The Direct Answer: Meter Setup and Safety Categories
To test a transistor, you do not measure resistance; you measure the forward voltage drop of its internal semiconductor junctions. A good silicon Bipolar Junction Transistor (BJT) will show a forward voltage drop between 0.600V and 0.750V across its base-emitter and base-collector junctions, and read 'OL' (Over Limit) in reverse bias. A good MOSFET will show a 0.400V to 0.600V drop across its intrinsic body diode.
Meter Setup Block
- Dial Position: Set to Diode Test mode (the symbol looks like an arrow pointing into a perpendicular line: ➔| ). Do not use the Ohms (Ω) setting, as the test voltage is often too low to forward-bias a silicon junction.
- Lead Jacks: Black lead into COM. Red lead into V/Ω/mA (or the dedicated diode/jack on your specific meter).
- Range: Auto-ranging is standard for diode mode. The meter will output a small DC voltage (typically 3V to 9V open-circuit) and display the voltage dropped across the junction in millivolts or volts.
Step-by-Step: Testing Bipolar Junction Transistors (BJTs)
A BJT (like the common 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 junctions.
Testing an NPN Transistor
- Base to Emitter (Forward): Place the Red probe on the Base and the Black probe on the Emitter. Expected reading: 0.600V to 0.750V.
- Base to Collector (Forward): Keep Red on Base, move Black to Collector. Expected reading: 0.600V to 0.750V (often slightly lower than the B-E reading).
- Reverse Bias Check: Swap probes. Black on Base, Red on Emitter, then Red on Collector. Expected reading: 'OL' (Over Limit) for both.
- Collector to Emitter: Place probes across C and E in both directions. Expected reading: 'OL' both ways. (If it reads near 0.00V, the transistor is shorted internally).
Testing a PNP Transistor
For a PNP transistor, the internal diodes are flipped. Simply reverse the probe polarity from the NPN steps above: Black on Base, Red on Emitter/Collector for the forward-bias readings (0.6V-0.75V), and 'OL' when reversed.
| Test Points | Probe Polarity (Red/Black) | Good Reading (Silicon) | Bad Reading (Replace Part) |
|---|---|---|---|
| Base to Emitter | Red on Base (NPN) | 0.600V - 0.750V | 0.00V (Short) or OL (Open) |
| Base to Collector | Red on Base (NPN) | 0.600V - 0.750V | 0.00V (Short) or OL (Open) |
| Reverse Bias (Any Junction) | Black on Base (NPN) | OL (Over Limit) | Any numeric voltage drop |
| Collector to Emitter | Either direction | OL (Over Limit) | 0.00V to 0.200V (Shorted) |
Note: Germanium transistors (rare, mostly found in vintage audio gear) will show a much lower forward voltage drop, typically between 0.200V and 0.300V. For a deeper look at semiconductor physics, refer to the SparkFun Transistor Tutorial.
Testing Power MOSFETs and Common Pitfalls
MOSFETs (like the ubiquitous IRFZ44N or IRLZ44) are voltage-controlled devices, not current-controlled like BJTs. A standard multimeter cannot fully test a MOSFET's amplification characteristics, but it can definitively identify a blown device by checking the intrinsic body diode and the gate insulation.
The N-Channel MOSFET Test Sequence
- Body Diode Check: Place Black on Source and Red on Drain. You should read the body diode forward drop: typically 0.400V to 0.600V.
- Reverse Block Check: Swap probes (Red on Source, Black on Drain). The meter should read OL.
- Gate Insulation Check: Place probes between Gate and Source, then Gate and Drain, in both directions. All four combinations must read OL. If you read anything else, the gate oxide layer is punctured and the MOSFET is dead.
The 'Gate Charge' Trick to Verify Channel Turn-On
Because a MOSFET gate acts like a tiny capacitor, you can use your meter's diode test voltage to temporarily turn the device on and verify the Drain-Source channel:
- With Black on Source, touch the Red probe to the Gate for one second. This charges the gate capacitance.
- Move the Red probe to the Drain (keep Black on Source). The reading should drop to near 0.00V (or a very low millivolt reading), indicating the channel has turned on.
- To turn it off and discharge the gate, touch all three pins together or short the Gate to the Source with a screwdriver.
Mistakes That Give Misleading Readings
- The Skin Resistance Trap: If you hold the bare metal probe tips and the transistor leads simultaneously with your bare fingers, your body creates a parallel resistance path (dry skin is roughly 50kΩ to 200kΩ). When checking for an 'OL' reverse-bias reading, the meter might display 2.5MΩ instead of 'OL', leading you to falsely believe the junction is leaking. Always hold probes by the insulated grips.
- In-Circuit Testing: Testing a transistor while it is still soldered into a PCB is notoriously unreliable. Parallel traces, bleeder resistors, and snubber capacitors will pull your voltage drop readings down. A good junction might read 0.300V instead of 0.650V because current is bypassing through a parallel 1kΩ resistor. Always desolder at least the Base (or Gate) leg to isolate the component, or remove it entirely.
- Ghost Shorted MOSFETs: Beginners frequently test a MOSFET, accidentally brush the Gate with the Red probe, and then read a dead short (0.00V) across Drain and Source. They assume the MOSFET is blown and throw it away. In reality, they just charged the gate. Always short the Gate to the Source before your final Drain-Source verification.
Frequently Asked Questions
How to test transistors with multimeter without removing them from the circuit?
Strictly speaking, you cannot reliably test a BJT or MOSFET in-circuit using only a standard multimeter's diode mode. Parallel components (like base pulldown resistors or snubber networks) will skew the forward voltage drop readings, making a good transistor look leaky or a shorted one look healthy. If you must test in-circuit, you need a dedicated transistor tester or an oscilloscope to observe the device under active operating conditions. If you only have a multimeter, use a soldering iron and desoldering braid to lift at least the Base or Gate pin off the PCB pad to isolate the junction before testing.
How to test a MOSFET with a digital multimeter in diode mode if it reads shorted?
If your N-channel MOSFET reads 0.00V (a dead short) from Drain to Source in both directions, do not discard it immediately. First, take a piece of wire or a screwdriver and short the Gate pin directly to the Source pin. This drains any residual static charge or gate capacitance that might be holding the internal channel open. Wait five seconds, then repeat the Drain-to-Source diode test. If it now reads 0.400V to 0.600V in one direction and 'OL' in the other, the MOSFET is perfectly fine. If it still reads 0.00V in both directions after discharging the gate, the silicon is physically melted and the part is destroyed.
What does an OL reading mean when testing a transistor base?
'OL' stands for Over Limit (sometimes displayed as a standalone '1' on the far left of the LCD on older or budget meters). It means the resistance or impedance between the two probe points is higher than the meter can measure, effectively acting as an open circuit. When testing a BJT, an 'OL' reading is exactly what you want to see when the probes are reverse-biased (e.g., Black on Base, Red on Emitter for an NPN). However, if you get an 'OL' reading when the probes are forward-biased (Red on Base, Black on Emitter), it means the internal semiconductor junction has cracked or burned open. The transistor is dead and must be replaced.
