How to Measure the Internal Resistance of a Multimeter on Its Current Range
Question
Build a discharge circuit using a flashlight bulb and a battery. Intend to connect a multimeter in series into the circuit to measure current. After connecting the multimeter in series, the bulb dims noticeably. This indicates that the multimeter's internal resistance produces a voltage drop. How can we find the internal resistance of the multimeter's current range? Can another multimeter be used for this measurement? What if the internal resistance of the second multimeter is less than 0.1 Ω? My two multimeters seem incapable of this, as their minimum resolution is only 0.1 Ω. Are there any other practical solutions?
Answer
The internal resistance of your test‑lead probes is relatively large. For accurate measurement, draw wires directly from the jack terminals (instead of probe tips) and connect them in series into the test circuit. This improves measurement precision. The internal resistance can be calculated by measuring the voltage drop across the series‑connected multimeter with a second meter.
Use a multimeter with accurate 200 mV range, or obtain a constant‑current source (e.g. home‑built with LM317). First adjust the constant‑current source to an accurate output of 1 A or several milliamperes (this method fails if the reference ammeter is inaccurate). Next, use the 200 mV‑range multimeter to measure the voltage across the ammeter under‑test, which is short‑connected to the constant‑current source. Calculate resistance value from the known current and measured voltage.
I connected two digital multimeters in series in one circuit and measured their internal resistances respectively. Results fell within 0.3‑0.5 Ω, which basically corresponds to the resistance of their own test leads. It shows that the multimeter‑body internal resistance is extremely small, possibly well below 0.1 Ω, since both multimeters have a minimum resolution of 0.1 Ω.
