Open Source Meter

[Piedtyper]

I’m an Electronic Engineering Tech student in my second year and I’m planning on a relatively simple meter for my cap stone project.

I’m almost as old and dirt and was a computer programmer/IT type for about 20 years.  It’s been very enjoyable competing with the significantly younger minds!  I’m confident in any of the programming needed but, of course, I’ll be challenged on the electronic design portion.

I’ve tried to keep the specs simple to start with the hope I can add to them in the future.  This will be completely open source and since it’s for school I’m not looking for anyone to do the work on at least the first cut.

As to the future…a hand held with much the same capabilities outlined by Dave in the video would be a long term goal but based on the many comments it might not be reasonable.  Time will tell.

Initial specs

1.   High input impedance in the range of 10’s of M Ohms for voltage and low impedance … less than 10 Ohm for current.
2.   Auto-ranging between 200 to 0 AC/DC V and input protection for > 200 V
3.   Accuracy in the range of 4 ½ digits
4.   Two channel with fully isolated grounds
5.   Channel 1 measures V as stated above
6.   Channel 2 measures current up to 1 A with auto ranging and maximum resolution range of 0-100 mA
7.   Simultaneous measurement of V and I for power calculations


Design challenges and possible solutions:

1.   Auto-Ranging
a.   Start with 0-20V AC/DC only and design for a further attenuation stage before this stage.  All switching in this state would be solid state.  The stage in front would probably need to be a relay of some sort (digital I’d think) and would be designed to simply knock down the input voltage to the 20V range.
2.   Channel Isolation
a.   Combination of buffers and opto-isolators for I2C communication between the micro on channel 1 and the ADC/measurement circuit on channel 2.  Channel 2 will need to auto range the current up to 1 amp.
b.  Thanks to the input of David Hess I'm planning on using a DC-DC isolated converter to power the second channel.
3.   User Interface/Output
a.   VERY simple to start.  Probably just an LCD displaying Voltage, Current and Power.  No user input except perhaps to display or not display power if both channels are in use.  It’s possible the measurements are discrete and shouldn’t be used for any power measurement.
4.   Input Protection
a.   After watching the post on meter input protection I'll go with a diode bridge for current and thermistor/MOV's for voltage.
5.   AC Voltage
a.   Use a RMS-DC chip.  It looks like this is possible.  Just feed in the appropriate level of AC and it spits out a DC value for RMS.

Questions:
1.   What’s the best way to handle negative voltage?  The ADC’s I’m considering have a range or 0-2.048V with an internal reference.  How should I handle this?  I’d much rather not have to create a negative supply.

If you are using isolated supplies so the inputs are galvanically isolated, then adding a negative supply is simple.

Instrumentation ADCs usually have no trouble handing negative inputs but the input can always be offset by half of the ADC input range.

I don't understand...yet.  I've got some ADC's on order from Digikey so I'll experiment some.

2.  There are a few examples of auto ranging voltage schematics including a couple from Intersil.  Is it possible to auto range from 200 "ish" down to zero without using relays?  If so where should I start to look?  Is the plan of starting with auto ranging 0-20V and then looking at a way to deal with 20-200V later at least reasonable?

Many Thanks for any input.  I've started a new thread as advised.