You test a Bipolar Junction Transistor (BJT) with a multimeter by treating it as two back-to-back diodes using the Diode Test mode. A good silicon NPN transistor will show a forward voltage drop between 0.500V and 0.750V across the Base-Emitter and Base-Collector junctions, and read "OL" (Over Limit / Open) in reverse bias and across the Collector-Emitter path. If you see 0.000V, the junction is shorted; if you see OL in forward bias, it is open.

This guide covers the exact bench procedure for verifying NPN and PNP BJTs, identifying misleading readings caused by parallel circuit paths, and selecting the right replacement part when a transistor fails.

Meter Setup and Safety Category Requirements

Before touching the component, configure your multimeter correctly. The standard resistance (Ohms) mode applies a variable test voltage that can yield inconsistent junction readings. You must use Diode Test mode, which sources a constant current (usually 1mA to 2mA) and measures the resulting forward voltage drop.

Meter Configuration Block

  • Dial Position: Diode Test (symbol: an arrow pointing into a perpendicular line, often sharing a dial position with continuity).
  • Lead Jacks: Black lead to COM, Red lead to V/Ω (or V/Ω/Diode).
  • Range: Auto-ranging (default). Expected display values are 0.200 to 0.800.
  • Open Lead Display: "OL" or "1" (depending on the meter brand, indicating infinite resistance).
Safety Category (CAT) Warning: If you are testing a transistor on a low-voltage DC breadboard or out-of-circuit, a CAT II rated meter is sufficient. However, if you are probing inside a mains-powered appliance, power supply, or motor drive, you MUST use a CAT III or CAT IV rated meter and probes. Never test a transistor in-circuit on a live mains board. De-energize the circuit, lock out the breaker, and bleed large filter capacitors before probing.

Identifying Pinout and Transistor Chemistry

The diode-test method relies on knowing whether your transistor is NPN or PNP, and whether it is Silicon or Germanium. Most modern transistors are Silicon, but vintage audio gear and specific RF circuits may use Germanium.

  • Silicon BJTs: Forward voltage drop reads between 0.500V and 0.750V.
  • Germanium BJTs: Forward voltage drop reads between 0.200V and 0.300V.

If you have an unmarked transistor and need to find the Base pin: The Base is the only pin that will show a forward voltage drop to both of the other two pins when the red probe is on it (for NPN) or when the black probe is on it (for PNP). According to All About Circuits, the BJT acts as two PN junctions sharing the Base region, making this identification method universally reliable for discrete BJTs.

Step-by-Step Probe Placement and Expected Readings

The following procedure assumes a standard Silicon NPN transistor (like the ubiquitous 2N2222 or 2N3904) with a known pinout. For a PNP transistor, simply reverse the probe polarities (Black to Base, Red to Emitter/Collector).

  1. Test Base to Emitter (Forward): Place the Red probe on the Base, Black probe on the Emitter. Expected: 0.500V - 0.750V.
  2. Test Base to Emitter (Reverse): Place the Black probe on the Base, Red probe on the Emitter. Expected: OL.
  3. Test Base to Collector (Forward): Place the Red probe on the Base, Black probe on the Collector. Expected: 0.500V - 0.750V.
  4. Test Base to Collector (Reverse): Place the Black probe on the Base, Red probe on the Collector. Expected: OL.
  5. Test Collector to Emitter (Both directions): Place probes on C and E, then swap. Expected: OL in both directions.
Expected Reading Matrix (Silicon NPN Transistor)
Red Probe (+) Black Probe (-) Good Reading Shorted (Bad) Open (Bad)
Base Emitter 0.500V - 0.750V 0.000V - 0.100V OL
Emitter Base OL 0.000V - 0.100V OL
Base Collector 0.500V - 0.750V 0.000V - 0.100V OL
Collector Base OL 0.000V - 0.100V OL
Collector Emitter OL 0.000V - 0.100V OL
Emitter Collector OL 0.000V - 0.100V OL
Pro-Tip for Darlington Transistors: If you are testing a Darlington pair (like the TIP120), the Base-Emitter forward voltage will be much higher—typically 1.0V to 1.5V—because the meter is forward-biasing two series-connected silicon junctions simultaneously. Do not discard a Darlington thinking it is faulty just because it reads 1.2V instead of 0.6V.

Decision Tree: Pass, Fail, or Misleading Readings

Use this decision path to interpret your meter readings and determine your next action.

Observation Diagnosis Action Required
B-E and B-C read ~0.6V forward; OL reverse. C-E reads OL both ways. Pass: Transistor junctions are intact. Reinstall or keep in parts bin. (Note: This does not test high-voltage leakage or Beta/hFE gain).
Any junction reads 0.000V to 0.100V in both directions. Fail: Junction is shorted internally. Desolder and discard. Check associated base resistor for damage.
B-E or B-C reads OL in the forward direction. Fail: Junction is blown open (usually from overcurrent). Desolder and discard. Investigate collector load for shorts.
C-E reads a low voltage drop (e.g., 0.400V) instead of OL. Fail: Collector-Emitter punch-through. Discard. The silicon die has thermally failed.
Readings are erratic, or show ~0.3V when testing in-circuit. Misleading: Parallel PCB traces or bleed resistors are altering the meter's test current. You must remove the transistor from the circuit (or lift at least the Base and Collector legs) to get a valid reading.

Common Mistakes That Give Misleading Readings

  • Testing In-Circuit: A 10kΩ pull-down resistor on the base will provide an alternate path for the multimeter's test current, yielding a false low reading. Always test out-of-circuit.
  • Touching the Metal Tips: If you hold the metal probe tips with your bare fingers while testing a reverse-bias junction, your body's resistance (roughly 50kΩ to 100kΩ) will parallel the junction. The meter might read 0.400V instead of OL, making you think the transistor is leaking. Hold only the insulated probe handles.
  • Confusing MOSFETs for BJTs: This diode-test method is strictly for BJTs. If you are holding a MOSFET (like an IRF520), the Gate will read OL to all pins, and the Drain-Source will read as a body diode (approx 0.5V one way, OL the other). For MOSFET testing procedures, consult the manufacturer's specific gate-charge testing guidelines.

When to Replace: Concrete Part Swaps and Microcontroller Upgrades

If your transistor fails the decision tree above, do not waste time trying to salvage it. Thermal runaway and internal die cracking are progressive; a "borderline" transistor will fail again under load.

When selecting a replacement, match the package (TO-92, TO-220), the polarity (NPN/PNP), and verify the $V_{CEO}$ (Collector-Emitter breakdown voltage) and $I_C$ (max continuous collector current) on the manufacturer datasheet. As noted by Fluke's testing guidelines, ensuring the replacement meets or exceeds the original power dissipation ($P_D$) rating is critical to prevent immediate repeat failures.

Default Bench Replacements (The Concrete Picks)

Stop guessing which part to order. Use these exact part numbers for 95% of general-purpose bench repairs and DIY builds:

  • For low-power signal switching (< 200mA, TO-92): Use the 2N3904 (NPN) or 2N3906 (PNP). They are cheap, widely available, and have excellent hFE consistency.
  • For medium-power switching (< 600mA, TO-92/TO-18): Use the PN2222A (NPN) or PN2907A (PNP).
  • For linear audio amplification: Use the 2N3904 but bin-select for higher hFE, or step up to the BC547 (NPN) / BC557 (PNP) which offer lower noise figures.
  • For power loads (1A - 5A, TO-220): Use the TIP31C (NPN) or TIP32C (PNP). Ensure you use a silicone thermal pad and mount to a heatsink if dissipating >1W.
The Microcontroller Upgrade Path: If you are replacing a BJT that is being driven directly by an Arduino, ESP32, or Raspberry Pi GPIO pin, stop using BJTs. BJTs are current-controlled and require base resistors that waste power and stress the GPIO pin. Replace the dead BJT with a Logic-Level N-Channel MOSFET like the IRLZ44N or FQP30N06L. These are voltage-controlled, draw virtually zero steady-state current from your 3.3V/5V microcontroller pin, and handle vastly higher loads without a heatsink.