Practical case: Test diode with multimeter in diode mode

Materials

• 1× Silicon diode (e.g., 1N4148 or 1N4007)
• 1× Digital multimeter with diode-test mode (DMM)
• 2× Short jumper wires or alligator clips (optional, for hands-free probing)
• 1× Breadboard (optional)

Wiring guide

• Set the multimeter to diode mode (look for the diode symbol).
• Identify the diode:
• Anode (A): unmarked end.
• Cathode (K): end with the band.
• Abbreviations used:
• DM+ = DMM red probe (positive in diode mode).
• DM- = DMM black probe (COM).
• V_F = forward voltage drop indicated by the DMM in diode mode.
• Forward-bias test:
• Connect DM+ to the diode anode (A).
• Connect DM- to the diode cathode (K).
• Reverse-bias test:
• Swap the probes: DM+ to the cathode (K) and DM- to the anode (A).
• Important: Remove the diode from any live circuit before using diode mode. Never connect the DMM in diode mode to a powered circuit.

Schematic

                 +V
                  │
               ● DMM+
                  │          A
                 ┌┬┐         D1: Diodo 1N4148
                 │││
                 │││
                 └┴┘         K
                  │
               ● DMM−
                  │
                 GND

Measurements and tests

• Forward-bias reading (V_F):
  • Connect as in the forward-bias step (DM+→A, DM-→K).
  • Expected V_F (typical at ~1 mA from the DMM):
  • Silicon signal/power diode (1N4148/1N4007): about 0.55–0.75 V.
  • Schottky diode: about 0.15–0.35 V.
  • LED (if you test one): about 1.6–2.2 V (red) up to ~3.0 V+ (blue/white).
  • If the DMM shows “OL” or no reading in forward-bias, the diode may be open.
• Reverse-bias check:
  • Swap probes (DM+→K, DM-→A).
  • Expected: “OL” (overload) or no conduction.
  • If you read a very low voltage or the meter beeps as if shorted, the diode may be shorted.
• Interpreting abnormal results:
  • “0.000 V” (or near zero) in both directions: diode is likely shorted.
  • “OL” in both directions: diode is likely open or you have poor contact.
• Optional in-circuit hint:
  • In-circuit tests can be misleading due to parallel paths. If readings look odd, desolder one lead and retest out of circuit.

Common mistakes

• Not removing power from the circuit before testing in diode mode.
• Holding probe tips poorly or touching both diode leads with fingers, causing unstable readings.
• Confusing the banded end (cathode K) with the anode (A).
• Using voltage or resistance mode instead of diode mode, leading to unclear results.

Safety tips

• Always power down and discharge capacitors before testing components.
• Do not use the diode mode across energized circuits; you can damage the meter or the component.

Improvements and further checks

• To confirm behavior at higher current, you can later build a simple test jig with a low-voltage supply and series resistor to measure V_F vs. current (not with the meter in diode mode).
• Keep a small chart of expected V_F for different diode types (silicon, Schottky, LED) for quick comparison.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Who we are