I am trying to find an error in a circuit I was given. I have been told that his is the schematic for a working piece of hardware that I need to replicate. I have breadboarded the given schematic but the ADC it is feeding is giving different/invalid results using the same software as the working unit. I am wondering if I was given a schematic with an error in it. Can anyone help me analyze this design for function/errors? I am having difficulty getting through it. The goal is to measure current to the 24vac solenoid.

 

The LM358A will not be happy with the input signal going BELOW ground.

Absolute maximum input voltage is -0.3 Volts max, from the data sheet.
Common mode voltage range does not include negative voltages.

If you need to replicate this, better to fix the bad design too.

 

hi avg'.
This is a LTSpice simulation of the circuit you have posted, it needs redesign
I have assumed a relay coil current of +/-1 Amp.

Is this a college assignment.?
E

 

In schematic posted the diode seems to be misplaced/not needed. Or
was there to clamp an input trying to go below ground ? In any event
does not look wired right.

Gain in circuit also does not look right. You need to find peak V across
sense resistor, then divide that V into max ADC input V to find G you
need.

A NI OpAmp stage looks like -



Regards, Dana.

 

The LM358A will not be happy with the input signal going BELOW ground.

Absolute maximum input voltage is -0.3 Volts max, from the data sheet.
Common mode voltage range does not include negative voltages.

If you need to replicate this, better to fix the bad design too.

Sorry, that designation on the schematic is wrong. The eagle property specifies an OP262.

 

hi avg'.
This is a LTSpice simulation of the circuit you have posted, it needs redesign
I have assumed a relay coil current of +/-1 Amp.

Is this a college assignment.?
E

No college assignment, just a project. Real life current is about 200ma after settle. Any suggestions?

 

In schematic posted the diode seems to be misplaced/not needed. Or
was there to clamp an input trying to go below ground ? In any event
does not look wired right.

Gain in circuit also does not look right. You need to find peak V across
sense resistor, then divide that V into max ADC input V to find G you
need.

A NI OpAmp stage looks like -

View attachment 151790

Regards, Dana.

Agreed. And the prior version of this circuit looked more like that but did include a schottky diode on the output in the same position of the simple rectifier specified in the "current" design that they claim works. I assume it is for rectification of the output where the ADC expects a more constant single ended voltage. I have seen a diode in this position on other current sense designs and am trying to brush up on my op-amp skills. It has been a while since I have worked with them.

 

hi avg'.
This is a LTSpice simulation of the circuit you have posted, it needs redesign
I have assumed a relay coil current of +/-1 Amp.

Is this a college assignment.?
E

Also, V+ is 3.3vdc

 

hi avg
OK, 3.3Vs
If 200mA when settled, any idea of Ipeak .? [ there is no suppression diode across the relay coil]
What Vadc to you want for 200mA.??

E

 

hi avg
OK, 3.3Vs
If 200mA when settled, any idea of Ipeak .? [ there is no suppression diode across the relay coil]
What Vadc to you want for 200mA.??

E

I will have to look into the code to see what voltage it is expecting. I was hoping not to have to do that. The max current available from the 24vac supply is 1A. The spice model for these parameters is interesting. If you could update with these parameters, I would greatly appreciate it!!!

 

hi avg,
Using only a single 3.3Vdc supply with LM358, to measure a current range of +/-1Amp AC, this is one option.
Your original circuit is a non starter.
E

 

hi avg,
Using only a single 3.3Vdc supply with LM358, to measure a current range of +/-1Amp AC, this is one option.
Your original circuit is a non starter.
E

Thank you for that but I do not have the option to completely redesign this circuit yet. My assumption is that the circuit is "mostly" correct but something simple is wrong. Perhaps a value is incorrect or a part is misidentified. I have confirmed that parts are in place matching the schematic but cannot yet desolder everything to confirm values. The circuit does use the OP262 instead of the lm358 identified in the schematic so I know there is at least on error. Can you think of a simple change to make this circuit work? The designer is very slow to respond and I need to move forward. Thanks!

 

I am trying to find an error in a circuit I was given. I have been told that his is the schematic for a working piece of hardware that I need to replicate. I have breadboarded the given schematic but the ADC it is feeding is giving different/invalid results using the same software as the working unit. I am wondering if I was given a schematic with an error in it. Can anyone help me analyze this design for function/errors? I am having difficulty getting through it. The goal is to measure current to the 24vac solenoid.

Isn’t it just that pins 2 & 3 of the op-amp need to be transposed in the original posted circuit?

 

Isn’t it just that pins 2 & 3 of the op-amp need to be transposed in the original posted circuit?

If so, they made that mistake on two versions of this circuit.

 

hi avg'.
This is a LTSpice simulation of the circuit you have posted, it needs redesign
I have assumed a relay coil current of +/-1 Amp.

Is this a college assignment.?
E

E,

I am trying to duplicate your results....and learn LTSpice at the same time. I have created the schematic but am struggling to recreate your 24v, current and Run setting to duplicate the results of your first model. Can you provide more detail on your settings? The only difference in my schematic is using a LT1498 op-amp to match the rail-rail version on the actual board.

Thanks in advance!!!

 

hi avg,
Is this easier to follow.?
R2R OPA's
E

 

hi avg,
Is this easier to follow.?
R2R OPA's
E

Can't thank you enough! Just one error. Can't find potentiometer CA3140 on load. I searched for it but only found references to obsolete op-amps for the part. Is this a custom part you built?

 

hi avg,
Is this easier to follow.?
R2R OPA's
E

Also, these asc files appear to be your second models. Can you send the 1st one you built that matches my original schematic. FYI, I am trying to find a way to make that circuit work with minimal changes since I am forced to work with the existing PCB.

Either way, THANKS!

 

hi avg,
Is it post #3 diagram.?
E

 

hi avg,
Is it post #3 diagram.?
E

Yes.