How to Measure Internal Resistance of Multimeter Ammeter Range
Question
A flashlight bulb and a battery form a discharge circuit. I want to connect a multimeter in series into the circuit to measure the current. However, after connecting it in series, the bulb becomes obviously dim. This is because the multimeter's internal resistance produces a voltage drop. How can I find out the internal resistance of the multimeter's current range? Can another multimeter be used for the test? But if the multimeter's internal resistance is less than 0.1 ohms, my two multimeters seem unable to measure it, as their minimum precision is only 0.1 ohms. Are there any other methods?
Answer
The internal resistance of your test leads may be quite large. If you want to take the measurement, you can draw leads from the jacks (instead of the test leads) and then connect them in series into the circuit for testing. This improves measurement precision. Then use another meter to measure the voltage drop across the multimeter connected in series, and you can calculate its internal resistance!
If you have a multimeter with accurate 200mV range: find a constant current source, or build one yourself using LM317. First use an ammeter to adjust it to an accurate constant current of 1A or some milliamperes - you will not be able to proceed if the ammeter is inaccurate. Next use the 200mV multimeter to measure the voltage across the ammeter connected in series to the constant current source. With known current and voltage, calculate the resistance value.
I connected two digital multimeters in series into the circuit and measured the internal resistance of each. The results obtained were between 0.3 and 0.5 ohms. Basically, these readings included the resistance of each meter's own test leads. This shows that the internal resistance of the multimeter itself is extremely small, possibly even less than 0.1 ohms, because the minimum resolution of both meters is 0.1 ohms.
