Louis at Scullcom Hobby Electronics has a new video updating the milliohm meter, and features the board I produced in this post.
NEW Link to updated video on YouTube
NEW Version 1.5 board available at OSH Park requires additional 0.01 uF ceramic capacitor, 2 pin header, and 250 mA PTC fuse.
NEW Updated project post with Milliohm Meter version 1.5 board.
It uses a 4 wire kelvin measurement system that supplies a precise 100 mA current to the device under test using one pair of leads, and then measures the voltage drop across the device under test using a second pair of leads.
As designed it can be used to measure low resistances of under 2 ohms (Ω). Some of the design changes I made to fit my needs were:
- Change the number of resistors needed in the current source circuit
- Use higher precision resistors with a low temperature coefficient
- Add separation and shielding between input, output, and supply circuits
- Use lower ESR and higher value capacitors for the MAX680 power supply
- Add additional decoupling capacitors on the INA106, and MAX680 IC’s
- Change the voltage regulator from a TO220 package to a SOT-223 package
OSH Park link to shared project page for V1.4 Milliohm meter
The LED display was purchased on Ebay from seller coldfusionx and is a 4 1/2 digit blue LED digital 2V Volt meter. The presented specifications are:
Accuracy: +/- 0.1% «
I was very happy with the performance of the display meter, and it looks much better than is shown in the photos. The battery assembly is comprised of 6 AA 1.5 volt Energizer Ultimate Lithium cells making a nominal 9 volt supply to the board. The lithium cells do very well with the + 130 mA load and had a very low voltage drop in use.Component placement inside case.
To power the display meter I added a separate buck converter to supply a dedicated 5 volts from the 9 volt battery.
On the V1.1 board I ended up with a bad footprint for the LM2940 5 volt regulator and bodged a TO220 version to the board. This has been corrected in versions after V1.1.
The completed meter works well for my purposes and I have found that it is used much more than expected for things that I would normally use my Fluke 189 multimeter for, and I don’t have to put up with the low resolution.
Link to EagleCAD Milliohm V1.41 schematic and board files ZIP
A big Thanks! to Louis from Scullcom Hobby Electronics for designing and presenting this project in an informative and professional video.
There is an updated project post with Milliohm Meter version 1.5 board.