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# Need help with op-amp circuit

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#### Jack_K

##### New Member
The problem involves paralleling two gas gauges that each require different resistance sending units.
The current gauge has a sending unit that reads 10 ohms when full and about 210 ohms when empty. The one I want to parallel reads 10 ohms when full but only 70 ohms when empty. That means the problem ends up being how to convert one series of voltages to a lower series of voltages and maintain a semblance of linearity (it’s only a gas gauge).

I was able to measure the voltage across the motorcycle's gas gauge sending unit.
Ohms Volts
10 --- 5.85
20 --- 6.23
40 --- 7.00
70 --- 7.99
100 -- 8.80
150 -- 9.83
175 -- 10.25
200 -- 10.65
215 -- 10.85

The other gauge I want to add produced the following voltages:
Ohms Volts
10 --- 0.40
20 --- 1.33
30 --- 1.92
40 --- 2.52
50 --- 2.94
60 --- 3.24
70 --- 3.50

I’ve attached the circuit I built to convert the higher voltages to the lower ones. Unfortunately it doesn’t work.
1) With no op-amp in the socket all voltages are as expected
a) 5.85 v on V1
b) 4.3 v at V2
c) 1.85 v at pin 3
d) 3.32 v at pin 2
e) 3.32 v at pin 1
2) With an LF353/TL082 op-amp (direct replacement for LM358) plugged in I got
a) 5.85 v on V1
b) 4.22 v at V2
c) 1.85 v at pin 3
d) 2.054 v at pin 2
e) 1.443 v at pin 1

The circuit is supposed to be a non-inverting op-amp with inverting positive reference.
https://www.electro-tech-online.com/custompdfs/2009/09/slod006b.pdf A.3.5
That means it should subtract the voltage on pin 2 from two times the voltage on pin 3. That’s all just great but why did the voltage on pin 2 drop from 3.32 to 2.054 volts? What have I done wrong?

Jack

#### Attachments

• Gas Gauge Circuit-Subtract-New-4.jpg
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Without analyzing your entire circuit, if the first opamp is not saturated, then the voltage at pin 3 MUST equal the voltage at pin 2 (virtual ground).

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