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Old 4th November 2014, 11:32 AM   #1
akis is offline akis  United Kingdom
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Default Determining current flow

Hello

I have attached two circuits below. The purpose is to use a SENSE resistor to monitor current flow into a variable load, shown as LOAD. The LOAD can change from around 50R to 200R. The SENSE resistor is 1R or 2R. The voltage developed on the sense resistor is (a) referenced to ground, (b) amplified and (c) rectified to produce a DC monitor voltage (not shown here).

As you can see the LOAD is fully floating with respect to GROUND as far as the differential op-amp is concerned.

In addition the voltage is a pure sine wave at 70V peak (140V peak to peak).

When I tried the circuit shown in the first picture, the op-amp cannot cope and inverts etc. Another op-amp I tried (a high voltage FET op-amp) blew up.

I then used the circuit in the second picture which works fine.

The problem is I was hoping that the single stage op-amp would be able to do two things at once: (a) refer the SENSE voltage to GROUND and (b) amplify it.

However the circuit in the second picture only achieves (a) ie refer to ground. I then need another stage to do the amplification.

Could anyone suggest a better way of doing this?

Thanks
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File Type: jpg HighVoltageDiffAmplifier-2.jpg (35.9 KB, 126 views)
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Old 4th November 2014, 11:50 AM   #2
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Maybe this way.
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Old 4th November 2014, 12:11 PM   #3
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Is the measuring circuit connected to mains earth at all? And what about the source? You probably need to make the measuring circuit fully floating as you may be exceeding common mode input voltage.
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Old 4th November 2014, 12:54 PM   #4
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There are a few options.

One is to use a high-common-mode input voltage opamp like the INA117. Its inputs can float up/down 200V or so, while still amplifying the differential voltage across the sense resistor.

Second option is to use a dedicated current sense amplifier - these are specifically designed for these applications, and several have a calibrated sense resistor inside their package.

Jan
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Old 4th November 2014, 02:29 PM   #5
AndrewT is offline AndrewT  Scotland
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LT have a video explaining the advantages of their fairly new opamp 6016 that can be used for low side or high side current monitoring.

6016 looks like a very useful opamp.
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Old 4th November 2014, 02:31 PM   #6
Ketje is offline Ketje  Belgium
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Perhaps you can use a Norton amplifier ?
Have a look at this (page 13)
Mona
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Old 4th November 2014, 02:47 PM   #7
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At that frequency u can use a current transformer which will isolate the amplifier from the current in the circuit u want to measure. I use Pulse P820X and find it very accurate.
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Old 4th November 2014, 02:59 PM   #8
akis is offline akis  United Kingdom
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Thanks for all the ideas, which I need to digest. I have a question though. Looking at the first picture, the voltages that the op-amp sees at its inputs with respect to the earth, are around 300mV/500mV and with respect to each other input they peak at around 800mV.

I do not understand how the op-amp has an issue with that. The problem is solved by raising the rails to around +/-20V, which also indicates that the op-amp inputs, somehow, are seeing large voltages.

But how?
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Old 4th November 2014, 03:16 PM   #9
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by akis View Post
Thanks for all the ideas, which I need to digest. I have a question though. Looking at the first picture, the voltages that the op-amp sees at its inputs with respect to the earth,are around 300mV/500mV and with respect to each other input they peak at around 800mV.

I do not understand how the op-amp has an issue with that. The problem is solved by raising the rails to around +/-20V, which also indicates that the op-amp inputs, somehow, are seeing large voltages.

But how?
Were do you see that ? LOAD and SENSE are floating = undefined voltage.
Mona
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Old 4th November 2014, 03:22 PM   #10
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Quote:
Originally Posted by AndrewT View Post
LT have a video explaining the advantages of their fairly new opamp 6016 that can be used for low side or high side current monitoring.

6016 looks like a very useful opamp.
Indeed, the LT6016 is almost a no-brainer for this.

Jan
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