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Current sensing circuit problem

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




RE: Current sensing circuit problem

Your "output" being specifically pin 5?

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

(OP)
Yes pin 5 is the out that goes to the comparator.  I have went over it a hunder times.  I floated the REF and it went to a fixed 24volt output.  

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

(OP)
I did just notice in a lot of application notes that they are showing -Vs where I am only using a single rail  24v and Gnd.  That could be an issue I suppose.

RE: Current sensing circuit problem

Cept their battery checking example seems to be that design.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current sensing circuit problem

Originator,

I think that you have guessed the problem in that the common mode input voltages do not incluce voltages near the negative rail.

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