To test a bipolar junction transistor (BJT) using a multimeter, set your meter to the diode test mode and measure the forward voltage drop across the base-emitter and base-collector junctions. A healthy silicon NPN transistor will read between 0.500V and 0.800V when forward-biased, and display 'OL' (open loop) when reverse-biased. While many modern multimeters feature a dedicated hFE socket, the diode test method remains the most reliable way to identify pinouts and detect internal shorts or opens without relying on the meter's low-voltage hFE bias current.
Meter Setup and Safety Category Requirements
Before probing any semiconductor, you must configure your multimeter correctly to source the small constant current (typically 1mA to 2mA) required to bias the PN junctions.
Meter Configuration Block
- Dial Position: Diode Test (symbol: an arrow pointing into a line,
->|). Do not use the standard Ohms (Ω) range, as the test voltage on resistance ranges is often too low (<0.3V) to forward-bias a silicon junction. - Lead Jacks: Black lead to
COM, Red lead toV/Ω. (Note: The red lead is positive and sources current in diode mode on 99% of modern DMMs). - Range: Auto-ranging is standard. If using a manual meter, select the dedicated diode range, not a low-voltage DC mV setting.
Step-by-Step Probe Placement for BJT Transistors
A BJT acts as two back-to-back diodes sharing a common anode (NPN) or cathode (PNP). The Base is the common terminal. To test it, you must systematically check all six possible probe combinations across the three pins.
Pinout Note: For a standard TO-92 package (like the common 2N3904 NPN or 2N3906 PNP), hold the flat side facing you with the pins pointing down. The pins from left to right are Emitter, Base, Collector. For the European BC547/BC557 series, the order is Collector, Base, Emitter.
Testing an NPN Transistor (e.g., 2N3904, 2N2222)
- Base to Emitter (Forward): Place the Red probe on the Base, Black probe on the Emitter. Expect ~0.600V.
- Base to Collector (Forward): Place the Red probe on the Base, Black probe on the Collector. Expect ~0.600V (often slightly lower than B-E, around 0.550V).
- Reverse Checks: Swap the probes (Black on Base, Red on Emitter/Collector). Both must read 'OL'.
- Collector to Emitter: Place probes across C and E in both polarities. Both must read 'OL'.
Testing a PNP Transistor (e.g., 2N3906, BC557)
- Base to Emitter (Forward): Place the Black probe on the Base, Red probe on the Emitter. Expect ~0.600V.
- Base to Collector (Forward): Place the Black probe on the Base, Red probe on the Collector. Expect ~0.550V to 0.600V.
- Reverse Checks: Swap the probes (Red on Base, Black on Emitter/Collector). Both must read 'OL'.
- Collector to Emitter: Place probes across C and E in both polarities. Both must read 'OL'.
Expected Readings: Good vs. Bad Transistors
The table below defines the exact numerical thresholds for silicon BJTs. Germanium transistors (like the vintage AC128) will show much lower forward voltage drops, typically between 0.200V and 0.300V. For deep-dive theory on PN junction biasing, Electronics Tutorials provides excellent foundational material.
| Junction Tested | Probe Polarity (NPN) | Expected Good Reading (Si) | Bad Reading (Short) | Bad Reading (Open) |
|---|---|---|---|---|
| Base - Emitter | Red on Base, Black on Emitter | 0.550V - 0.800V | 0.000V - 0.100V | OL (both directions) |
| Base - Collector | Red on Base, Black on Collector | 0.500V - 0.750V | 0.000V - 0.100V | OL (both directions) |
| Collector - Emitter | Either direction | OL | 0.000V - 0.400V | N/A (OL is the only good state) |
Pro-Tip for Identifying Unknowns: If you don't know the pinout, find the single pin that shows a forward voltage drop (~0.6V) to both of the other two pins. That pin is the Base. If the Red probe was on the Base to get those readings, it's an NPN. If the Black probe was on the Base, it's a PNP.
Common Mistakes That Give Misleading Readings
Even with a high-quality meter, operator error can lead to false condemnations of good parts or passing bad ones.
- Testing In-Circuit: Never rely on a diode test while the transistor is soldered to a PCB. Parallel traces, bleeder resistors, and protection diodes will create alternate current paths, pulling your 0.600V reading down to 0.200V or lower, making a good transistor look shorted. Always desolder at least two pins (or remove the component entirely) before testing.
- Finger Resistance Interference: When holding a tiny TO-92 transistor, your skin's resistance (typically 50kΩ to 500kΩ) can parallel the junction. While this won't heavily skew a forward diode drop, it can cause reverse-bias readings to show a ghost voltage (e.g., 1.2V instead of 'OL') rather than a true open. Hold the plastic body, not the metal leads.
- Confusing MOSFETs with BJTs: A MOSFET (like the 2N7000 or IRF520) has an insulated gate. The diode test will read 'OL' between Gate-Source and Gate-Drain in both directions because there is no PN junction to bias. To test a power MOSFET, you check the internal body diode between the Source and Drain (reads ~0.5V one way, 'OL' the other). Applying standard BJT logic to a MOSFET will lead you to throw away perfectly good FETs.
- Ignoring the hFE Socket Limitations: The hFE socket on budget multimeters supplies very low base current. A power transistor (like a TIP31) might show an artificially low gain (e.g., hFE of 15) in the socket because it isn't receiving enough base current to fully turn on, leading you to falsely believe the part is degraded.
Frequently Asked Questions
Can I test a transistor using a multimeter without removing it from the circuit?
For a definitive pass/fail diagnosis, no. In-circuit testing is only useful for finding dead, catastrophic shorts. If your multimeter reads 0.000V across the Collector and Emitter while in-circuit, the transistor is almost certainly shorted and blown. However, if you get a low but non-zero reading (like 0.350V), you cannot tell if the transistor is leaking or if a parallel resistor on the PCB is pulling the voltage down. You must lift at least the Base and Collector leads from the solder pads to isolate the junctions for an accurate diode test.
What is the difference between testing a silicon and germanium transistor?
The testing procedure is identical, but the expected numerical values change drastically due to the semiconductor material's bandgap. Silicon transistors (the vast majority of modern parts like the 2N3904, BC547, or TIP120) have a forward junction voltage drop of roughly 0.550V to 0.800V. Germanium transistors (mostly found in vintage audio amplifiers and fuzz pedals, like the AC128 or 2N139) have a much lower forward voltage drop, typically reading between 0.150V and 0.300V on a multimeter's diode range. Furthermore, germanium parts are notoriously leaky; a reverse-bias reading of 1.5V instead of 'OL' is often normal for vintage germanium, whereas it would indicate a failing silicon part.
Why does my multimeter show 'OL' in both directions when testing a MOSFET?
If you are probing the Gate pin against the Source or Drain, 'OL' in both directions is the correct, expected reading. The Gate of a MOSFET is separated from the channel by a thin layer of silicon dioxide (SiO2), acting as a capacitor with near-infinite DC resistance. The diode test cannot forward-bias an insulator. To verify a MOSFET is functional, you must test the intrinsic body diode located between the Source and Drain. On an N-channel MOSFET, placing the Red probe on the Drain and Black on the Source should read 'OL', while swapping them (Red on Source, Black on Drain) should read the body diode drop of roughly 0.400V to 0.600V.
Does the hFE socket on my multimeter give a better result than the diode test?
No, they serve entirely different purposes. The diode test checks the structural health of the transistor's internal PN junctions (looking for shorts or opens). The hFE socket measures the DC current gain (Beta) by injecting a known base current and measuring the resulting collector current. You should always perform the diode test first. If the junctions are shorted, plugging the part into the hFE socket is pointless. If the junctions test good, the hFE socket can help you match pairs for audio amplifiers, but be aware that bench-tester hFE readings often differ from the gain measured under real-world operating voltages and currents on the actual PCB.
