that gives a clean answer:
- Set the meter to diode mode.
- Red on the anode, black on the cathode — note the reading.
- Swap the probes — the reading should be OL.
- Compare the forward reading with the table above.
Results and what they mean:
- Drop one way, OL the other — good.
- Near zero both ways — shorted.
- OL both ways (for a diode that should conduct) — open.
- A drop one way and a lower-than-expected reading the other way — leaky. Schottky diodes leak a little by nature, but a significant reverse reading in an ordinary diode means damage.
Testing a diode in circuit
On a board, the diode shares its nets with other parts, and those parts can change the reading:
- A forward reading close to the expected value and OL or a high value in reverse usually means the diode is fine.
- A lower reading in reverse does not prove a fault — a resistor or another junction may be in parallel.
- A reading near zero in both directions is suspicious, but check whether the two nets are connected by something else, such as a very low-resistance rail or an inductor.
The boardview helps here. Click each end of the diode and see which parts share the net. If the diode bridges two rails with many parts on them, an in-circuit reading tells you little; lift one end to be sure.
Schottky diodes in power paths
Schottky diodes appear where a low voltage drop matters: in older power paths, in the RTC battery circuit, and in some bootstrap and protection circuits. Their forward drop is lower than a silicon diode's, so a reading of around 0.2–0.3 V is normal and not a sign of a short.
They also leak more in reverse, especially large ones and especially when warm. A Schottky diode showing a high but not OL reading in reverse is usually fine. One that reads close to its forward value in both directions is shorted. Background on this type is in the glossary entry for schottky diode.
Zener diodes
A Zener diode conducts forward like any diode, and in reverse it conducts once the voltage reaches its rated Zener voltage. A multimeter's diode mode tests only up to its own open-circuit voltage, often around 2–3 V. So:
- A Zener rated above that voltage reads OL in reverse — this is normal.
- A low-voltage Zener may show a reading in reverse that is close to its Zener voltage.
To confirm the actual Zener voltage, use a bench supply with a series resistor to limit the current and measure the voltage across the diode. For in-circuit checks, the most useful test is simply whether the diode is shorted.
TVS diodes and ESD arrays
Transient voltage suppressors protect connectors and power inputs from spikes. They sit between a line and ground and stay idle until a surge pushes the voltage above their breakdown level, then they conduct hard to clamp it.
They are among the most commonly damaged parts on laptop boards, because they take the hit when something goes wrong: a faulty charger, a static discharge on a USB port, liquid bridging pins.
- A unidirectional TVS reads like a Zener: a drop forward, OL in reverse.
- A bidirectional TVS reads OL in both directions at meter voltage.
- A shorted TVS reads near zero in both directions and pulls its line or rail to ground.
On the power input, a shorted TVS diode is a classic cause of a charger that shuts off as soon as it is plugged in. The glossary entry for tvs diode explains the clamping behaviour.
ESD arrays are small multi-pin chips that protect several data lines at once, for example the lines of a USB or HDMI port. Measure each protected line to ground in diode mode and compare lines that should be similar — D+ with D−, one HDMI pair with another. A line that reads very differently, or near zero, points at a damaged array or a damaged chip behind it.
LEDs
An LED's forward drop is higher than a silicon diode's, and it depends on colour: red and orange LEDs have lower drops, blue and white LEDs higher ones. Many meters' diode modes cannot reach the drop of a white or blue LED and show OL in both directions even for a good part. If your meter makes a red LED glow faintly in diode mode, you know the LED works.
For a reliable test of any LED, use a bench supply at a low voltage with a series resistor and watch whether it lights.
How diodes fail
- Short — the most common failure after a surge or overheating. Often visible as a cracked or discoloured body.
- Open — after a large overcurrent that burned the junction or bonding wire.
- Leaky — partial damage, showing a reverse reading that should not be there; heat makes it worse.
- Corroded terminals — after liquid damage, the diode may be fine but its joints are not.
Example: a charger that cuts out instantly
A laptop's charger LED goes out the moment the plug touches the DC jack. With the board unpowered, the adapter input rail reads almost zero ohms to ground. The schematic shows a TVS diode right after the DC jack. Lifting one end of the TVS makes the input rail read normally, and the TVS on its own reads near zero in both directions. It is shorted. The board works with a new TVS — and the customer is told to stop using the cheap replacement charger that probably caused the spike. The full troubleshooting sequence for this symptom is covered in Diodes and TVS Diodes on Laptop Boards.
Replacing a diode
Match the type, the voltage rating, the current rating and the package. For Schottky diodes, a different part with a higher forward drop may cause heat or voltage loss. For TVS diodes, the breakdown voltage must match: a lower rating will clamp the normal supply voltage and fail again, and a much higher one will not protect the circuit. Always observe polarity — the bar marks the cathode, both on the part and on the footprint.
Transistors contain diode junctions too, so the same diode-mode technique extends to them, as described in How to Test a Transistor with a Multimeter: BJT and MOSFET.
FAQ
Why does my diode read OL in both directions?
The diode may be open, or its forward drop is higher than the meter's diode range — common with LEDs and bidirectional TVS diodes. Check the type before calling it faulty.
What reading means a diode is shorted?
A reading close to zero in both directions, measured out of circuit. In circuit, confirm by lifting one end, because other parts can create a low-resistance path.
Can I test a diode with resistance mode instead of diode mode?
Roughly, but the readings are less meaningful because the test current is different and changes with the range. Diode mode gives a standard, comparable number.
Is a slightly lower forward drop a problem?
Not usually. Forward drop varies between parts, with temperature and with the meter's test current. Compare with an identical diode if one is available.
Why do TVS diodes fail so often?
Their job is to absorb surges. A large or repeated surge can exceed their limit and leave them shorted, which protects the rest of the board but takes down the rail they guard.
Bottom line
A good diode conducts one way and blocks the other. Know the expected drop for each type, test out of circuit when an in-circuit reading is ambiguous, and treat a near-zero reading in both directions as a short. Pay extra attention to TVS diodes and ESD arrays near connectors — they are the parts most often sacrificed to protect the board.