Operating Method for Inductance Measurement with a Multimeter
① Selection of Measurement Range Since the DC resistance of the copper wire in an inductor coil is very small and almost conductive, digital multimeters use this characteristic. Select the lowest resistance range and judge the quality of the inductor by measuring continuity.
Switch the digital multimeter to the range marked with the diode symbol, and select the range with the diode and buzzer function.
② Measurement Operation With the laptop mainboard powered off (passive state), touch the two test leads of the digital multimeter to the solder joints at both ends of the inductor. The inductor has no positive or negative polarity, so the test leads can be connected either way. Do not touch the solder joints for too short a time during measurement; otherwise, misjudgment may easily occur.
③ Observe and Judge the Measurement Result If the stable reading on the LCD is "0" or close to "0", and the buzzer sounds continuously during testing, the tested inductor is good. If the reading shows overflow symbol "1" (i.e. ∞), the inductor is open-circuited and damaged. If the numbers on the LCD flicker and the buzzer beeps intermittently, in most cases there are cold solder joints or detached solder joints on the component. Resolder and retest.
The above three cases are common in maintenance and inspection. Two special cases are explained below:
First: If the digital reading is about 10nΩ or above 10Ω even with continuous buzzer sound, the inductor cannot be judged as good. All inductors on laptop motherboards cannot have resistance around 10Ω. Flat storage inductors and transformers used in high-power and high-current power circuits adopt thick copper windings with low resistance. Small chip inductors used in low-power, low-current power circuits or signal circuits have thin copper windings but few turns and short length, so their resistance is also low.
Therefore, a reading of about 10Ω or higher is not the normal resistance of an inductor. It may indicate an open-circuited inductor, and the reading comes from the resistance of other parallel circuits. It may also mean the inductor winding is burnt open, and the reading is contact resistance caused by inter-turn short circuit.
Second: If the buzzer only makes a short beep at the moment the test leads touch the two solder joints and then stops suddenly, this usually occurs in high-current power circuits. This phenomenon means not only that the inductor is open-circuited, but also that there is a severe short circuit (earth fault) in the circuit at one end of the inductor. The short beep of the buzzer is caused by instantaneous charging of large capacitors in the circuit by the voltage across the test leads.
For this reason, the test leads should stay in contact with the solder joints of the inductor for sufficient time when testing inductors.
