Op amp circuit

[rkovach12]

I am trying to build a meter to display pressure. I have a transducer that has an output from .53 volts at 0 psi to 4.53 volts at 500 psi. I want the meter to read 0 volts at 0 psi and 5.00 volts at 500 psi. I have used Ltspice to simulate the circuit and it shows no problems. When I build the actual circuit, the output is not correct. The readings I am getting are OK from 125 psi to 500 psi. (they are only off by about 2 psi which is close enough for me). The problem is the lower readings are off a lot more. At 0 psi the reading is .14, at 10 psi the reading is .21 and at 50 psi the reading is .57. I have tried to adjust the resistors, but this does not help. Is this because the simulator uses an "ideal opamp" and I am useing a 741 opamp? I have attached the schematic to show the values I am useing. I am a novice so please do not get too technical in your replies.

 

[audioguru]

Your opamp is missing a power supply so it will do nothing. It might be a dual-polarity or a single positive supply.

A lousy old 741 opamp needs a total power supply of at least 10V. It has inputs that do not work if they are closer than 4V from the positive or negative supply voltage. It has an output that cannot get closer than 3V from the positive or negative supply voltage.

 

[rkovach12]

I am useing a dual-polarity power supply from two 9 volt batteries. Would a different opamp perform better for this circuit?

 

[audioguru]

A 741 with two 9V batteries powering it and making plus and minus 9V works fine. Where the two batteries join must be connected to the circuit's ground. Each battery needs a bypass capacitor of 10uF to 100uF.
The input can be from -5V to +5V and the output can go down to 0.0V.
The load for the 741 opamp can be no lower than 2k ohms.

 

[vne147]

I'm doing a similar project. Before starting I did some reading and found this application note that helped me a lot. The range of pressure and amplifier gain discussed are a little different than what you're working with but the concepts are basically the same I think.

 

[MRCecil]

I am trying to build a meter to display pressure. I have a transducer that has an output from .53 volts at 0 psi to 4.53 volts at 500 psi. I want the meter to read 0 volts at 0 psi and 5.00 volts at 500 psi. I have used Ltspice to simulate the circuit and it shows no problems. When I build the actual circuit, the output is not correct. The readings I am getting are OK from 125 psi to 500 psi. (they are only off by about 2 psi which is close enough for me). The problem is the lower readings are off a lot more. At 0 psi the reading is .14, at 10 psi the reading is .21 and at 50 psi the reading is .57. I have tried to adjust the resistors, but this does not help. Is this because the simulator uses an "ideal opamp" and I am useing a 741 opamp? I have attached the schematic to show the values I am useing. I am a novice so please do not get too technical in your replies.

Read your post this evening and I can see multiple issues with your circuit. I came up with an alternative, which should much more closely meet your criteria with a great deal more accuracy. The schematic and DC sweep plot (0.53V-4.53V) of the 4 significant voltage points are attached below.

Calibration of the two output end points is as follows using very accurate DMM (+/- 0.1% DC or better):

1. With the sensor attached at 0 lb. pressure and power applied, measure voltage across inputs, NOT junction, of R5 & R6. Adjust R2 for 0.0000V.
2. With the sensor attached at 500 lb. pressure and power applied, measure voltage from output to ground. Adjust R9 for 5.0000V with expected load on output.
3. Repeat step 1 to ensure the voltage is still 0.0000V then measure output again as in step 2.
4. The output should then follow the sensor output, upon which the linearity of the output depends.

A few last remarks. I would strongly recommend bypass caps, .1uf, at all supply points. If you use the MC33079 that would take 2 at pins 4 & 11 and another on the 5V supply to U1 at R1.

I'd be happy to answer any questions you may have regarding the circuit. Good Luck with your project.

 

[rkovach12]

I want to thank everyone for your replies. I tried the circuit suggested by MRCecil with the following changes. I replaced U1, R1 and R2 a and b with a 50k pot. I also replaced U2 and U3 with 1458 opamps. I made these changes, to test the circuit, because I allready had these items at home. The circuit works fine with these components. I am now wondering if I should use the components I allready have, or replace them with the parts in the original circuit, as suggested?

 

[CafeLogic]

Wouldn't it be easier just to feed the signal into a differential amplifier with a gain of 1.25 and a .53 ref on the negative rail?