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  • Author Author: Jan Cumps
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Programmable Electronic Load - Measurements Part 1: Control Circuit

Jan Cumps

Analog Time !

I've prepared the DC Load to measure the behaviour of the circuit. In this blog series I'll show dynamic aspects of the instrument.

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The first blog shows the points of interest and some initial teaser measurements. If you want a particular measurement to be done, ask in the comments.

 

 

Test Points

 

I've wired the main points of the control circuit - an opamp integrator:

  • the inputs of the integrator (set point and measured feedback)
  • the output of the integrator
  • the gate of the power mossfet.

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This should be enough to get a view into the dynamics of the circuit. In hindsight I could have also captured the inverting input of the integrator but I didn't.

 

First Setup: Square Wave input signal.

 

has documented the behaviour using a simulator. One of the exercises was the reaction of the controller circuit on a square wave input.

Programmable Electronic Load: Dynamic Behaviour: Part 1 Overview

 

I've replicated this by passing a pulsed 0.6 V signal to the dc load and try to keep the current to 200 mA.

On the captures below, the

  • yellow signal is the square wave 0.6 V 20 Hz input voltage to the dc load.  It isn't a perfect square because there is output capacitance on supply.
    (I don't have a square wave generator that can be loaded, so I used the TI SWIFTTm Power Module EVM module from the RoadTest I did a while ago and pulsed the output via the EN signal)
  • purple is the TTL 20 Hz signal that I used to pulse that voltage.
  • light blue is the gate voltage to of the mosfet.
  • dark blue is the measured current, taken after the opamp. Inverted because the opamp has a negative gain.

 

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The first capture shows a complete cycle:

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The next one is triggered on the rising edge:

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The last one is triggered on the falling edge:

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That's it for this blog. It's more food for thought than actually analyzing what's happening. I think it shows enough info to start a discussion though.

Parents Comment Children
  • in reply to Jan Cumps

    This is a capture of the DAC switching from 0 to raw DAC value 10000 (represents 1.165 A)

    The difference is that here, I measured over the current sense resistor with decent probing technique.

    image

    (for the next calculations, don't look too far into the decimals. the scope isn't very precise and I've measured visually using cursors)

    With the cursors, I measured -58.80 mV over the 0.05 Ohm resistor -> 1.176 A.

    Measurement at the ADC = 398.625 mV (raw ADC value: 2125)

    That means that the gain of opamp U3C and U3B is 6,77933673469388.

    (note that the schematic gives -7.8 for U3C but that is without R32. I mounted R32 which turns the gain to - 6.8)

     

     

    edit: the yellow line was supposed to be the input voltage measured by a differential probe. I had it turned off to save battery life and forgot to turn it back on. Vin = 5 V.

    Here's the same one with the probe turned on. I left the cursors in place and glad that they still match the measuremens. You never know these days ...

    image