Auto ranging bipolar current meter

185

01:25:26

The scoped output shows an auto ranged  voltage proportional to the current that the current source on the top left puts out.
The 3 ranges are:
0-50uA     > 0-10V (1V/5uA)
50-500uA > 1-10V (1V/50uA)
0.5-5mA   > 1-10V (1V/500uA)
The voltage developed across the 200R-resistor by the input current is rectified and amplified x10 by the full wave rectifier. 
That voltage is compared with a threshold (100mV) by the 2 compairing opamps.
The FET-switches and logic gates makes a crude analog multiplexer (ie 4052) that sets the gain of the leftmost opamp to x1, x10 or x100.
More ranges could be added if more comparators and FET-switches would be used,  just keep the relatively high burden voltage developed by the input resistor in mind (up to 1V in this case).
I think im gonna breadboard this thing and find out to what extent real-world imperfections like opamp offset voltage and input current will affect accuracy, if it works at all that is :)
The input current should be set to under 50uA in order to make the simulation start.

EDIT: Bug fixes

published 6 years ago

LeButch

6 years ago

Nice idea, but I prefer manually ranging meters.

gravityx

6 years ago

After breadboarding i can state that the circuit works. First I tried an MC33078 for the rectumfryer but the input bias current was unbearable. Replaced it with a 072 and trimmed the offset voltage away on the +inputs which worked way better. Sure, it suffers from some temperature dependency but in my application that's ok

gravityx

6 years ago

@LeButch I can respect that, but I intend to use the meter in my HV capacitor leakage tester along with an analog panel meter and LED indicators. When using the tester I will turn a pot to control the HV regulator while monitoring the current, and therefore it is practical with an auto ranging thing. It doesn't require a whole lot of accuracy to be useful which simplifies the design.

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