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  • Author Author: Jan Cumps
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Programmable Electronic Load - Analyse the Summing Node Zero Point

This blog documents investigates the feedback node of the electronic load that , and are designing.

It's an important spot in the load's design. It measures the set point and the feedback from the output.

When the output is driven to 0, it should be on a potential as close as possible to 0 V.

On the first prototype it's -0.2 V. Not so much off, but the negative value  influences our ADC measurements.

This document checks how we can get this node to 0 V.

image

 

Because this document is evolving, some comments below may be out of sync with the content. That's because the content is adapted based on the conversation.

The measurements taken here are based on the original design, without R32 in place and U3B + tied to ground.

The current sense side of R7 is connected to ground, and a variable negative voltage from 0 V down is applied to the current sense side of R8 to simulate current being sensed.

 

The circuit isn't complex. The set point is driven by a DAC. It's set to 0 for this test.

The second input to this node is OpAmp 3C. It has both inputs tied to ground so should theoretically have 0 V at the output.

On my board I measure a potential of -0.212V at the left side of R33.

I hope to get this closer to 0 V to ease the ADC a bit - its performance degrades with negative voltage at its inputs.

Like the other blogs for the electronic load, this is a working document that will be updated with findings from anyone who wants to chime in.

 

Behaviour at 0V

 

buzy image

  • in reply to jc2048

    I found it: all resistors on my bodge board are 100 Ohm instead of 100K. They overshout the 100K feedback on the summing node.

     

    my envelope witk 100K resistors contained 100 Ohm resistors.

    image

  • in reply to Jan Cumps

    I can't see anything obvious.

     

    The output of the op amp is a low impedance (much lower than the 100k). The op amp output open-loop isn't more than 100R at low frequencies and the feedback should bring it much, much lower. If you see a static voltage at the output it shouldn't be any different to what you see with the DAC providing that voltage. Is it static? What does a 'scope show?

  • in reply to Jan Cumps

    , do I make a dummy mistake here with the impedances?

    When I use the output voltage of a DAC, through a 100K resistor, the eload reacts linear(ish).

     

    When I use that output through a *1 opamp, with a voltage on the - input for zero compenation.

    The output of that opamp is a, through a 100 K resistor, to the same point as  in the original design.

    As soon as the output of the opamp, measured before the 100K resistor (same point as where I measured the DAC voltage) reaches 0, the load goes to full drive.

  • in reply to Jan Cumps

      wrote:

     

    No success yet.

    I am able to drive the output after R5 to 0 by playing with DAC 1 and 2, in esssence taking away the offset.

    But any little voltage I go over that voltage fully drives the FET, as if it pushes the integrator off balance.

    I 'll have to re-check connections, double check if I used the right resistor value for R5, measure the whole circuit probably.

    The fact that I've been working and changing components on the PCB often doesn't help with reliability either...

    I removed the compensation circuit and all works as before again.

     

    image

     

    On one side that's good news - I don't have to troubleshoot the original PCB.

    But something is wrong with my compensation circuit.

     

    image

     

    R2 gets the original DAC 1 outout,

    R1 uses DAC2 output to compensate the offset at 0

    R5 is connected to the summing node.

    In the original design, the opamp was not there and DAC 1 was connected to the summind point directly via a 100K resistor:

    image

     

    At least that is simple enough circuit to troubleshoot fairly easy ... It worked with a 471 on a breadboard ...

  • in reply to Jan Cumps

    ... I should of course measure before R5 ...

  • in reply to Jan Cumps

    No success yet.

    I am able to drive the output after R5 to 0 by playing with DAC 1 and 2, in esssence taking away the offset.

    But any little voltage I go over that voltage fully drives the FET, as if it pushes the integrator off balance.

    I 'll have to re-check connections, double check if I used the right resistor value for R5, measure the whole circuit probably.

    The fact that I've been working and changing components on the PCB often doesn't help with reliability either...

  • in reply to Jan Cumps

    The compensation circuit is now bodged in.

    image

    It's the wires on left and right that disappear through the mounting holes.  The other wires are there to probe the circuit when the LCD is mounted on top.

     

    image

  • in reply to jc2048

      wrote:

     

    That's an impressive meter, being able to graph like that.

     

    I realised a week or two ago that my old Fluke 8808A (only 5-and-a-half digits) has a RS232 connector on the back. When I looked in the manual, the 'Installation Test' involved typing '*IDN?', so it's SCPI. One project in the coming months will be to find a convenient way to get it connected to a PC. Rather than try and find a USB-to-RS232 converter, I wondered about building a box with reed relays controlled by a small SBC and doing some kind of input multiplexor, as well as the SBC talking via RS232 to the meter.

     

    Is it possible to set your meter to run the test many times and then draw error bars at each vertex so that you can see visually the variation?

    If you enroll in the Analog Discovery 2 + LabView Home Bundle, and you are selected image - you could use its LabVIEW serial driver.

    If you'd would consider putting an RS323-over-USB connector in (you can make one of a MSP432 LP and 'convert to 3.3V' circuitry without programming - if you only need the TX and RX) , you can make the meter-PC connection look like a COM port.

    If your PC has a serial port, that 'd be even better. Rumour says that most motherboards in PCs still have a serial port connector hidden somewhere.