Summary of Universal Measurement Functions of Digital Multimeters
① Measurement of DC Current First, insert the black test lead into the "COM" jack. For current above 200 mA, insert the red test lead into the "10A" jack and turn the rotary knob to the DC "10A" range. For current below 200 mA, insert the red test lead into the "200mA" jack and set the knob to an appropriate DC‑current range within 200 mA. After setup, connect the multimeter in series into the circuit, keep it steady and take the reading. If "1" is displayed, select a higher range. If a "‑" symbol appears on the left side of the reading, it means current flows into the multimeter through the black test lead.
② Measurement of AC Current The measurement procedure is the same as item ①, except that the rotary knob shall be switched to the AC‑current range. After completing current measurement, move the red test lead back to the "VΩ" jack. Forgetting this step and directly measuring voltage will destroy the meter or power supply.
③ Measurement of Resistance Insert test leads into the "COM" and "VΩ" jacks. Turn the rotary knob to the required resistance "Ω" range. Touch the metal terminals of the resistor with the test probes. You may touch one terminal of the resistor by hand during measurement, but avoid touching both terminals simultaneously, as human‑body resistance will affect measurement accuracy. Maintain good contact between probes and the resistor when reading values. Pay attention to units: the unit is "Ω" for the 200‑range; "kΩ" for ranges from 2 k to 200 k; "MΩ" for the 2 M and higher ranges.
④ Measurement of Diodes A digital multimeter can measure light‑emitting diodes, rectifier diodes and other diode components. Keep the test‑lead sockets the same as for voltage measurement, and turn the selector knob to the diode test position. Connect the red test lead to the diode anode and the black test lead to the cathode; the forward voltage drop of the diode will be shown on the display. The voltage drop of a Schottky diode is approximately 0.2 V; ordinary silicon rectifier diodes (1N4000, 1N5400 series, etc.) are around 0.7 V; light‑emitting diodes are roughly 1.8‑2.3 V. Swap the test leads. If "1" is shown on the screen, the diode is normal due to its high reverse resistance. Otherwise, the diode has been broken down.
⑤ Measurement of Transistors Use the same test‑lead jack positions as for diode measurement; the working principle is also similar. Assume pin A is the base. Connect the black test lead to pin A, and touch the remaining two pins respectively with the red test lead. If both readings are approximately 0.7 V, then connect the red test lead to pin A and the black test lead to the other two pins. If "1" is displayed for both measurements, pin A is the base; otherwise repeat the measurement, and the device is a PNP transistor.
