As the first version of Half Ohm milliohm meter design had some fatal design flaws, I designed a new board. The problems with the old one was that the small BR1225 battery was rated to maximum of 1mA and couldn't handle the 5mA current draw of my circuit. The second problem was that the pinout of the op-amp was wrong. For whatever reasons the pinout standards for sub 3€ op-amps and above 3€ op-amps is different.
The new schematic is basically the same, only the board got rerouted and sized up to accommodate bigger and more standard BR2032 batteries with maximum output current of 10mA. I milled the board with my 3d printer using visolate (it truly violates my eyes). The board looks weird but works very well, so I already ordered some from the factory.
When connected to the multimeter, turned on and not probing anything, the meter shows the battery voltage. When probing, then the multimeter displays mΩ in mV range. So 135mV means 135mΩ of resistance. Since the probes have considerable resistance (In my case 77mΩ) it is nice to have relative/offset/zero button on your multimeter, because then you don't have to subtract the probe resistances. I could have done it with four wire measurement, but I wanted to keep things simple. I tested my meter against commercial 4 wire meter and the result stayed in calculated 1% error. I am pleased.
A bit of technical details: The board now draws about 4.7mA so I should have about 40 hours of use. I tried to measure the 1.24V±0.5% voltage reference with my VICHY VC 97 (0.5% + 4 digits) and I got 1.247±0.006V as a result. Remeasured it with Mastech MY-64(0.5% + 1 digit) and the result was 1.241±0.004V. I tend to agree with the Mastech result a bit more, as Mastech isn't some weird ebay brand.
Here are some random measurements I did in the lab.
- 77mΩ - Test probes
- 19mΩ - 18AWG Silicone cable with soldered cold connectors
- 125mΩ - Tweezers
- 5mΩ - 1cm long 2mm wide tin plated track
- 5mΩ - GND plane over 3cm of pcb
- 26mΩ - 1cm of 0.2mm trace
- 2mΩ - a 0.3mm via
As you can see it is quite useful for measuring tracks. I have already used it couple of times to find short circuit. One of my friend in lab used it to determine if his new idea of making vias is good enough. Quite awesome little tool. Next up - manufacturing units and recalculating the error with better formulas.