Ohms is kinda working

A project log for HydraMETER

Open-source multimeter with some unique features

john-duffyJohn Duffy 09/02/2023 at 04:390 Comments

Ohms range is turning out to be a pain.  To have some measure of input protection, I'm basiclaly doing a 3-wire sense with the juntion at the positive jack.  The ohms "force" has a ~1K resistor, switch, ssr, small fuse, and some low voltage movs.  Getting that to be really well controlled would be difficult, so instead I have a known current going into that, the voltage path is high-impedance so most of that goes through the dut, and we just measure the voltage.  Instead of an actual current source, it effectively just uses a resistor going to a boosted voltage (~8V above common, but there's significant drop before the output), measure the voltage across said resistor, then we know the current.  Hopefully that made some sense, I'll post schematics soon.  

I am working to keep down the number of opamps and other actives, kinda for price kinda for accuracy, kinda just because.  This has made measuring the voltage drop failrly difficult, the adc only has a range of +-1.5V from common, so I have to do a differential measurement of one low and one high-Z node, which can be from 0 to almost 9V apart, then shift that ~9V down to ground and put it within the adcs range.  And the high voltage supply is noisy because it's switched. All of these are fairly simple individually, but getting a setup to handle everything well, without like seven opamps, is challenging (which is half the point of this project anyway, so I'm not taking the easy way out, dammit!). 

Anyway it's mostly working using an array of resistors and pfets, and a single opamp and pfet to source a current proportional to the voltage across one of those resistors.  That current goes through a smaller resistor to common, which both puts it in the right range and relative to common.  Problem is that if there's high voltage across the high-side resistor, there's not enough overhead for both.  This still works but requires more range resistors.

Any case I'm mostly accepting ohms range as is for now and moving to putting it all into KiCad, I'll finalize the design and put up schematics soon.  

Mean time does anyone knows of a component with a roughly constant voltage drop (looking for about 3V, but could chain a few) even down to the uA range?  That would be helpful for this.  So like an ideal diode (where "ideal" includes a ~0.7V drop) would work fine by just putting a few in series, problem is real diodes drop goes down close to 0 once you get below a few dozen uA.