I have a low resistance resistor that I would like to use to measure the current through another circuit, but first I need to accurately determine its resistance. (A Bourns # CSS4J-4026R-1L00F, 4-Terminal Current Sense Resistor, 1 mOhms ±1% 4W, 4026).
Its data sheet says it is 0.001 ohm +/- 1%
A basic setup is to apply a voltage to the resistor, measure the current going through it and the voltage across it, using the set up in the diagram above.
I am having some trouble understanding my current and voltage measurements, and how the probes may effect the circuit.
The first thing is measuring the voltage. If I touch my probes together, I get a reading of 0.02 mV. Is this something I need to offset in my measurement? For instance, if I measure 0.16 mV, should I subtract the 0.02 mV and record the measurement as 0.14 mV?
Secondly, when my power supply is OFF, my multimeter current measurement reads 3.7 mA, so as I vary the current limit, the true measurement is 3.7 mA less than what the power supply indicates. For example, if set to a 200 mA current limit, the multimeter reads 197.3 mA.
The idea is to use one multimeter to verify the current to the resistor, as the power supply reading may not necessarily be accurate. However, as I write this I realise my power supply has an OFF button for the voltage output and for the device itself. When the device is OFF, the current reading is 0 mA, when it is ON, but the output is OFF, the reading it 3.7 mA, and when the output is turned ON (with the current limit 200 mA) the current reading is 197.3 mA.
So I reason that the current reading on the multimeter is probably fine as-is. It is just when the supply is ON, even if the output is off, there is a small current leaking through.
But I am still not sure about the voltage.
Edit The diagram I meant was this, with 2 separate meters one for current one for the voltage across the resistor.