In this video I'm going to show you how you
can use a cheap multimeter to measure resistances the milliohm range with a
decent amount of accuracy. Most of the time a resistance of a few
milliohms doesn't matter. But if there are high currents in your
circuit a few milliohms can cause a substantial voltage drop and power loss
in the wires. For example let's say you're designing
a coilgun or winding a solenoid or you have a project with long battery wires
and you want to know the exact resistance of a piece of wire. If you try to use your multimeter's
resistance function you'll get a useless reading. Zero ohms... 0.3 ohms... I have no
idea what's going on here. Even an excellent multimeter will
just say something like 0.2 ohms when I know for sure the resistance is a
lot less than that. Part of the problem is that you're
measuring the resistance of the multimeter's cables and part of it is that most
multimeters are not designed to measure resistances this small. The solution to the problem is called
four wire measurement, also sometimes called Kelvin resistance measurement. And
it's really simple. Let's start out with a constant current
source of one ampere, and I'm going to show you how to set that up later. Since it's a constant current source, no matter what resistance I connect it to,
the current flowing through the circuit will always be exactly one ampere. Now let's say I connect an unknown
resistance and let's call that resistance "R". And let's connect a multimeter
across the resistance and use it to measure the voltage across R. And let's call that voltage "V". Ohm's law is
Volts = Current x Resistance and rearranged that gives you
Volts divided by Current = Resistance Now because we know the current in the circuit
is exactly one ampere you end up with V = R. In other words the resistance in ohms
is the same as the number of volts across R. So let's say you measure fifty millivolts
across the wire you are testing... that means that the resistance of the
wire is fifty milliohms. Here I am measuring the voltage across a
small piece of wire and since the reading is fourteen millivolts I know
that the wire has a resistance of fourteen milliohms. You will never get this kind of accuracy
with the traditional two wire measurement approach. Now another cool thing about this circuit
is how the resistances of the test wires and alligator clips are not a problem anymore. The current is always exactly one ampere so
as long as I measure the voltage directly across the device under test I
can calculate its resistance. And because the input impedance of the
multimeter is so high (well into the megaohm range) almost no current flows
into the multimeter so the meter doesn't affect the circuit. Okay now here is the easiest way to do
this in real life. I strongly recommend that you have a
bench power supply with current limiting functionality. If you don't have one you should check
out www.MPJA.com Start out by setting the voltage to
about two volts. The exact number doesn't matter but keep
it a low voltage because we're going to start short circuiting things soon. Start with the current limiting dial set
to zero. Now connect the multimeter set to
measure current directly across the power wires. Now we just need to increase the current
limit so that we get exactly one ampere. Doing this with the multimeter gives you
a little more accuracy than just reading the bench supply's display. Okay now we have a constant current
supply of one ampere. And you just need to connect your
current supply wires to the thing you want to measure, then measure the voltage across it. Let's measure the resistance of
this inductor. 47 millivolts means the inductor
has a D.C. resistance of 47 milliohms. And this piece of speaker wire has a
resistance of seven milliohms. Okay now what if you don't have a
current limiting power supply like I do? Well you can build a constant current
source out of an LM317 using this circuit. In theory this should give you a
constant current of one ampere regardless of what you have connected to it. In reality you'll never find a
precise 1.25 ohm power resistor. So the current won't be exactly one ampere. So here's what you have to do. First use your multimeter to measure the
actual current that you're getting out from the circuit. In my case I got 0.92 Amps. Next you measure the voltage across your
wire as before. Let's say it's 80 millivolts for a
certain piece of wire. Now you apply Ohm's law again but since the current isn't exactly one
ampere, you can't do the easy volts to ohms conversion. Instead, take your measured voltage and
divide it by your measured current and you'll get the exact resistance of your piece
of wire. That's it for four wire measurement and
now your death ray designs can be more efficient than ever!