Why Are My 4-Point Probe Measurements Failing?

[rury_cruz]

4 point probe measurement help!

I will really appreciate if somebody can help me:

I was trying to measure the electrical conductivity of some conductive polymer films using the 4 point probe technique. Altough is widely described and seems easy to arrange, I have been unable to measure my samples due to the following problems:

1.- As soon as I turn on the power source the films burns out (like a fuse)due to the joule effect, I guess, even when I set the power source in the "constant current" mode, fixed to the minimum.
2.- When I tried to measure the electrical conductivity of the semiconducting samples, (or at least I thought so...), no voltage drop is detected between the inner probes, and no current flows at all. It seems to be "too insulating" to close the circuit
3.- The digital multimetter is always changing the voltage drop measurement, whether the current source is on or off. Could it be due to the connections?... some bi-metalic joint effect?

I hope some of you have done this kind of measurements before, I really need to know the conductivity of my samples. If you need further information feel free to contact me...

 

[nbo10]

For the first problem you could put a shunt resistor in the cicuit to limit the current through the sample.

If the sample is insulating then you won't read a votlage across the inner leads.

A perferred method is to use a lockin-amp and measure the AC-resistance. It will give you a better S/N ratio. Otherwise you might need a nanovolt meter you get decent results.

How are you attaching your leads to the sample? This was a major problem that I ran into while making these type measurements.


JMD