In earlier note on How to Check Electronic Components on a PCB Using a Multimeter, I only briefly outlined about how to test MOSFETs on PCB board. I should have mentioned where to connect or touch the MOSFET pins with the COM and LIVE probe of the multimeter and so here is note on that.
When testing a MOSFET with a multimeter in diode mode, the placement of the probes depends on the test you're performing. Here's how to connect the ground (COM) and live (positive) probes correctly:
1. Gate (G) to Source (S) Test
Purpose: Check for gate charge or leakage.
Probe Placement:
Live (Red) probe → Gate (G)
Ground (Black) probe → Source (S)
Expected Reading:
OL (open circuit) if the MOSFET is discharged.
If it shows a voltage (e.g., ~1-3V), the MOSFET may be charged (touch G to S briefly to discharge it).
2. Drain (D) to Source (S) Test (Body Diode Check)
Purpose: Verify the built-in body diode.
Forward Bias (Diode conducts):
Live (Red) probe → Source (S)
Ground (Black) probe → Drain (D)
Expected Reading: ~0.4V to 0.7V (diode forward voltage).
Reverse Bias (Diode blocks):
Swap probes:
Live (Red) probe → Drain (D)
Ground (Black) probe → Source (S)
Expected Reading: OL (no conduction).
Key Notes:
✔ On PCB: Other components may interfere—desolder MOSFET for accurate testing.
✔ Discharge MOSFET before testing (short G to S with a wire or resistor).
✔ N-Channel MOSFET assumed (for P-Channel, polarities reverse).
