Electronics > Projects, Designs, and Technical Stuff
Programmable Electronic Load, 0-5A
JeanLeMotan:
--- Quote from: Kleinstein on February 24, 2019, 11:16:52 am ---For current sharing it might be better to have a separate OP to drive each MOSFET and thus a separate current regulating loop. Just a source resistor may have to be relatively large if the FETs are not well matched. The separate OPs also have it easier to drive the gate capacity.
The TO220 FETs are only useful for low power - there rating is absolutely theoretical. I would not count on more than about 40 W from a TO220 case. The FET given is likely also not good at higher voltage, like more than 12 V - there is a chance of thermal runaway inside the chip.
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I confirm, I went through a few FETs when stressing the load with >20V and 1-2A. They kept failing short until I learnt what the SOA is.
For this reason I ordered the IXTP80N075L2-ND which seem way more appropriate for a load.
--- Quote from: Kleinstein on February 24, 2019, 11:16:52 am ---For a low voltage version with a low value shunt, the TL071 is not really a useful choice: it's rather high drift and quite some low frequency noise. The OP27 is much better - as a cheaper alternative an OP07 (slow) or even NE5534 (with offset adjustment and cap for compensation) should be Ok. With offset adjusted at the NE5534 (and most other BJT based OPs) the drift also gets better.
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I agree, I went for the TL071 because I could get them fast from Amazon (next day delivery) and they were the only ones available with offset trimming.
The OP27 since it's almost pin compatible except for the offset trimming - and the schematic/PCB allows it. I will try it once it arrives.
Thanks for the advice.
JeanLeMotan:
--- Quote from: exe on February 24, 2019, 04:10:55 pm ---Looks awesome! Can you please share STL files? I'd like to see how panels are connected as I'm designing enclosure for my PSU.
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Sure. I'll actually post the STL source as well in case you want to modify it. I use DesignSpark (free) for modelling.
I'll upload it in the next 30 min on github.
JeanLeMotan:
--- Quote from: aiq25 on February 24, 2019, 08:10:26 pm ---Someday I would like to build my electronic load, I think I will start off with constant current dummy load though.
Here are couple of comments:
- What is the reason for wanting 1mA/1mV resolution? By the way, I might be nitpicking but I think you meant accuracy, not resolution. :)
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I wanted to stay away from saying 1mV/mA accuracy as I don't have enough equipment to calibrate for that. With my switching, cheap bench supply and UNI-T multi-meter I'm not sure I can reach that. Factor in the temp coefficients, PWM filtering, lack of ref voltage and everything else I'm probably missing - it seemed unfair to claim that accuracy.
But the load can definitely distinguish each individual mV/mA, actually better than that since it has auto ranging and the ADS1115 is 16 bit.
--- Quote from: aiq25 on February 24, 2019, 08:10:26 pm ---- At 30V/5A your looking at 150W., your going to be dissipating a lot of heat. Assuming 2.5A load on each FET, looking at the Safe Operating Area of the IRFB3006, its only spec'ed for 0.2A for DC operation at 30V. While it is probably okay, just keep it in mind. I don't know if these are any better but I have been looking at ON Semi FDP075N15A or Nexperia BUK755R4-100E. I only have limited knowledge of MOSFET's, so maybe someone else here can here help if I made any mistake. The Nexperia part is not a logic level FET though.
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I ordered some linear FETs to replace the IRFB3006. After killing a few of them, I started reading and got to the SOA part and finally understood what it stands for.
--- Quote from: aiq25 on February 24, 2019, 08:10:26 pm ---- Typically I don't like to use 1Meg Ohm resistors due to noise issues with this high value. Considering lowering these to less than 150k Ohm or put couple in series.
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Do you think the noise will be that significant? There should be no current flowing through those resistors. What is the cause of the noise?
exe:
--- Quote from: JeanLeMotan on February 24, 2019, 09:43:36 pm ---Do you think the noise will be that significant? There should be no current flowing through those resistors. What is the cause of the noise?
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Afaik, mainly thermal noise (https://en.wikipedia.org/wiki/Johnson–Nyquist_noise) for metal film resistors. There are other types of noise, but I can't find a good article on them. So, I assume, if one doesn't use carbon resistors, thermal noise is dominating type of noise for thin-film resistors.
aiq25:
--- Quote from: JeanLeMotan on February 24, 2019, 09:43:36 pm ---I wanted to stay away from saying 1mV/mA accuracy as I don't have enough equipment to calibrate for that. With my switching, cheap bench supply and UNI-T multi-meter I'm not sure I can reach that. Factor in the temp coefficients, PWM filtering, lack of ref voltage and everything else I'm probably missing - it seemed unfair to claim that accuracy.
But the load can definitely distinguish each individual mV/mA, actually better than that since it has auto ranging and the ADS1115 is 16 bit.
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Oh ok. Yeah I'm sure the load can distinguish it. For me electronic load doesn't need to be this precise, it does all depends on the application though.
I realized now your testing CR123A batteries, yeah you might want this kind of resolution.
--- Quote from: JeanLeMotan ---I ordered some linear FETs to replace the IRFB3006. After killing a few of them, I started reading and got to the SOA part and finally understood what it stands for.
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What part did you end up ordering?
--- Quote from: JeanLeMotan ---Do you think the noise will be that significant? There should be no current flowing through those resistors. What is the cause of the noise?
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--- Quote from: exe on February 24, 2019, 10:34:05 pm ---Afaik, mainly thermal noise (https://en.wikipedia.org/wiki/Johnson–Nyquist_noise) for metal film resistors. There are other types of noise, but I can't find a good article on them. So, I assume, if one doesn't use carbon resistors, thermal noise is dominating type of noise for thin-film resistors.
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I'm guessing no, however since your setting 1mV/1mA resolution I would say it could matter. My experience is based on the automotive and test equipment industry where this does matter.
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