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  • Author Author: Jan Cumps
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Programmable Electronic Load - Current Sense Circuit

Jan Cumps

Detailed review of the current shunt circuit of the Programmable Electronic Load.

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The circuit and measurements are discussed here.

 

Circuit

 

The current measurement circuit reads the voltage over a 50 mΩ shunt resistor that's in series with the load circuit.

 

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That voltage (always negative, because the resistor is "below" the circuit's analog ground) is amplified by a non-inverting OpAmp circuit, with gain 7.8.

 

Test Setup

 

A DMM measures the voltage of the shunt and the output of the OpAmp. Both relative to that analog ground.

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The measurement is automated.

At every iteration, the shunt voltage and the output of the OpAmp are logged.

A relay switches the DMM probe between the two measurement points.

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Automated flow detail: switch DMM probe, then take a sample.

 

I've also added a gain calculation.

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Gain is calculated as the ratio between output and input. Then logged.

 

Measurement Results

 

All data is written to the attached spreadsheet.

The extract below shows 101 measurements.

The DAC that controls the eLoad is set to 0.

A first measurement of current (I take that from the PSU), shunt and OpAmp output voltage are written.

Then, the DAC is 100 times increased, each time with 100. ANd the measurements are repeated.

The attached version (9.xlsx) has more measurements (All eLoad ADCs, and some more stats from the PSU).

 

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101 samples. PSU = 5 V, DAC initial = 0, steps 100, step 100

 

 

Here's a graph of the output vs input. The line represents the gain.

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And a graph of the OpAmps gain, with as axis the shunt voltage.

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Attachments:
eload_dp832a_shunt_circuit_characterisation_9.xlsxeload_dp832a_shunt_circuit_characterisation_9.xlsx
eload_dp832a_shunt_circuit_characterisation_18.zip
eload_dp832a_shunt_circuit_characterisation_19.zip
eload_dp832a_dmm6500_current_2 - workingcopy.zip
Parents Comment
  • in reply to Andrew J

      wrote:

     

    Will you give this a go?  I read about the thermal differences between metals somewhere else early this week and thought that it was losing game!  I guess re-calibration of instruments involves things like measuring and tweaking those potentiometers.  Interesting paper.

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    I've glued a  multi-turn trimmer to the underside.

    One side will be connected to -5, the other to +12.

    I'll use some MΩ sized resistor between wiper and OpAmp node...

Children
  • in reply to Jan Cumps

    I‘m finalising a prototype schematic at the moment so I’ll be interested to see how this pans out; I can provide spacing to add something similar in before I get the PCB made.  Did you do the maths or are you just going for it?!

  • in reply to Andrew J

    I used what I have here. I don't run a big stock.

  • in reply to Andrew J

    The components are mounted. All yellow wires are circuit additions after the initial design. The thicker grey one is a measurement breakout wire.

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    I've placed it in such a way that the trimmer is accessible when the PCB is stacked.

     

    Tuned the trimmer so that I start off with 0 V , then soldered the OpAmp inverted in circuit...

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    Next step is to get the preciser DMM out again, trim the offset away, then run the automated tests ....

  • in reply to Andrew J

    This won't necessarily help Jan much, but it might help you Andrew, or at least give you even more to think about. It's all documented elsewhere, but it's getting so hard to wade through it all I'll repeat it here.

     

    I used a different op amp to Jan. We were constrained in choice because Peter's initial choice for the circuit had an unusual [wide] footprint, but we wanted to replace it with one with better specs. Jan chose one with FET input transistors (very low current bias, fairly quiet, voltage offset not too good). I would have tried the same, but when I came to buy it couldn't get one so selected a part with bipolar transistors [LT1125] on the input instead (higher bias currents, better offset voltage spec, fairly low noise).

     

    I got around the poorer bias currents by using a differential amplifier configuration [ie I used the additional resistor placement Jan put on the board] to balance the load on each input better, and by dropping the feedback resistor values considerably [6k8 and 1k]. Reducing the feedback resistors means higher currents in the feedback resistors which reduces the contribution from the amplifier bias currents. I could afford to do that because the shunt is only 50mR. [Peter was concerned about circuit protection, and I'm compromising that, so don't necessarily copy what I've done without thinking of the other aspects of it.]

     

    I must have been lucky with which way the voltage offset went [I'm struggling to remember now], if there was even much offset at all [datasheet says 100uV max, but you'd be very unlucky to get one at the extreme of the bell curve]. I didn't trim the shunt amplifier, instead I trimmed the integrator op amp to take out the small overall offset round the current-control loop [the integrator amp has an offset too, so adjusting the current sense amplifier to read right won't be enough to get rid of the overall loop offset].

     

    This was my offset trim. [BTW I wouldn't particularly recommend the Suntan presets. They're cheap, but there's a lot of slop in the lead screw mechanism. Very frustrating if you're trying to adjust something.]

     

     

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    On a slightly different tack, that picture has reminded me that I put 20M across the integrator capacitor. That's to limit the gain at low frequency. I had a problem that the flicker noise from the op amp input transistors was taking the integrator on a random walk at very low load currents [below a milliamp or two]. The resistors don't entirly get rid of the effect, but do help to reduce it. That might be less of an issue for Jan with his FET input transistors. If you don't do a true PID scheme, with a pure integrator, you might not see it anyway.

  • in reply to jc2048

      wrote:

     

    ..... I didn't trim the shunt amplifier, instead I trimmed the integrator op amp to take out the small overall offset round the current-control loop [the integrator amp has an offset too, so adjusting the current sense amplifier to read right won't be enough to get rid of the overall loop offset]...

     

     

    I compensate both at the moment - the current sense one with the trimpot just added here, the integrator by injecting DAC2 output.