Current sensing with OP AMP

AnalogKid

Joined Aug 1, 2013
8,251
I don't think any of the TS posts state the voltage level of the sensed current. The LM358 input stage common mode range does not include its positive rail.

ak
 

Thread Starter

LewisMF

Joined Nov 15, 2014
100
I don't think any of the TS posts state the voltage level of the sensed current. The LM358 input stage common mode range does not include its positive rail.

ak
The voltage level of the sensed current is between 1.2 and 30V DC.
I didn't state it because I didn't find it relevant in order to resolve the question I was asking.

Anyway after tying the GND's of the PSU and the op amp's together like stated in my previous post, it's working fine now :)
 

AnalogKid

Joined Aug 1, 2013
8,251
The problem is that op amp 1 is not working as it should
For the LM358 to function correctly, the voltages at both input pins *must* be a couple of volts less than the voltage on the V+ pin. The value is on the datasheet. Your input voltage range exceeds both 12 V and 24 V operating voltages.

ak
 

Thread Starter

LewisMF

Joined Nov 15, 2014
100
For the LM358 to function correctly, the voltages at both input pins *must* be a couple of volts less than the voltage on the V+ pin. The value is on the datasheet. Your input voltage range exceeds both 12 V and 24 V operating voltages.

ak
I as not aware of this.
Where about's in the datasheet does it state this parameter?
 

AnalogKid

Joined Aug 1, 2013
8,251
Electrical Characteristics table: Input Common Mode Voltage Range. The minimum is listed as 0 V, an astounding feat in 1971. Note that while the 358 is described as a single-supply part, and even has its negative rail pin labeled GND (a disappointing mistake), it can function just fine as a traditional dual-supply part.

ak
 

Plamen

Joined Mar 29, 2015
98
I'm currently designing a short circuit detection circuit for a bench lab PSU.
When the PSU's output goes above 1.5A the short circuit detection circuit will disconnect the load from the output and latch so until the load has been removed.

This is my current circuit:

View attachment 173502

Basically I pass the output through a 1Ohm power resistor and feed the voltage on both sides of the sense resistor into an op amp in differential configuration. After that the output of op amp 1 goes into the non-inverting input of a second op amp that is configured as a comparator against a reference voltage adjusted by a pot .

The problem is that op amp 1 is not working as it should and I think the issue might have something to do with the grounding.

The op amps are supplied by a separate 12V supply but the two voltages going into the differential op amp are referenced to the PSU ground.

Is it ok to feed in voltages that are referenced to a separate ground?

My question might sound a bit obvious to some people but I'm really not sure what the issue is.

All help is welcome :)
 

Thread Starter

LewisMF

Joined Nov 15, 2014
100
Electrical Characteristics table: Input Common Mode Voltage Range. The minimum is listed as 0 V, an astounding feat in 1971. Note that while the 358 is described as a single-supply part, and even has its negative rail pin labeled GND (a disappointing mistake), it can function just fine as a traditional dual-supply part.

ak
Thank you AK, I was not aware of this.

I think the best solution would be to adjust the output voltage of the LM317 to 31V DC to supply the op amps, this way I would be over the measured voltage.

When the PSU is outputting 30V DC the supply voltage of the op amps would only be 1V over (not 1.5V as specified in datasheet) but I don't think this would have a huge effect on the circuit.

I could adjust the LM317 output voltage to 32V DC and then I would always guarantee being 1.5V over the measured voltage but I don't really want to push the LM385 maximum supply voltage...
 
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