# High side current sensing

I want to sense 30 amps of current using op amp in high side. I know that there are many dedicated current sensing amplifiers for this purpose. But I'd like to try it with an op amp. I have tried high side current sensing using the below circuit:

But my output gradually decreases when applying load. Is the above circuit correct? Or where I am going wrong?

EDIT 1 Sorry i forget to say, i want to measure the current in bldc drive. Since there will be problem in measuring current because it contains switching devices.

• Which op-amp are you using? An op-amp that will work at 40V, will usually not have a common mode input range that goes all the way to Vcc (40V in your example). – Mattman944 Jul 3 at 11:06
• Is the opamp 40V rated ? What part number. What common mode range? | Scale your opamp Vins down into the opamps common mode range. If eg R2 = R1 = 50K tghen opamp Vin max ~~= 40V/3. If R1 = R2 = 200k opamp Vinmax ~= 13V. If ... – Russell McMahon Jul 3 at 11:21
• Nihal - PLEASE answer the questions so that we can help you. The system says that you last logged in 1 hour ago but you have not answered questions asked 18 hours ago. Help us to help you. – Russell McMahon Jul 4 at 6:05
• Sorry for late reply friend. I am using LM318 and i supplied 36 voltage for that IC. 30amp is enough for the measurement. – Bud Jul 5 at 6:48

Is the opamp 40V rated?
What part number.
What common mode range?

Scale your opamp Vins down into the opamps common mode range.
If eg R2 = R1 = 50K tghen opamp Vin max ~~= 40V/3.
If R1 = R2 = 200k opamp Vinmax ~= 13V.
If ...

The main aim of this circuit is to take a floating differential voltage proportional to current and produce a ground (or other) referenced voltage proportional to current.

A secondary aim is to produce a voltage gain between Vshunt and Vout. Ideally an amplifier that handles the Vhigh and has a common mode range that equals its Vdd is available. If not then a means of reducing Vsenses into the opamps usable range is required.

The above has the effect of reducing the voltage gain of the sensor BUT achieves ground referencing. Additional amplification can be added subsequently if desired.

• Would the downvoter like to share their thought processes for my edification? Really . ||| If say R2=R4 = 100k then opamp non inverting input is at 40V/2 = 20V. This greatly reduces the Vout per current - but does allow current to be measured. – Russell McMahon Jul 3 at 11:54
• I wasn't the downvoter. However, I had flag your post as "NAA" before your edit, because at that point, your post was just some questions, and one general advice without any explicit relationship to OP's problem (and rather confusely written I must say). I suspect the downvoter had the same thoughts. Since your edit, I retracted my flag. – dim Jul 3 at 12:13
• @dim OK. Thanks. I can see I need to make my thought processes clearer. The questions targeted possible problems which were affecting his circuit and knowing the opamp would greatly help. || The "general" advice was aimed at getting the opamp into its legal operating range IF it was outside it. – Russell McMahon Jul 3 at 13:47
• For my own curiosity I have been searching in Digikey for 40+V op-amp that has a rail-to-rail common mode input range. No luck so far. High operating voltage is a special feature, rail-to-rail common mode input range is a special feature. Very hard to find both. – Mattman944 Jul 3 at 13:56