Determining current flow - diyAudio
 Determining current flow
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diyAudio Member

Join Date: Jul 2008
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
Attached Images
 HighVoltageDiffAmplifier-1.jpg (36.1 KB, 128 views) HighVoltageDiffAmplifier-2.jpg (35.9 KB, 128 views)

diyAudio Member

Join Date: Feb 2006
Location: Ashland,Ky U.S
Maybe this way.
Attached Images
 ScreenHunter_08 Nov. 04 06.49.jpg (26.4 KB, 125 views)

 4th November 2014, 12:11 PM #3 Did it Himself diyAudio Member     Join Date: Nov 2003 Location: Gloucestershire, England, UK 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. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more.
 4th November 2014, 12:54 PM #4 diyAudio Member     Join Date: May 2002 Location: Great City of Turnhout, Belgium Blog Entries: 7 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 __________________ If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham Check out Linear Audio!
 4th November 2014, 02:29 PM #5 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders 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. __________________ regards Andrew T.
diyAudio Member

Join Date: Dec 2012
Location: Flanders
Perhaps you can use a Norton amplifier ?
Have a look at this (page 13)
Mona
Attached Files
 LM3900-norton.pdf (682.3 KB, 8 views)

 4th November 2014, 02:47 PM #7 diyAudio Member   Join Date: Jan 2002 Location: nowhere 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.
 4th November 2014, 02:59 PM #8 diyAudio Member   Join Date: Jul 2008 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?
diyAudio Member

Join Date: Dec 2012
Location: Flanders
Quote:
 Originally Posted by akis 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

diyAudio Member

Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
 Originally Posted by AndrewT 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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