Hi, I have a significant voltage drop across a power supply to the load, and I want to measure the resistance of the connections. There is about 6 ft of wiring from the power supply to the load, and with some connections/junctions along the way.
I calculated roughly 0.3 ohm of resistance (based on supply voltage of 1V, voltage and current at load (0.7V and 1A)) which I believe is way too high. The copper wiring has a cross section of 0.75mm^2.
I want to measure the resistance using this multimeter:
I wonder if the fact that the leads on the multi-meter being so long would give me inaccurate measurements? Would the resistance of the leads be added to the actual resistance of whatever you are measuring?
I would think that your calculated resistance would be more accurate than anything you measure with that meter. All the meter does is pass some current through the wire and measure he voltage. For such a low resistance you need quiet a bit of current to get an accurate measurement, so, provided your measurement of current is accurate, measuring the voltage across the ends of the wire and calculating the resistance is probably better.
paulwece:
There is about 6 ft of wiring from the power supply to the load, and with some connections/junctions along the way.
I calculated roughly 0.3 ohm of resistance...
Can't just be the wire then. That length/diameter should be about 0.1ohm round-trip.
What junctions?
Leo..
If you know the load current, you just measure the voltage between the ends of each wire at the power supply, and at the load. You may need to extend the multimeter lead, or measure one part then another.
If you have already done that, then you have measured the resistance. It cannot be different. If you want to know which connections are a problem, measure voltage between one side and the other. This is the correct way to determine small resistances, by passing a large current through them,
If you can't put the load and the supply close enough to use the Multimeter's leads to measure the voltage drop, you can use ohm's law to determine the cable's resistance.
Measure the voltage at the supply and then measure the voltage at the load. That's the voltage drop across the cables. Now measure the current (at either end, should be the same). Now you have the voltage and the current needed to calculate the cable's resistance.
To measure small resistances you need to use a "Kelvin" or 4-wire measurement. Current passes through one pair of wires, voltage is measured using the other pair (which carry no current).
The is easy with a bench power supply with configurable current limit, you connect to the wire and set 1A,
then just measure the voltage across the wire with multimeter.
You connections/junctions are likely to have more contact resistance than the wire unless they are
soldered or screw terminals.
