Attempt to implement an x100k ohmic scale in an analog multimeter.

[Avelino Sampaio]

I have an analog multimeter and its schematic is shown in the image below. I would like to create a circuit capable of adding the x100k ohmic scale, using the 50µA/0.1v input, which gives direct access to the galvanometer. If I were to use the same logic employed in the x10k ohmic scale (see the schematic), I would need to have a 125v source (b2) with a 2M resistor (R23). I would like to know if you know of any technique to implement this x100k scale without using high voltage? I was unable to access the schematics, which would give me the opportunity to observe how it was designed, without using high voltage.

[Avelino Sampaio]

I found a draft schematic of an analog multimeter, sold here in Brazil, containing the x100k scale. Note that a transistor is used on the x100k scale as well as only the 3v source. I still haven't been able to decipher how this transistor is acting as a voltage divider and with which resistor. Can someone explain to me how this transistor is acting? And how the designer managed to match the VBE (hfe) curve of the transistor with the ohmic scale?

[Avelino Sampaio]

After several failed attempts, I finally managed to design a circuit with good precision. The diagram is below.

And congratulations to the designer who managed to solve this with a transistor and two base resistors. Until now, I have not been able to figure out how this circuit works.

image 2: 10M
image 3: 4M7
image 4: 2M

[Avelino Sampaio]

I made an alternative project, using the Linear Scale Lv(v), which goes from 0 to 3. Full scale up to 30M, with each line corresponding to 1M. I also obtained good precision! Below is an image of the schematic and some readings.

Image 2: 20M
Image 3: 10M
Image 4: 4M7

[Avelino Sampaio]

I realized in this project that it would be possible to implement an even larger scale (x1M) but I would have to choose an Opamp with smaller input bias currents than the LF356. I tested with the OPA2197 and the OPA1678 and preferred to choose the latter, as it is cheaper. The next challenge will be to implement the circuit internally to the meter, using the 9v battery. Below is an updated schematic.

[Avelino Sampaio]

Adaptation in progress.

image 1: New schematic
image 2: Original multimeter - Having to remove the base of the spare fuse to fit the board.
image 3: Phenolic board used.
image 4: I cut the board, making the most of the space. Note that I will use the base of the cover's fixing screw to secure the PCB.

The next step will be to attach the components to the PCB.

[Avelino Sampaio]

Project practically finished. Tomorrow I will bring some sample measurements of each scale.

[Avelino Sampaio]

Another update was necessary, aiming to guarantee the integrity of the opa1678, from a possible overvoltage of a charged capacitor, put under test.