OpAmp wont go change output Voltage

Thread Starter

KaPex

Joined May 5, 2024
5
Hello,
i hope this is the right place to post this.

Im using TS358CD C3G opamp to measure the current thru a resistor (1R).
the poblem im having is that the output voltage wont change when the current increases.
In a previous version, i was using the TLV9352IDDFR opamp which worked fine, the opamp is the only change ive made since i switched from SMD to THT.

Supply for the opamps is VCC (10V).
measured inputs for U1B:
5: 5.01V
6: 5.00V
7: 1.5V


what i have checked:
routing, pinout, correct resistor values

Maybe im just missing something obvious so id be more than happy if any of you could give me a hint.
 

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Thread Starter

KaPex

Joined May 5, 2024
5
So what is the voltage at Vin?

Post the measured voltages at all the nodes.
Thanks for the quick reply!

The voltages are:
Vcc=Vin=9.99V
Pin 7: 1.409V
Pin 6: 5.01V
Pin 5: 5V
Pin 3: 9.99V
Pin 2/1: 8.62V


This is with no Load on the Output so the Voltage difference at the 1R resistor is close to 0
 
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Thread Starter

KaPex

Joined May 5, 2024
5
Resistors used are 20k insted of the 18k since i dont have 18k THT but it shouldnt change anything.
Resistor values are (measured):
R13: 19.97k
R42: 19.94k
R15: 19.90k
R14: 20.05k
 

Thread Starter

KaPex

Joined May 5, 2024
5
I meant the voltages at every node in the circuit, not just U1B.
This problem exists even when i only solder the parts shown in the screenshot. Ill attach a picture of how the opamp is connected right now (VCC and GND are connected but not shopwn in the pic).

If you still want me to post everything let me know! but it will take me a few moments to measure all.

also: sorry for the file names, didnt realise you can read them haha
 

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Ian0

Joined Aug 7, 2020
10,277
Thanks for the quick reply!

The voltages are:
Vcc=Vin=9.99V
Pin 7: 1.409V
Pin 6: 5.01V
Pin 5: 5V
Pin 3: 9.99V
Pin 2/1: 8.62V


This is with no Load on the Output so the Voltage difference at the 1R resistor is close to 0
The common mode input voltage of an LM358 only extends to 1.3V below the positive supply.
If the inputs are higher than Vcc-1.3 it won't work (and in your case the inputs are at Vcc)
 

Ian0

Joined Aug 7, 2020
10,277
JFET input op-amps such as TL071 have a common-mode input range that exceed the positive supply. However, their offset voltages aren't too good.
 

DickCappels

Joined Aug 21, 2008
10,247
Because your circuit deals with both positive and negative voltages. Try and negative power supply for the negative power supply input to the op-amp and tell me what happens.
 

crutschow

Joined Mar 14, 2008
34,855
The circuit should work with a single supply if you remove U1A (connect node U1A-1 to U1A-2).
The problem is that the TS358CD C3G can't operate with an input near the supply voltage.
A buffer is not needed since the impedance at that point is very low (1Ω), so all you need is U1B.

The other problem is that the 358 is an very old design (near 50 years) and has poor output current sink capabilities so will not go below 1V while sinking the current through R42.
Increasing the 18kΩ resistors to 100kΩ will, help, reducing the minimum to about 0.5V, but you should really use a better RR op amp.
(Alternately a negative supply voltage will also allow the output to go to 0V.)

Why did you change to that old design op amp when you had one that worked?
Op amps vary significantly in their operating characteristics and limitations, unlike ideal amps, and are not randomly interchangeable.

As a replacement, the TLV915x (x = 1, 2, or 4) is a modern, low-cost RR I/O op amp with good offset and frequency characteristics, and it will pull the output down to 0V with a single supply.
 
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