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Building an Electronic Load inspired by Louis Scully

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kalj:
I am currently in the process of designing and building an Electronic Load, inspired by the fantastic video series by Louis Scully:



As part of this, I want to understand some parts of the schematic, and see if some simplifications can be made. Currently, I have the following questions which I need to find the answer to:

[Q1] In Part 6 (https://youtu.be/9auu8hH4IPM?t=63), he describes how he uses some feedback transistors to control limit the load of each individual MOSFET. This seems a bit unusual to me. Wouldn't it be much simpler to equip each MOSFET with its own opamp control loop? That seems like a common solution, such as the one used in this DIY kit (https://www.aliexpress.com/item/32859047052.html):



[Q2] In Part 7 (https://youtu.be/_JReIwVH7eo?t=154), he says that he was not able to get the current down to 0, and therefore added what he calls a "Current Null Calibration" circuit, i.e. a small negative offset on negative opamp input:



Unfortunately, he doesn't actually go into detail about the cause of the problem. What could be the reason? The AD8630 has a very small DC offset, so that can probably be ruled out. Could it be that the op amp is using GND as V-? I.e., could the problem be solved by instead using, ±5V rails for the op amp?

[Q3] Similarly, later in Part 7 (https://youtu.be/_JReIwVH7eo?t=946), he says that he couldn't get the voltage and current readings down to 0, motivating him to add a similar negative offset to the inputs to the ADC:



Again, this seems like a hack to me, and it would be interesting to understand the cause. Can it be caused by the same problem with the op amp not being able to go completely down to the GND negative rail?

EDIT: Can't for the love of god get the inline attachments to show up...
EDIT2: Now they're here. Thanks MarkF!

MarkF:
I think you should have a buffer between the adjustment pot and the op-amps:

   https://www.eevblog.com/forum/projects/constant-current-dummy-load-ran-a-gutter/msg3182924/#msg3182924

Vovk_Z:
Q2 is typical elecgronic load problem and typical decision.

MarkF:

--- Quote ---EDIT: Can't for the love of god get the inline attachments to show up...
--- End quote ---

The inline attachments has problems.  Dave has been working on it for a while.

What I've been doing:
  - Attach photo to end of post
  - After posting your message, click on the photo to see it full size
  - Save URL of full size photo
  - Edit your message again
  - Now place your cursor at position you want the photo and 'insert photo' (i.e. button under the BOLD button)
That is it

Example:  {img width=value} photo.url {/img}
  Replace '{ }' with '[ ]' and the 'width' is optional
 
It's best if the 'img' tag is on its own line.

Vovk_Z:
I just post an attachments and that's ok ( for me). MarkF way should work.

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