mechg

Here is some shameless self-promotion:  I make and sell gauss meters that use MSP430G series chips:
 
http://gravitastech.weebly.com/index.html
 
My customers include slot-car racers who use them to tune motors,  guitar makers who build magnetic pickups, and Model-T Ford owners who rebuild their own generators.   The design is not open source.
 

Here is some shameless self-promotion:  I make and sell gauss meters that use MSP430G series chips:
 
http://gravitastech.weebly.com/index.html
 
My customers include slot-car racers who use them to tune motors,  guitar makers who build magnetic pickups, and Model-T Ford owners who rebuild their own generators.   The design is not open source.
 

Here is some shameless self-promotion:  I make and sell gauss meters that use MSP430G series chips:
 
http://gravitastech.weebly.com/index.html
 
My customers include slot-car racers who use them to tune motors,  guitar makers who build magnetic pickups, and Model-T Ford owners who rebuild their own generators.   The design is not open source.
 

Here is some shameless self-promotion:  I make and sell gauss meters that use MSP430G series chips:
 
http://gravitastech.weebly.com/index.html
 
My customers include slot-car racers who use them to tune motors,  guitar makers who build magnetic pickups, and Model-T Ford owners who rebuild their own generators.   The design is not open source.
 

Heads up to anyone trying the lower shunt resistors - I was just helping someone diagnose a problem where current readings in the mA range were almost twice what they should be, and we discovered that his range switches had more contact resistance than the ones I purchased.   Identical  brand and model of switch, but his have a resistance of almost the same value as the shunt resistor for the mA range (.1 ohm).   This effectively doubled the voltage seen by the op-amp.
 
On the PowerScope that I have been using in my daily work, I had to set the calibration constant all the way down to 811 to calibrate the milliAmp range.   Dividing .1 ohm by 811/1000 gives .123, which shows the switch resistance is about .023 ohms.   The C&K switch spec states a contact resistance of .070 ohms max, so I guess I lucked out with .023 ohms.
 
If you get a switch with a high contact resistance, you can:
 
1) Use the calibration feature to set the calibration constant really low. A value of 500 would compensate
     for a contact resistance of .1 ohms.
2) Use the original, higher shunt resistors and live with the higher burden voltages.
3) Order more switches and hope they have lower contact resistance.
4) Change the value of the shunt resistor in the source code from .1 to whatever value 
     gives you correct mA readings.  If you have a milli-ohm meter you can measure the
     switch resistance and add .1 ohm to it.
5)  Bypass the switch and hardwire the PowerScope to one of the two ranges.
 
If you do # 4,  remember that in the uA range there is a connection from the switch to 
the MPU that goes to ground when in the uA range to tell the software what range
to use.   See Eagle screen shot below.
 
http://www.flickr.com/photos/28541023@N05/9099381635/
 
If you decide to set the calibration constant low to compensate, you can hold the button down and it will decrement automatically.
 
When using the switch, it will probably be necessary to calibrate the PowerScope periodically in case the switch contact 
resistance changes over time due to oxidation.  And since it is a movable contact, it can vary slightly each time you cycle the switch.
 
Using the lower shunt resistors kind of pushes the PowerScope up to the limits of what it was designed to do.  Maybe in a future iteration I will redesign it so the switch is not included in the measurement.
 
    Greg

It was pointed out to me that the 4-pin male header on my parts list,  3M9323-ND, is actually 2mm pitch, not .1" as is needed for the PowerScope. This one has the correct pitch:  3M9449-ND
 
I have updated the parts list on the project site with this change.
 
Anyone who purchased the 2mm header can certainly use it if they can break the pins apart and solder them individually.

Here is a good article on various approaches to current measurement.   Of the approaches discussed, the PowerScope falls into the category of low-side current sensing using an instrumentation amp.
 
 
http://www.eetimes.com/design/industrial-control/4236380/A-Current-Sensing-Tutorial-Part-II--Devices

Ok,  I found the issue.   When I originally tested the storage/retrieval of the calibration constants,  I must have been working at a lower clock speed.
 
It appears that the "information" flash segment, where I store the calibration constants, does not like being written-to at 16MHz.   By dropping down to 1MHz, just while data is being saved, all of the constants are being stored and retrieved correctly.
 
I had to add this code around the call to storeCalibrationData() in main() :
 
---------------------------------------------------------------------------------------------
BCSCTL1 = CALBC1_1MHZ;          // Set DCO to 1MHz during flash write
DCOCTL  = CALDCO_1MHZ;         //  as 16MHz is too fast
 
storeCalibrationData();
 
BCSCTL1 = CALBC1_16MHZ;         // Set DCO back to 16mHz
DCOCTL  = CALDCO_16MHZ;
--------------------------------------------------------------------------------------------
 
When they said the chip could run at 16MHz,  I assumed they meant everything would work at that speed.  
 
Bad assumption !
 
     Greg

43oh Store link   I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.    http://code.google.com/p/powerscope/   This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.   Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.    The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com I don't make any money from oshpark.   Greg Glenn
http://gravitastech.weebly.com/

43oh Store link   I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.    http://code.google.com/p/powerscope/   This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.   Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.    The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com I don't make any money from oshpark.   Greg Glenn
http://gravitastech.weebly.com/

43oh Store link   I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.    http://code.google.com/p/powerscope/   This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.   Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.    The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com I don't make any money from oshpark.   Greg Glenn
http://gravitastech.weebly.com/

43oh Store link   I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.    http://code.google.com/p/powerscope/   This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.   Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.    The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com I don't make any money from oshpark.   Greg Glenn
http://gravitastech.weebly.com/

43oh Store link   I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.    http://code.google.com/p/powerscope/   This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.   Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.    The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com I don't make any money from oshpark.   Greg Glenn
http://gravitastech.weebly.com/

43oh Store link   I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.    http://code.google.com/p/powerscope/   This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.   Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.    The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com I don't make any money from oshpark.   Greg Glenn
http://gravitastech.weebly.com/

43oh Store link   I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.    http://code.google.com/p/powerscope/   This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.   Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.    The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com I don't make any money from oshpark.   Greg Glenn
http://gravitastech.weebly.com/

43oh Store link   I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.    http://code.google.com/p/powerscope/   This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.   Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.    The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com I don't make any money from oshpark.   Greg Glenn
http://gravitastech.weebly.com/

43oh Store link   I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.    http://code.google.com/p/powerscope/   This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.   Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.    The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com I don't make any money from oshpark.   Greg Glenn
http://gravitastech.weebly.com/