To check a transistor with a multimeter, set your meter to Diode Test mode. For a standard silicon NPN Bipolar Junction Transistor (BJT), place the red probe on the Base and the black probe on the Emitter; a good transistor reads between 0.500V and 0.700V. Reverse the probes, and it should read OL (Open Loop). If it reads 0.000V (short) or OL in both directions (open), the junction is dead.
Testing transistors on the bench is a fundamental diagnostic skill. Whether you are salvaging parts from a blown power supply or verifying a batch of TO-220 power transistors before soldering them into an amplifier, relying on the multimeter's internal test voltage is the fastest way to separate good silicon from e-waste. Below is the exact bench procedure for both BJTs and MOSFETs.
Meter Setup & Safety Categories
Before touching any probes to silicon, configure your digital multimeter (DMM) correctly. Using the wrong mode is the most common reason for false failures.
- Dial Position: Diode Test (symbol: →| ). Do not use Continuity or Resistance (Ω). Continuity mode only outputs ~1V, which is not enough to forward-bias a silicon junction or charge a MOSFET gate.
- Lead Jacks: Black probe to COM, Red probe to V/Ω/Diode.
- Range: Auto-ranging is standard. The meter will display the forward voltage drop in volts (e.g., 0.650V).
How to Check a BJT (Bipolar Junction Transistor)
A BJT (like the common 2N2222, 2N3904, or TIP31) is essentially two diodes sharing a common anode (NPN) or cathode (PNP). We test it by checking the Base-Emitter (B-E) and Base-Collector (B-C) junctions.
Testing an NPN Transistor
- Forward Bias B-E: Red probe on Base, Black probe on Emitter. Note the reading.
- Forward Bias B-C: Red probe on Base, Black probe on Collector. Note the reading.
- Reverse Bias B-E: Black probe on Base, Red probe on Emitter.
- Reverse Bias B-C: Black probe on Base, Red probe on Collector.
- Collector-Emitter Check: Probe C to E in both directions. Both should read OL.
Testing a PNP Transistor
Reverse the probe colors. Black probe on Base, Red probe on Emitter/Collector for forward bias. Red on Base, Black on Emitter/Collector for reverse bias.
Expected Reading Table: BJT
| Junction Tested | Probe Configuration (NPN) | Good Reading (Silicon) | Bad Reading (Fail State) |
|---|---|---|---|
| Base to Emitter | Red on B, Black on E | 0.500V - 0.750V | OL (Open) or 0.000V (Short) |
| Base to Emitter | Black on B, Red on E | OL (Open Loop) | Any numeric voltage drop |
| Base to Collector | Red on B, Black on C | 0.500V - 0.750V | OL (Open) or 0.000V (Short) |
| Base to Collector | Black on B, Red on C | OL (Open Loop) | Any numeric voltage drop |
| Collector to Emitter | Either direction | OL (Open Loop) | Any numeric voltage drop (Shorted) |
Note: Germanium transistors (rare, mostly vintage audio) will show a lower forward drop of 0.200V to 0.300V. For deeper theory on junction behavior, see the Electronics Tutorials BJT guide.
How to Check a MOSFET (Metal-Oxide-Semiconductor FET)
MOSFETs (like the IRF540N or IRLZ44N) do not behave like two diodes. They have an insulated Gate and an intrinsic body diode between the Drain and Source. Testing requires a two-part process: checking the body diode and performing the "Gate Charge Trick" to verify the channel can actually switch on.
Testing an N-Channel MOSFET
- Discharge the Gate: Briefly touch the Black probe to the Gate and the Red probe to the Source. This bleeds off any residual gate capacitance.
- Body Diode Check (Forward): Red probe on Source, Black probe on Drain. You should read the body diode drop (typically 0.400V to 0.650V).
- Body Diode Check (Reverse): Red probe on Drain, Black probe on Source. Should read OL.
- The Gate Charge Trick (Turn ON): Keep the Black probe on the Source. Touch the Red probe to the Gate for 2 seconds. The meter's internal ~3V to 9V battery will charge the gate capacitance, turning the MOSFET channel ON.
- Verify Channel ON: Move the Red probe to the Drain (Black remains on Source). The reading should drop to near 0.000V (or a very low millivolt drop), indicating the channel is conducting.
- Discharge to Turn OFF: Touch Black to Gate and Red to Source again. Re-test Drain to Source; it should return to OL.
Expected Reading Table: N-Channel MOSFET
| Test Step | Probe Placement | Good Reading | Bad Reading |
|---|---|---|---|
| Body Diode (Forward) | Red on Source, Black on Drain | 0.400V - 0.650V | OL or 0.000V |
| Body Diode (Reverse) | Red on Drain, Black on Source | OL | Any numeric voltage |
| Gate to Source/Drain | Either direction | OL (Infinite resistance) | Any numeric voltage (Gate oxide punctured) |
| Channel ON (Post-Charge) | Red on Drain, Black on Source | 0.000V - 0.050V | OL (Channel failed to open) |
Common Mistakes That Give Misleading Readings
Even with the right dial setting, bench environment and technique can skew your results. Watch out for these specific failure modes:
- Testing In-Circuit Without Isolation: If you test a transistor while it is still soldered to the PCB, parallel copper traces, bleed resistors, and transformer windings will create alternate current paths. A perfectly good BJT might read 0.200V or show continuity across C-E because of a parallel 100Ω base bleed resistor. Always desolder at least the Base/Gate pin to isolate the component.
- Finger Resistance Interference: If you hold the metal probe tips and the transistor leads simultaneously with your bare fingers, your skin resistance (typically 50kΩ to 500kΩ) will parallel the junction. In reverse-bias tests, this can cause the meter to display a phantom voltage drop instead of OL. Hold the plastic probe barrels or use alligator clip leads.
- Using Continuity Mode for MOSFETs: The continuity setting outputs roughly 1V to 1.5V. This is below the threshold voltage (Vgs_th) of most standard MOSFETs (which require 2V to 4V to begin conducting). The Gate Charge Trick will fail, and you will falsely conclude the MOSFET channel is dead.
- Ignoring the Meter's Battery: If your DMM's 9V battery is low, the open-circuit voltage in Diode Test mode drops. It might output only 1.8V instead of 3V, failing to forward-bias high-current power diodes or charge logic-level MOSFET gates adequately. If known-good transistors read OL, change the DMM battery.
Frequently Asked Questions
How to check a transistor with a multimeter without removing it from the circuit?
You can perform a preliminary "go/no-go" check in-circuit, but you cannot definitively prove a transistor is good. Set the meter to Diode Test and measure the Base-Emitter and Base-Collector junctions. If you read 0.000V (a dead short) in either direction, the transistor is almost certainly blown and must be replaced. However, if you get a normal 0.6V reading, it might still be bad (e.g., an internal open circuit or a leaky Collector-Emitter junction masked by parallel PCB traces). For a guaranteed diagnosis, desolder the component.
Can I use the hFE setting on my multimeter to test a transistor?
The hFE (DC Current Gain) socket on many cheap multimeters is useful for binning matched pairs of small-signal BJTs (like sorting 2N3904s by gain), but it is terrible for fault-finding. It only tests low-power signal transistors that fit the tiny ZIF socket, and it cannot test MOSFETs or TO-220 power BJTs. Furthermore, a transistor with a shorted Collector-Emitter junction might still show a phantom hFE reading. Stick to Diode Test mode for pass/fail diagnostics.
Why does my multimeter show a short circuit across all three transistor pins?
If your meter reads 0.000V (or beeps continuously on continuity mode) between Base-Emitter, Base-Collector, and Collector-Emitter, the transistor has suffered a catastrophic thermal or over-voltage failure. Internally, the silicon die has melted or arc-flashed, welding the junctions together into a single lump of conductive material. This is incredibly common in horizontal deflection output transistors in older CRT monitors and in switching MOSFETs in failed motor controllers. The part is destroyed and must be discarded.
