[SOLVED] How to provide multiple ranges for current monitoring...

[prasimix]

I'm opening a new topic with a question started here and apologize if such duplication makes any trouble to anyone (and hope that Kleinstein, Strada916, Kean and others will follows up ). Just in short: I'd like to introduce multiple ranges (two for the beginning will be just fine: 0-500 mA and 0-5 A) for current monitoring that can be used in power supply for control loop and measurement.
It started with an idea that each shunt has it's own low Rds(on) mosfet in a series as to switch from one range to another in a "make-before-break" fashion. Mosfets will be highly saturated to provide the lowest specified Rds(on) and yes, that mean that Rds(on) has to be added into account for measurement. That should be a problem if calibration is accomplished and that Rds(on) doesn't vary a lot with current and temperature. I don't talk here about "reference instrument" that has to be very accurate, but rather something that can be used in DIYers/hobbyist bench top.

Many thanks in advance for your timely response.

[jaromir]

Perhaps you can find some answers in one of Dave's videos

especially in the second half of the video, but watching it all can't hurt.

[eblc1388]

Place the two shunts in series, and use just one MOSFET to "bypass" the higher resistance one when changing to 0~5A range.

Obviously you have to also switch the voltage drop across the range but with just three wires going to the shunts, you'll need to switch between two wires only, while the remaining one being the common which needs no switching.

Note: the Rds of the MOSFET does not contribute to any of the shunt voltage drop