Making a 4 Probes Resistance Measurement Setup

A four probe resistance measurement setup is used to measure precisely the resistance of a sample and suppress the influence of the measuring device (contact and wire resistance). It is generally used to measure sheet resistance (a good explanation of sheet resistance is available here: https://www.ossila.com/pages/sheet-resistance-theory) but can be used for more specific needs as you will see along this page.

In this instructable i introduce the way i made my own setup and share a few tips as well as some 3D files that you may find useful. However i designed this setup with a 3D printer and the limited furniture's i had available. So this is basically my answer to the question: How do you do if you need a 4 probe measurement setup but you only have a 20 dollars budget and a few metal scraps laying around...

Tools:

• 3D printer
• soldering iron
• drill
• Threading tools

3D models:

• https://www.thingiverse.com/thing:5585731
• (optional: https://www.thingiverse.com/thing:5540256)

Furniture:

• 32mm metal tube
• Metal plaque
• Spring loaded pogo pins *4 (i used the ref TH116 from: https://www.aliexpress.us/item/2255801110658817.html)
• Female banana connector *4 (4 mm connector)
• 6 POM wheel (https://www.amazon.com/Creality-3D-Printer-Plastic-Bearing/dp/B07KXPD6XZ)
• Some epoxy resin
• Various M4 M5 and M6 srews and nuts

The first step for consist in making the setup base composed of a metal plaque and a tube fixed perpendicularly to serve as a guide for the probe arm. I decided to use a 5mm aluminum plate as it is light and easy to machine (and i had one laying around).

As i want to use various "adds on" to facilitate the measurements, i pierced the work area of my plate with a array of holes threaded for M3 screws. In my design the holes are dispersed every 3cm. The tube is fixed to the plate from the inside to minimize the footprint.

To fix the tube, print the "tube insert" part and check that it fits snugly into the tube. Place the insert on the plaque and draw the three 4mm holes. Pierce the holes and prepare a chamfer on the back side. Fix the insert to the plaque with M4 conical head screws and bolts. Use long screws (30 to 45 mm) so that they exceed the insert. Clean the tube to remove any grease on the inside and place it over the insert. Prepare 100 to 200 ml of epoxy. Check that the junction insert/tube is tight and clog the holes with tape or hot glue if required. Check the positioning of the tube and fix it straight if it is not perpendicular. When everything is fine pour the epoxy in the tube and let it cure. You should be left with a strong junction.

In this design the "slider" is the part that slides along the tube and allow a smooth translation for the whole probe arm.

To prepare this part, just print the "slider" file and 12 "spacers". Attach the wheels with M5 screws placing one spacer on each side of each wheel. Preferably you should use 25mm long screws for the fix wheels and 30mm long screws with nuts for the adjustable ones. Fix the wheels, try the fit and adjust the tightness if necessary.

After that it is time to prepare the probe arm. Two versions of the "probe arm" file is available. The one noted "electrode" corresponds to the one shown in the pictures here. I designed it to measure the resistance of simple electrodes i make and am trying to optimize. To use this setup to measure sheet resistance, use the file "sheet resistance". The only difference is the position on the holes for the pogo pins.

First solder some cables at 90 degree to the pogo pins. At this step 25 cm cable or so are perfect. To help organizing the cables using 4 different cable colors is helpful. At that point use a small hammer to force the pogo pins into the holes at the extremity of the probe arm. The holes are designed to be tight an to hold the pins without the need for glue. It the resistance is to strong, use your soldering iron to slightly heat the pin and soften the plastic. Make sure to align the 4 pins to the best you can.

From that point on be cautious of not hitting the connectors to much when manipulating the probe.

At that point you can fix the arm to the slider and place it on the base to securely maintain the probes while you are working. The arm attaches to the slider with a tongue and groove system. It is secured in position by a M6 screw and nut placed on the arm to lock the tongue in place. You can ultimately use this system to adapt the position of the probe to the hight of your sample.

To complete the connections you need to print one "connector holder" and four "cable organizer" parts. Screw the banana connectors to the "connector holder" part, and fix it to the arm with 4 M3 screws. Cut the cables to length and attach then to the connectors. The "cable organizer" parts go by pair and help maintain the cable in position. They are also fixed with M3 screws.

At that point you setup is functional but the probe can still rotate around its axis.

You may print the "guide" part to block the rotation of the arm. This part attaches to the plate with 2 M3 screws. The exact position can be fine tuned as the screw holes are grooves. Additionally this part is designed to "clip" around the arm and maintain it up while you charge your sample bellow.

In general sheet measurement doesn't require to aim for a precise part of your sample but if you use this setup to characterize a material, an aiming system can be useful. You can print and fix with 2 M3 screws the two "Aim 1" and "Aim 2" parts according to those pictures. The alignment is done using the screw slots.

You can retract the aim or directly carry out the measurement through it.

To hold you sample in position you can use the quick clamp system i designed.

More details here:

https://www.thingiverse.com/thing:5540256

https://youtu.be/CKmVyAYNHIk