Current sensing circuit problem
Current sensing circuit problem
(OP)
I went over thiis plan in detail with an Analog Device engineer, and it appears I have the exact same schematic, but somehow this isn't working:
link 1 is my schematic, link 2 is the AD drawing.
http://melrosemagazineonline.com/ad628schem.jpg
http://melrosemagazineonline.com/ad628.gif
This concept was ok when I was using a BB2604 (audio op amp) in the same manner. I could get that older version to output a volt or so which was proprotional to the voltage drop across the shunt.
Now with the AD device, there is still a volt dropping across the shunt, but no change on the output of this new "fancy" IC I decided to use instead. The output is sitting a 7.84 volts , and drops maybe .02 when I strain the motor. It should be going up not down.
I tried a diffrent IC but no change.
Any suggestions on what went wrong?
Thanks
Todd Chapman
link 1 is my schematic, link 2 is the AD drawing.
http://melrosemagazineonline.com/ad628schem.jpg
http://melrosemagazineonline.com/ad628.gif
This concept was ok when I was using a BB2604 (audio op amp) in the same manner. I could get that older version to output a volt or so which was proprotional to the voltage drop across the shunt.
Now with the AD device, there is still a volt dropping across the shunt, but no change on the output of this new "fancy" IC I decided to use instead. The output is sitting a 7.84 volts , and drops maybe .02 when I strain the motor. It should be going up not down.
I tried a diffrent IC but no change.
Any suggestions on what went wrong?
Thanks
Todd Chapman





RE: Current sensing circuit problem
You have checked your wiring right? (Where I botch it up).
Weird scuzmatic on the motor.. I don't understand it. Does it go to ground somewhere.
You get what when you short R4?
Short R1 see what you get.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Current sensing circuit problem
Shorting the 1.2 ohm shunt resitor gives the same output!
Shorting R1 gives 23 volts. Pin 1 is the 24v supply. Pin 8 is off the motor side of the shunt, and is dropping from 23.85 down to 22.4 when a heavy load is on the motor.
The output pin five with the current gain set is putting out 7.88, and DROPS .02 to 7.86 on a heavy load. This is not right as it should go up, which would make it seem the inputs are reversed rights?
I pulled out the fileter cap just to check but ne change. I changed R1 to 15k, it just dropped the output to 1.7 volts, with still only the -.01 change on a load.
Makes no sense. I pulled the comparator out too just to make sure it wasn't screwy.
I'll call them tomorrow and see whats up. I let this one slip putting an unfamiliar ic on these new boards without breadboarding first...
RE: Current sensing circuit problem
RE: Current sensing circuit problem
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Current sensing circuit problem
I think that you have guessed the problem in that the common mode input voltages do not incluce voltages near the negative rail.
RE: Current sensing circuit problem
Worked it out with AD that Vref has to be a posotive voltage. I had it at ground for both the pin 3 and off the Rext2 location as well. Nice working chip, programmable gain plus a filter.
Thanks for the tips.