hi, so i am trying to make a current limiting lower supply but im having issues with the current limiting portion. what i am using to measure the current is a TLC272/LM358 (I've tried both) op amp differential amplifier across a 1 ohm resistor and it should output a voltage equal to the current flowing. The issue is the output voltage is wayyyy off ex. 20mA running and it outputs 100mV. any ideas why? is it the wrong type of op amp? i have also attached a picture of the circuit.

 

is it the wrong type of op amp?

Yes.
You need a rail-rail input op amp.
If you look at the specs of those you used, you will see that the input voltage range does not extend to the supply voltage, as shown below.

 

R1 in your drawing shows a 1 K Ohm resistor on Vcc and to the LM 317 Input. The output of the LM 317 also is tied to ground? I also see Crutschow just commented so nothing else for me to add.

Ron

 

Crutschow, he's got dividers on the inputs, so the inputs should be OK.

Bob, a circuit like this is going to be very sensitive to resistor matching. Any mismatch will translate to a large offset error. You can try matching all 4 of the 10K resistors by hand or go with 0.1% resistors.

In addition, you are expecting the output of that opamp to be zero with no current flowing. That is not going to happen unless you use at least a couple of volts negative on pin 4.

 

he's got dividers on the inputs, so the inputs should be OK.

You are right, of course. I didn't look closely enough at the circuit.

And as you also noted, resistor mismatch could be the problem.
At a Vcc of 10V, a 1% mismatch will give an offset of 10V × .01 = 100mV at the output.

Bob, what's the current sense output voltage with no current?

What you could do is replace one of the 10k resistors with a 9.76kΩ resistor in series with a 500Ω pot.
You then adjust the output for 0V with no current.

 

Hey, thanks for the responses they have been very helpful.

At a Vcc of 10V, a 1% mismatch will give an offset of 10V × .01 = 100mV at the output.

despite the error in my schematic the resistor i'm using to measure current is 1 ohm (not 1k) so wouldn't to offset voltage be the voltage drop across the resistor *0.1. so if i was running 1.5A than 1.5 * 0.1 = 0.15mV offset at a max?

Bob, what's the current sense output voltage with no current?

The sense output voltage with no current flowing is 700mV which is wayyy to high.

What you could do is replace one of the 10k resistors with a 9.76kΩ resistor in series with a 500Ω pot.
You then adjust the output for 0V with no current.

even if i add a 10k pot in series with a 10k resistor and try to adjust to voltage drop i can only get it down to 100mV at no current.

- also quick note the video I am building the supply is from here:

he used to TLC272 op amp and seemed to have no problem with 1% 100k resistors.

 

I have configured the port for input...I want to give input as logic1. For that I am connecting that pin to 5v directly..but it's not working.do I need to add any register to make that input.

 

Bob, try putting -5 volts on pin 4 of your LM358. You really need a negative supply if you expect the output of the op amp to go to zero.

 

Bob, try putting -5 volts on pin 4 of your LM358. You really need a negative supply if you expect the output of the op amp to go to zero.

only issue is i only have a single supply dc power supply. so i cant get a negative voltage and any options to convert positive to negative seem to be complex for something that seemed to work in the video i linked.

 

There are a number of simple chips that can be used to generate a negative voltage from a positive. For example: ww1.microchip.com/downloads/en/DeviceDoc/21469a.pdf.

 

despite the error in my schematic the resistor i'm using to measure current is 1 ohm (not 1k) so wouldn't to offset voltage be the voltage drop across the resistor *0.1.

No.
The offset error is due to imbalance of the 10kΩ bridge resistors.
It has nothing to due with the sense resistor.

even if i add a 10k pot in series with a 10k resistor and try to adjust to voltage drop i can only get it down to 100mV at no current.

Not surprising.
That's why I had a 9.76kΩ resistor (the value is not arbitrary).
You can't adjust the offset correction in either direction if all the bridge resistors are 10kΩ.
And since the offset can be either polarity, you need to be able to adjust both plus or minus.
With a 1k pot, the series resistor should be about 9.5kΩ.

 

That's why I had a 9.76kΩ resistor (the value is not arbitrary).

ok so i tried a 2 4.7k resistors in series to get 9.4k than put a 5k pot in series also but it doesn't matter what way i turn it the value is 300mv+ offset. I feel like im missing something in the video i linked above it worked flawlessly and nothing i do seems to works

 

put a 5k pot in series also but it doesn't matter what way i turn it the value is 300mv+ offset.

Then something is wrong with your circuit.
Post the exact circuit you have with the pot connection.

All all the resistors 5% types as the 4.7k resistor apparently are?

 

ok so update I have tried using some 0.1% resistors and it doesn't seem to change much for the values. i still have the output to be around a gain of 6 which should be only 1. am i missing something obvious cuz nothing i do seems to work

 

ok so i figured out the problem. thanks to everyone for the info and help but the issue was a faulty breadboard... i changed it out with a new one and BAM it worked. that is 3 days i will never get back lol

 

that is 3 days i will never get back lol

You might. Painful learning experiences like this one can immunize us from making the same mistake again. It’s called wisdom, and should never be underestimated.

 

ok so i figured out the problem. thanks to everyone for the info and help but the issue was a faulty breadboard... i changed it out with a new one and BAM it worked. that is 3 days i will never get back lol

You may have spent 3 days learning so you can live 6 extra days.