## DIY Milliohm Meter

25 08 2011

I asked Joe Khachan this question:

Did you go to great lengths to reduce the resistance of the coil path?

He responded:

Yes we try and use a thick enough wire so that the resistance is small but it cannot be too thick in order to be able to have many turns. More importantly is the contact resistance when connecting to the wires. We try to keep this low. From this perspective, a milliOhm meter is helpful.

Milliohm meters start around \$200 on ebay. Too much.

Then I found this excellent tutorial on Kelvin (4-wire) resistance measurement.

Luckily I had a current sensing resistor on hand for the current reading:

I used the resistor on the right.

So current sense resistor plus two voltmeters and BINGO: milliohm meter. Results:

The coil path is 0.35Ω.

The dummy coil is 0.185Ω

As a calibration I tested the 0.001Ω resistor. Result: 0.00088888

Now I have excellent tool to troubleshoot resistance in the coil path. AWESOME.

### 3 responses

25 08 2011

Moin

Nicely improvised.
I often use the same technique to measure resistances below 10Ohm.
Even though my multimeter is not the bad, it’s only not that precise for low resistances.

One other thing you should measure is the inductance of the coils.
Even if the inductance is very small, it might still have a great influence of your coils impedance since you use high currents and voltages.
Also you can calculate the the theoretical waveform for you pulse an compare it to your measurements.
An other important fact is that the impedance limits your maximum pulse frequency and if I remember correctly the experiment involves firing the coils repeatedly and fast.

Just some late night thoughts from Germany.

Greetings
Sebastian

25 08 2011

Silver has a lower resistance (about half that of copper) however it is obviously more expensive. Plus it doesn’t normaly come lacquered.

27 08 2011

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