OPamp as a differential amplifier issue

[RenesasT]

Hi everyone,

Hope you guys are doing well.

I am trying to meaure the current through the motor using sense resistor of 10mohm.

I am using LM324 as a differential Op-amp.

The circuit description is as follows,

1) The sense resistor is connected in between 12V positive supply and motor . It means I am measuring the current at higher side.

2) The 12V positive side sense resistor is connected to non inverting terminal of Op-amp using 1K resistor.

3) The motor side sense resistor point is connected to inverting terminal of Op-amp using 1K resisitor.

4) The feedback resistor and the resitor at non inverting terminal to Gnd is 4.7K.

5) The LM324 is supplied with single supply of 5V I did not able to measure the current. Then I used single 12V supply then also I did not able to measure the current.

Please help me.

 

[KlausST]

Hi,

a schematic - even hand drawn - is more informative than a lot of words.

Please next time give a detailed error description!

One thing is rather obvious:

The voltage at the non inverting input is 12V * 4k7 / (4k7 + 1k). Which is about 10V
Read datasheet about "Common mode input voltage range".
--> It says your LM324 need a supply voltage of more than 10V + 1.5V = 11.5V
5V supply surely won´t work.


****
Another thing to consider:
The motor current may be noisy. Please decide which upper frequency you are interested in. Then use a low pass filter to suppress all frequencies above this limit.
Best if you do this at the OPAMP input.

****

I recommend to use free simulation software like LTspice.

Klaus

 

[asdf44]

Yes I think Klauss nailed the main problem: your inputs exceed their input current range.

I suggest you consider a 'current sense amplifier' or 'high side current sense amplifier'. There are many parts which are designed to solve this problem and they'll run off 5V while monitoring 12V happily.


A quick fix would be two more resistors. Like 500, between the opamp +/- inputs and GND. If you calculate or simulate this I think you'll see this will divide down the sensed signals into the acceptable common mode input range of your amplifier while having no effect on gain (perhaps unintuitively). This has a negative impact on performance for a few reasons but it might get things working for you.

 

[crutschow]

If you want to measure the high side current, then it's best to use a rail-rail op amp powered by the motor supply voltage.
The circuit below is typically used for high-side current measurement as it avoids the common-mode error of a standard differential amp circuit caused by resistor mismatch.

What's the maximum motor current you want to measure?