To perform a reliable transistor test, set your digital multimeter (DMM) to the diode test mode. A healthy silicon Bipolar Junction Transistor (BJT) will show a forward voltage drop between 0.500 V and 0.800 V across the base-emitter and base-collector junctions, and read "OL" (open loop) in reverse. If you see 0.000 V (a dead short) or OL in both directions on a junction, the semiconductor is destroyed. This guide provides the exact bench procedures, expected numerical values, and decision paths for testing both BJTs and MOSFETs without guessing.

Meter Setup and Safety Category (CAT) Requirements

Before probing any silicon, you must configure your meter correctly. Using the wrong dial setting is the most common reason hobbyists misdiagnose a good transistor as bad.

Meter Setup Block:
  • Dial Position: Diode Test (symbol: an arrow pointing into a line, ->|). Do not use the Continuity (beeper) or Ohms (Ω) ranges. The diode test applies a known constant current (usually 1 mA to 2 mA) and measures the resulting voltage drop, which is exactly what you need to evaluate a PN junction.
  • Lead Jacks: Black lead in COM, Red lead in V/Ω.
  • Range: Auto-ranging is standard for diode mode. If manual, set to the 2V or 3V DC range.
Safety Category (CAT) & Mains Warning: If you are testing transistors on a low-voltage DC bench supply or an Arduino shield, a CAT II rated meter is sufficient. However, if you are probing the primary-side switching transistors in an offline Switch-Mode Power Supply (SMPS) or a mains-connected motor drive, you must use a CAT III or CAT IV rated meter and probes. Furthermore, never test in-circuit on an SMPS without unplugging the mains and physically discharging the bulk DC filter capacitors with a high-wattage bleed resistor. A charged 400V bulk cap will instantly destroy your DMM and poses a lethal shock hazard.

The Bipolar Junction Transistor (BJT) Test Procedure

A BJT (like the ubiquitous 2N3904 NPN or 2N3906 PNP) is essentially two diodes sharing a common anode or cathode (the Base). We test it by forward-biasing and reverse-biasing these internal diodes.

Testing an NPN Transistor

  1. Base-Emitter Forward: Place the Red probe on the Base (B) and the Black probe on the Emitter (E). Note the reading.
  2. Base-Collector Forward: Keep the Red probe on the Base (B) and move the Black probe to the Collector (C). Note the reading.
  3. Reverse Bias Check: Swap the probes. Put Black on Base, and Red on Emitter, then Red on Collector. Both should read OL.
  4. Collector-Emitter Check: Place probes across C and E in both directions. Both must read OL.

Testing a PNP Transistor

The procedure is identical, but the probe polarities are reversed. Black goes on the Base for forward bias, and Red goes on the Emitter and Collector. Reverse bias requires Red on the Base.

Expected Readings: Good vs. Bad BJT Values

When testing standard silicon transistors, the numerical readout on your DMM tells the whole story. Germanium transistors (rare today, like the AC128) will read much lower, typically 0.200 V to 0.300 V.

Junction / Test Point Probe Orientation (NPN) Expected Good Reading (Silicon) Bad Reading (Indicates Failure)
Base to Emitter Red on B, Black on E 0.500 V – 0.800 V 0.000 V (Short) or OL (Open)
Base to Collector Red on B, Black on C 0.500 V – 0.800 V 0.000 V (Short) or OL (Open)
Reverse Bias (Both) Black on B, Red on E/C OL (Open Loop) Any numerical voltage drop
Collector to Emitter Red/Black on C/E (both ways) OL (Open Loop) Any numerical voltage drop

Expert Caveat for Darlington Pairs: If you are testing a Darlington transistor like the TIP120, the Base-Emitter forward voltage will read between 1.200 V and 1.400 V. This is normal because a Darlington contains two series-connected base-emitter junctions. Do not throw it away thinking it is shorted.

MOSFET Testing: A Different Decision Path

Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) do not test like BJTs. The Gate is insulated by a layer of silicon dioxide, meaning a DMM will read OL between the Gate and Source/Drain in both directions. Instead, we test the intrinsic body diode and use the meter's internal voltage to charge the gate capacitance.

Here is the definitive test for an N-Channel MOSFET (e.g., IRFZ44N):

  1. Discharge the Gate: Touch the Black probe to the Drain and the Red probe to the Gate. This shorts any residual gate charge.
  2. Check Body Diode (Reverse): Put Red on Drain, Black on Source. It should read OL.
  3. Check Body Diode (Forward): Put Black on Drain, Red on Source. You should read a diode drop of roughly 0.400 V to 0.600 V.
  4. Charge the Gate: Keep Black on Source, and touch Red to the Gate. The DMM's internal 3V-9V battery will charge the gate capacitance, turning the MOSFET on.
  5. Verify Conduction: Move the Red probe from the Gate to the Drain (Black stays on Source). The reading should drop to near 0.000 V (or a very low resistance), indicating the channel is fully enhanced and conducting.
  6. Discharge to Turn Off: Touch Red to Source and Black to Gate. Re-testing Drain-Source should now show OL again.

Common Mistakes That Give Misleading Readings

Even with the right dial setting, bench environment and technique can ruin your data. Avoid these three traps:

  • Testing In-Circuit Without Isolation: If you test a BJT while it is still soldered to a PCB, parallel traces (like a 1kΩ base pull-down resistor or a snubber diode across C-E) will create alternative current paths. Your DMM will read a confusing 0.200 V or fail to reach OL. Fix: Desolder at least the Base pin, or lift the component entirely before testing.
  • The "Finger Resistance" Error: When holding a small TO-92 transistor (like a 2N2222) between your thumb and forefinger while probing, the moisture and salt on your skin creates a parallel resistance path. This can drag a reverse-bias OL reading down to 1.500 V, making you think the junction is leaky. Fix: Clip the transistor into a small bench vise or use a transistor test socket adapter.
  • Using the Ohms Range: The Ohms range applies a variable voltage and measures current to calculate resistance. It does not provide enough forward voltage to turn on a silicon PN junction (which requires ~0.6V). You will read OL across a perfectly good base-emitter junction. Fix: Always use the Diode Test mode.

Decision Tree: When to Trash the Part and Buy a Replacement

Do not waste time trying to "recover" a suspect power semiconductor. If a transistor fails the diode test, it is e-waste. Use this decision path to determine your exact replacement strategy.

Symptom / DMM Reading Diagnosis Action & Concrete Replacement Pick
0.000 V across C-E or B-E Hard Short (Thermal Runaway or Overvoltage) Trash it. For general-purpose < 3A NPN bench replacements, stock and use the TIP31C. For PNP, use TIP32C.
OL in forward bias (Red on B, Black on E) Open Junction (Bond wire snapped or overcurrent) Trash it. Replace small signal NPN with 2N3904 or 2N2222. Replace PNP with 2N3906.
Forward bias reads 0.100 V - 0.300 V (Silicon BJT) Severe Junction Leakage (Heat degraded) Trash it. A leaky BJT will cause thermal drift. Replace with exact OEM part or verified upgrade.
MOSFET Body Diode reads 0.000 V Drain-Source Short (Avalanche breakdown) Trash it. For 12V/24V DC-DC buck converters, default to a logic-level IRLZ44N (if gate drive is 5V) or standard IRFZ44N (if gate drive is 10V+).
MOSFET Gate reads 0.400 V to Source/Drain Gate Oxide Puncture (ESD damage) Trash it. Replace and add a 10kΩ gate-to-source pulldown resistor and a 12V Zener diode to protect the new part.

For deeper theoretical background on semiconductor junction behavior during these tests, refer to the bipolar transistor tutorials at Electronics-Tutorials or the official Fluke guide on testing transistors. Keep your DMM in diode mode, isolate the part from the circuit, and trust the numerical voltage drop over a simple continuity beep.