There is a constant current source circuit. The simulation is fine, but the actual circuit is incorrect. Can you help me take a look? Thank you!

sxbo88

There is a constant current source circuit. The simulation is fine, but the actual circuit is incorrect. Can you help me take a look? Thank you! [Copy link]

 

Please help me find the problem, thank you!

maychang

What is that U2, LT1021BCH5? The linear voltage regulator chip drops from 32V to 5V to power the op amp?

maychang

Does your op amp OPA297AZ allow the input potential to be lower than the negative power supply? From the values of the components in the figure, the potential of the op amp's non-inverting input terminal is already lower than the negative power supply terminal.

maychang

The current flowing through the load RL is not only the current flowing through R6, but also the current consumed by the op amp and the current consumed by R1R5. This is obviously incorrect.

Gen_X

There are errors in the two places of the grounding point, resulting in no negative feedback in the loop, and the negative end cannot be directly grounded, so it does not work properly.

There are many circuits like this. Why don't you just copy one? Why do you have to design it yourself?

1nnocent

Node 5 is grounded, and the inverting end cannot be grounded. Are you sure this schematic is correct?

1nnocent

Compare carefully

sxbo88

Thanks

sxbo88

I changed it according to the picture, but the current flowing through R6 is about 100mA, so it should be less than 10mA according to the calculation.

The positive and negative voltages of the amplifier are 1.9V, which is normal.

chunyang

The circuit in the main post is not correct at all, and it is impossible to "simulate without problems". The existence of current does not mean constant current. Constant current means that the current in the constant current circuit can remain unchanged even if the power supply voltage or load is changed.

sxbo88

I don't know where the problem is, thanks for your advice!

maychang

sxbo88 posted on 2022-4-13 11:41 I don’t know what the problem is, thank you for your advice!

"I don't know what the problem is, thank you for your advice!"

In the figure on the 9th floor, the load RL is placed on the drain of the MOS tube.

In addition, why does the 12V power supply need a rectifier bridge?

jimtien

sxbo88 posted on 2022-4-13 10:42 It was changed according to the figure, but the current flowing through R6 is about 100mA, which should be less than 10mA. The positive and negative terminal voltages of the amplifier are 1.9V, which is normal...

1.9V/10Ω= 190mA

chunyang

First, why did you design the circuit that way?

sxbo88

According to the new diagram, I changed the amplifier and it worked fine. The voltage on R5 and R6 was changing, indicating that the current changed with the V1 input. However, the current in RL did not change. It felt like there was a supplementary current from the ground. What I needed was that when V1 changed, the current in RL also changed.

PowerAnts

U2 outputs a stable 5V voltage, the negative terminal of the op amp is grounded, and the positive terminal is also 0V.

R5 current = R1 current + R2 current, R6 voltage = R5*R5 current

Young man, you put the circle for detecting current on the drain of Q4, it is a constant current source, otherwise you have to add a constant U2 to power U3.

PowerAnts

In your picture, the power supply of the power part is positive and negative floating, which will kill many people.

sxbo88

Originally I used this circuit, but there was a problem at the beginning, the current was extremely large.

The main purpose of using this circuit is to transmit signals over long distances using a constant current source (two-wire system, which transmits signals and provides power). I wonder if there is any good way to do this? Thank you!

PowerAnts

Open your eyes and look carefully at the 4 red circles below

sxbo88

ADA4091 can be powered by 5V or 15V