BJT Verification with a Multimeter
Pre-Test Notes
Set multimeter to diode test mode. Identify the three pins: Base (B), Collector (C), Emitter (E). No discharge step is needed between tests as BJTs have no gate capacitance.
NPN BJT Test Sequence
| Step |
Action |
Red Probe (+) |
Black Probe (−) |
Expected Reading |
Pass/Fail Indicator |
| 1 | Base-Emitter junction (forward) | Base | Emitter | 0.6 – 0.7 V | ✅ Forward voltage |
| 2 | Base-Emitter junction (reverse) | Emitter | Base | OL (open) | ✅ No conduction |
| 3 | Base-Collector junction (forward) | Base | Collector | 0.6 – 0.7 V | ✅ Forward voltage |
| 4 | Base-Collector junction (reverse) | Collector | Base | OL (open) | ✅ No conduction |
| 5 | Collector-Emitter (no bias) | Collector | Emitter | OL (open) | ✅ No conduction |
| 6 | Collector-Emitter (no bias) reverse | Emitter | Collector | OL (open) | ✅ No conduction |
PNP BJT Test Sequence
| Step |
Action |
Red Probe (+) |
Black Probe (−) |
Expected Reading |
Pass/Fail Indicator |
| 1 | Base-Emitter junction (forward) | Emitter | Base | 0.6 – 0.7 V | ✅ Forward voltage |
| 2 | Base-Emitter junction (reverse) | Base | Emitter | OL (open) | ✅ No conduction |
| 3 | Base-Collector junction (forward) | Collector | Base | 0.6 – 0.7 V | ✅ Forward voltage |
| 4 | Base-Collector junction (reverse) | Base | Collector | OL (open) | ✅ No conduction |
| 5 | Collector-Emitter (no bias) | Emitter | Collector | OL (open) | ✅ No conduction |
| 6 | Collector-Emitter (no bias) reverse | Collector | Emitter | OL (open) | ✅ No conduction |
BJT Failure Mode Reference
| Symptom |
Likely Cause |
| Both junctions show OL in both directions | Open transistor — internal break |
| Both junctions conduct in both directions | Shorted transistor — blown |
| One junction reads correct, other is shorted | Partial failure — replace |
| C-E conducts without base bias | Collector-Emitter short — blown |
| Junction voltage too low (<0.5 V) | Weakened junction — degraded transistor |
| Junction voltage too high (>0.8 V) | High resistance junction — suspect leakage or damage |
BJT Key Notes
- BJTs do not have gate capacitance like MOSFETs — no discharge step is needed between tests.
- The B-E and B-C junctions behave like regular diodes — forward voltage should be 0.6–0.7 V for silicon, 0.2–0.3 V for germanium.
- C-E should always read OL with no base bias applied — any conduction here indicates a fault.
- For a more thorough test, use your multimeter's hFE mode if available — insert the BJT into the socket and it will display the gain directly.
MOSFET Verification with a Multimeter
Pre-Test: Discharge Gate & Body Capacitance
Before any testing, short Gate to Source for 2–3 seconds to fully discharge both the gate oxide capacitance and any residual body capacitance. Repeat this reset between every test step.
N-Channel MOSFET Test Sequence
| Step |
Action |
Red Probe (+) |
Black Probe (−) |
Expected Reading |
Pass/Fail Indicator |
| 1 | Discharge gate/capacitance | — | — | Short G to S for 2–3 sec | Reset before each step |
| 2 | Body diode (forward) | Drain | Source | 0.4 – 0.7 V | ✅ Forward voltage |
| 3 | Body diode (reverse) | Source | Drain | OL (open) | ✅ No conduction |
| 4 | Turn ON gate | Gate, then Drain | Source | Low V or beep | ✅ Channel conducting |
| 5 | Verify ON state | Drain | Source | Low V (near 0) | ✅ Channel open |
| 6 | Discharge gate/capacitance | — | — | Short G to S for 2–3 sec | Clears stored charge |
| 7 | Verify OFF state | Drain | Source | OL (open) | ✅ Channel closed |
P-Channel MOSFET Test Sequence
| Step |
Action |
Red Probe (+) |
Black Probe (−) |
Expected Reading |
Pass/Fail Indicator |
| 1 | Discharge gate/capacitance | — | — | Short G to S for 2–3 sec | Reset before each step |
| 2 | Body diode (forward) | Source | Drain | 0.4 – 0.7 V | ✅ Forward voltage |
| 3 | Body diode (reverse) | Drain | Source | OL (open) | ✅ No conduction |
| 4 | Turn ON gate | Source | Gate, then Drain | Low V or beep | ✅ Channel conducting |
| 5 | Verify ON state | Source | Drain | Low V (near 0) | ✅ Channel open |
| 6 | Discharge gate/capacitance | — | — | Short G to S for 2–3 sec | Clears stored charge |
| 7 | Verify OFF state | Source | Drain | OL (open) | ✅ Channel closed |
Failure Mode Reference
| Symptom |
Likely Cause |
| Conducts in both directions (D↔S) at all times | Drain-Source short — MOSFET blown |
| No body diode conduction at all | Open junction — internal break |
| Gate has no effect on conduction | Gate oxide failure |
| Conducts after discharge (won't turn off) | Incomplete capacitance discharge — retry step 6, or leaky gate |
| Erratic/inconsistent readings | Residual charge — discharge longer, or multimeter leakage current too high |
Key Notes
- Body capacitance (Cgs, Cgd, Cds) can hold charge long enough to keep the channel on even after removing gate stimulus — always re-discharge between steps
- Some multimeters source enough probe current (~0.3 mA) to partially enhance the channel — if readings are inconsistent, try a different meter
- This procedure tests basic switching integrity only; parameters like Rds(on) and Vgs(th) require a curve tracer or dedicated component tester