Hi all,
My design is to control the current and it to be stable.
Current Range is 040A . What I made is in my PCB , I made a
wider portion of copper coating ( like shunt ) for feed back
and at two end I measure the voltage in term of mVs, and I need to amplify
0  10V ( 0  40A) . I used LM324 OpAmp to amplify . I got the nonlinear
problem .
Because of very high gain needed , I put 100 Ohms input resistor ( Very low
) .
1) because of that current region can I put low resistor like this .
2) up to 4 A current i got negative voltage O/P after postive volt
O/P is coming ,
( need to put offset volt control ) .
Is there any way to solve this problem .
Best regards
Ramesh

Non Linearity problem (OPAMP)
Started by ●June 25, 2003
Posted by ●June 25, 2003
Hi Ramesh,
I think your problem is the offset voltage of the LM324 that is around 3mV,
that in your case is problem.
I advise you to use the OPA277 that have a low offset voltage around
100uV.
For I help you better I need to see your schematic. If you need more some
thing send me your schemetic, may be I can
help you.
Best Regard,
Gilmar Ribeiro de Oliveira.
Development Engineer.

Posted by ●June 25, 2003
Hi Ramesh,
I think your problem is the offset voltage of the LM324 that is around 3mV,
that in your case is problem.
I advise you to use the OPA277 that have a low offset voltage around
100uV.
For I help you better I need to see your schematic. If you need more some
thing send me your schemetic, may be I can
help you.
Best Regard,
Gilmar Ribeiro de Oliveira.
Development Engineer.


Posted by ●June 25, 2003
First, are you sensing on the high side (near V+) or low side (near V )? If you use a conventional high side shunt then the easy way to get the measurement is with a Linear Technology LT1787 Precision High Side Current Sense amplifier. I use this with a 40A 050mV shunt on a DC motor. It has an internal gain of 8 so the 50 mV signal comes out at 400 mV. Multiplying by 5 and offsetting by 2.5V sets the signal in the center of a 05V A/D converter. In any event, you are measuring a differential voltage and would probably be well served to review the recent discussion on Instrumentation Amplifiers  also with a Subject of 'Linearity'. You can choose whether to build an IA with 3 Op Amps or just buy the thing already built. The Texas Instruments INA125 and INA126 look promising. The best discussion of single ended railtorail Op Amps for offset and scaling is in "Op Amps For Everyone"  a free book available at www.ti.com.  In , R Ramesh <ramesh_21_in@y...> wrote: > Hi all, > > My design is to control the current and it to be stable. > > Current Range is 040A . What I made is in my PCB , I made a wider portion of copper coating ( like shunt ) for feed back and at two end I measure the voltage in term of mVs, and I need to amplify 0  10V ( 0  40A) . I used LM324 OpAmp to amplify . I got the nonlinear problem . > > Because of very high gain needed , I put 100 Ohms input resistor ( Very low ) . > > 1) because of that current region can I put low resistor like this . > > 2) up to 4 A current i got negative voltage O/P after postive volt O/P is coming , > ( need to put offset volt control ) . > > Is there any way to solve this problem . > > Best regards > Ramesh > > >  > 
Posted by ●June 26, 2003
Thankx a lot .
Here I attached my simple circuit to control the current and stablize it .
For ur information SHUNT is the part that I have mentioned in last mail,
where I get the feedback . If u got any more Idea plz forword the circuits
,
best regards,
Ramesh
Type: application/pdf 
Posted by ●June 26, 2003
Without a description I am not certain what you are after with the circuit. I ran it through PSPICE (Student) {FREE by the way} and got the following results with both pots set at 10K. For 0V input: With a command of 0V output is 2.2V, command of 5V yields 7.7V and a command of 10V yields 13.2V. So the loop gain for the command is 11/10 or 1.1 with an offset of 2.2V. Or, for the form y = mx + b; Vout = 1.1 * Vin + 2.2. Looks very linear to me. Now, for a command of 5V which we know will have a DC output of 7.7V and an input AC signal varying from 20mV to +20mV (40mV pp) the output varies from 5.7V to 9.7V. Again, this is linear around the command input of 5V with a gain of 4V / 0.04 or 100. Of course the simulation is assuming ideal op amps and dual supplies (+15,15) and simulations are not the real hardware. Nevertheless, you can apply 0 signal and expect to get the results above. Give a command of 5V with a signal source of 40mV pp you should come close to the results of the AC analysis. As to how the MOSFETs react to the output  I didn't get into that. Perhaps the nonlinearity is in the drivers, not the amplifiers. Then again, simulations are sometimes just plain wrong!  In , R Ramesh <ramesh_21_in@y...> wrote: > Thankx a lot . > Here I attached my simple circuit to control the current and stablize it . > > For ur information SHUNT is the part that I have mentioned in last mail, where I get the feedback . If u got any more Idea plz forword the circuits , > > best regards, > Ramesh >  > 