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Electronic load control feedback loop - Use ACS712?
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SpottedDick:
Hi All,

I'm trying to build my own "open source" 20A/24V/500W max DC load.

This is about the feed back loop that controls the MOSFET gate.

This link:
https://www.instructables.com/Arduino-Programmable-Constant-Current-Power-Resist/
goes through his project in good detail building an 8A load. To get his current sense for feedback he used an array of parallel resistors to generate 100mV per amp. This gets fed into the op-amp for feedback for control of the load MOSFET.

Feedback from resistor array to MOSFET gate:




Is there a reason I'm missing I can't substitute the resistor array with a ACS712?

I'll link the datasheet, but by coincidence, it runs up to 20A and output 100mV per amp. It's cheaper than an array of resistors and uses less heat and lowers the component count.

Here's the full datasheet:
https://www.allegromicro.com/-/media/files/datasheets/acs712-datasheet.pdf

(I'll have to shift the output of this by 2.5V, but we'll leave that for later).

I think the ACS712 is basically an analog component, so I don't see any issues. Any input on this idea?

T3sl4co1l:
Use a lower ratio for R17/R18 and smaller resistors. No point in them being 10 freaking watts.

A lower noise op-amp would be desirable, since the input signal will be that much smaller.

Remember to repeat channels of these in parallel, to reach total capacity.  One amp per MOSFET is required.

Carefully mind any offset voltage along the ground return path, against which the op-amp is sensing.  A differential configuration may be preferable.

Tim
SpottedDick:
Hi Tim.

Those are 1W resistors, he's referencing that he has 10 in parallel.

Anyway, yes, there's plenty of ways to peel a potato, but I don't want to muddy the waters at the minute for why I'm choosing this approach.

I've spare op-amp channels I could use for example to amplify signals etc...

While I appreciate the feedback, I want to keep this thread on the topic. Is the ACS712 suitable to just swap out for that bank of resistors.
Gribo:
Nothing wrong with the ACS712, except that it is NRND. There is the ACS730 and the ACS71240 which are in production.
SpottedDick:
Thanks for letting me know it's NRND I wasn't aware. I'll assume the other parts are similar and check the datasheets shortly.

I suppose under the instructible, the reason the writer states for using all analog control of the MOSFET is that doing it from the MCU using pure PID control wouldn't react quick enough. I'm worried some buffering or logic in the ACS712 (or comparable part) will make the circuit unstable. This is the guts of the question. The block diagram seems to make it appear to be a more analog part, and so it should be fine.
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