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
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
Attachments
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
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
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?
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?
If you look at the first schematic, we reference the voltage developed across SENSE to earth via the 10K/91K resistors. At the junctions of the 10K/91K resistors, the voltages with respect to earth are tiny, a few hundreds mV.
Why is the op-amp objecting to this?
The magnetic field of the 2 ohm resistor is coupling to the wires that go to the opamp circuit. You are producing much more voltage than you think. (I assume you did not use a wirewound).
Take 3 pcs of a 6 ohm carbon comp resistor, tie them in parallel, the three bodies in a triangular pack. Attach one of your tap wires to one end, and the other tap wire to the second end. Feed the second wire through the resistors, then where it meets the first wire, twist them together all the way to the opamp circuit.
In the geometric center of the 3 pack of resistors, there is no magnetic field, so you will not trap any time varying field.
I had to do this at 22 Khz, nevermind 200khz..
jn
Yes yes yes you are right.
I am coming back to my laptop just now after I dumped the simulation and worked on a real oscilloscope and I saw large voltages where they should not be any.
So I then fixed it as follows:
1) the source voltage is provided by the SENSE resistor which is 1R. Therefore the source impedance is 1R.
2) We can "load" the source by 10 times its impedance without causing too much trouble. I decided to "load" it by 44 times to be even more accurate.
3) I used 2 * 22R resistors to reference the SENSE voltage to ground. As a result there is not enough magnetic fields and stray capacitive currents to withstand these low resistors and all the large voltages disappear.
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Chain this monkey to the keyboard long enough, and eventually I'll make a good guess.
I believe you are simply loading down the voltage source with the 22 ohmers. The excess voltage is being caused by mutual coupling, which has inductance associated as well as some coupling constant, loading it is a perfectly acceptable method.
As long as you don't change frequency too much, you can cal it for accurate results.
jn
That's why (see the drawing)
If you use a LT6015 the V may be much larger, it's made for big common mode deviations.
Mona
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Ideally there should be no connection between the two sides, like birds on a wire, if you see what I mean. The problem is at those frequencies, capacitive and inductive leakage. On the breadboard, the RSENSE is 5 cm away from the op-amp, and around 5 cm away from the resistors network, and yet, still the op-amp sees like +/- 20V on its inputs.
By loading the RSENSE the problem goes away. And the "load" is not really since it is 22R, 22 times less than the source, much better than rule of 10.
It is not hi-fi for fishes, it is a specially designed device which for most part resembles a hi-fi amp.
This circuit is better than single resistor loaded transformer.
1. maintains phase better
2. lower reflected impedance to the transformer primary
3. lower voltage seen on the transformer windings
Diodes are for amp protection in the absence of op-amp power
Resistor to ground has been required in some cases for stability.
Doug
1. maintains phase better
2. lower reflected impedance to the transformer primary
3. lower voltage seen on the transformer windings
Diodes are for amp protection in the absence of op-amp power
Resistor to ground has been required in some cases for stability.
Doug
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