How to measure the internal resistance of a multimeter when it is in the current setting?
question:
I used a light bulb and a battery to form a discharge circuit. I wanted to connect a multimeter to the circuit and measure the current. However, I found that the brightness of the bulb dimmed significantly after being connected to the circuit. This indicates that the internal resistance of the multimeter has produced a voltage drop. How do I know the current level of the multimeter? Can I use another multimeter to test the internal resistance? But if the internal resistance of the multimeter is less than 0.1 ohms, my two multimeters seem to be powerless because the minimum accuracy is only 0.1 ohms. Are there any other good ways?
answer:
The internal resistance of the test lead should be very large. If you want to measure it, you can only lead the wire from the place where the test lead is inserted and then connect it to the circuit for testing. This can improve the accuracy. Then use another meter to measure the voltage drop of the meter connected in series to the circuit. Got it!
Do you have a multimeter with an accurate point of 200mV? Find a constant current source, or use LM317 or something yourself
To do this, first use an ammeter to adjust the constant current to an accurate 1A or mA - if the ammeter is inaccurate, there will be no solution - and then use a 200mV multimeter to measure the current that is short-circuited to the constant current source.
The voltage across the flow meter, the known current, the known voltage, calculate the resistance.
I connected my two digital meters in series to the circuit and measured the internal resistance of the two multimeters respectively. The results were between 0.3-0.5 ohms. Basically it is the resistance value of the respective meter wires of the two meters. It means that the internal resistance of the multimeter itself is very small, maybe less than 0.1 ohms at all, because the minimum resistance accuracy of the two meters is 0.1 ohms.
4. Why doesn't the internal resistance of the multimeter's DC current range be the smallest?
For example, if the meter head is 1 mA and 100 ohms, the internal resistance of the 1A range is theoretically calculated to be 0, 1 ohm, but the internal resistance of the corresponding range of the multimeter is higher than the calculated value. Why?
This is determined by how the multimeter is used. The basic structure of a multimeter is a microampere meter, which is connected in series with a resistor to form a voltmeter; in parallel with the resistor, it forms an ammeter.
When measuring voltage, it is connected in parallel with the circuit under test. When shifting gears, the switch will be momentarily disconnected or have poor contact. At this time, there is no current flowing through the meter, and there is no problem.
When measuring current, the meter is connected in series with the circuit under test. If the parallel resistance of each gear is directly connected in parallel with the meter, then the gear must be changed during measurement. If the switch is momentarily disconnected or has poor contact during the shift, a large current will flow through the meter, which can easily burn the meter. , the contact resistance of the switch can also seriously affect the accuracy of the measurement.
Therefore, in the current range, instead of directly connecting different resistors in parallel with the meter, a resistor string is formed and fixedly connected in parallel with the meter, and then different current levels are drawn from the resistor string (as shown in the figure). This is a sacrifice.
It reduces the sensitivity of some meters, but ensures the safety of measurement; the impact of the contact resistance of the shift switch on the measurement is greatly reduced.
