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DC dummy load circuit calibration

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VEGETA:

--- Quote from: ledtester on May 29, 2018, 11:23:11 pm ---You might be interested in this kit:

https://www.aliexpress.com/item/DIY-Kits-150W-10A-battery-capacity-tester-adjustable-constant-current-electronic-load-discharge-Test/32870007246.html

The schematic looks very similar to what you are doing (see attachment).

Another forum member turned this kit into a completed project. You may be interested in what he and other forum members had to say about it:

https://www.eevblog.com/forum/projects/upgraded-lm324-based-150w-72v-10a-electronic-load/msg1506610/

Note, also, that the PCB has footprints for both TO-220 and TO-247 MOSFETS - this makes it easy to experiment with different load transistors.

--- End quote ---

Seems like a nice project.

However, I don't know how much voltage does the 10-turn POT outputs to LM324 in that project. Our project here has a disadvantage of having low current passing through the POT to negative rail which could result in induced voltage from the environment that affect reading.

I couldn't figure out the voltage there, they seem to have TL431 reference (set to 2.5v?) then 22K + 10K pot? If you know please clarify.

 

ledtester:
It's advertised as a 10A electronic load spread out over the four transistors - so 2.5A per transistor.

However, you can adjust the resistor values and pot to achieve a different range. In fact, you may need to do this as demonstrated in this video:



Also check out his other videos on this project - they are very informative.

VEGETA:
I've got a new idea to try to eliminate the shielding if possible, which makes the project immune to RF stuff. It is in the picture below in attachments.

Basically, instead of making the 10-turn pot has the 12uA current to set the output (which makes it weak vs RF as Ian said), now it has around 0.2mA of current (or even 2.5mA) which is x100 times better!!

The only problem is the divider, how to get the 52R resistor. Maybe we could get away with 47R and then adjust the zeroing pot to make for it... I guess it will work.

Waiting your opinion... I hope this really solves that issue or at least makes it weaker.

The only downside is that we have to get TL431 to be 2.5v ref (can it be 2v?) and one LM358 op-amp since we need 2 op-amps and this IC has them. These items are very cheap and affordable. Maybe TL431 is not as famous as LM358 but still dirt cheap from China.

ledtester:
By using a 2.5K resistor you can have the 10K pot tune a voltage between 0 and 2 volts using the TL431 as a 2.5 V reference.

I think it's time to actually build sometime and test it to see where the problems are. Then you will have a better idea of where you should spend your time improving on the design.

VEGETA:

--- Quote from: ledtester on May 31, 2018, 05:43:09 am ---By using a 2.5K resistor you can have the 10K pot tune a voltage between 0 and 2 volts using the TL431 as a 2.5 V reference.

I think it's time to actually build sometime and test it to see where the problems are. Then you will have a better idea of where you should spend your time improving on the design.

--- End quote ---

You are correct, I can use 1k + 1k to have 2k resistor -> 2.08A maximum which is a nice margin.

My point from the last reply is to propose a solution to RF noise issue. I hope Ian gets some time to give his opinion since I am confident it solved the problem or at least made it less effective.

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