CC Load to test Low Voltage/High Current Solar Cells - Page 1
Forum section: electronics / projects

author: BHopkins123
author_rank: Newbie
timestamp: October 23, 2018, 11:57:42 am
content: Hello there -I am currently looking to design a variable constant current load for testing a solar cell, however these cells are unique in that they will have a Voc ranging from 0 to 1V and Isc ranging from 0 to 3A.I was originally designing a circuit based off the video Dave did in episode 102. I had been having some issues with that circuit that when I applied feedback to the opamp from the sense resistor, i would get no output from the opamp to the gate of the transistor. In trying to find out the problem for this, I found that those kinds of circuits require the DUT to have a supply voltage greater than the voltage at which you're driving your opamp.Which led me to finding a project where another individual was testing solar cells and they used some sort of large capacitor/relay/bleed resistor setup, which I am not familiar with at all.https://www.instructables.com/id/IV-Swinger-2-a-50-IV-Curve-Tracer/]See that here: https://www.instructables.com/id/IV-Swinger-2-a-50-IV-Curve-Tracer/At the end of all this, is there a way to achieve a CC load given the parameters of my cells?

author: cbc02009
author_rank: Regular Contributor
timestamp: October 23, 2018, 12:09:26 pm
content: The link you share gives an IV curve for the panel, not a CC source. If you are trying to test long-term power generation, then I don't think that is what you are looking for.GreatScott on youtube has the design for a constant current load here:I've also seen some relatively cheap ones on amazon in the $20-$50 range if you search for constant current load, but I haven't tested them, so I don't know how accurate they are, or if they're any good.https://smile.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=constant+current+loadHope that helps! I'm also working on a solar project right now.

author: max_torque
author_rank: Super Contributor
timestamp: October 23, 2018, 12:18:02 pm
content: if you want you load to absorb current right down to zero volts (and are you sure this is the case?) then you're load will need a 'ground' reference that is less than zero volts, ie negative!!If say, more sensibly, you want to dump 3A at 100mV, then your load impedance clearly needs to be less (in total, including all wiring) than 33mOhm........ (which means careful design of your wiring and probably multiple control Fets and a very load value of sense resistor to avoid excessive burden voltage)

author: cbc02009
author_rank: Regular Contributor
timestamp: October 23, 2018, 12:35:51 pm
content: Dave actually uses one in yesterday's mailbag video to test the USB charger:https://youtu.be/OyIgZ549D1g?t=596

author: BHopkins123
author_rank: Newbie
timestamp: October 23, 2018, 01:10:36 pm
content: I'm sure it won't be 0v, maybe a few mV, I don't actually know yet. I doubt that it will be 0 volts though.

author: olsenn1
author_rank: Newbie
timestamp: October 24, 2018, 02:02:09 am
content: Different question, sorry. However, the topic seems related...I have two folding solar chargers:1. Instapark Mercury 10 -- 10 Watt dedicated USB charger2. Powerfilm F12-750 -- 12 Watt 15.4V chargerWhen I connect the Mercury 10 to my electronic load at CV (5V) I get 1.0 Amp of current (5W) and this is the most I can reliably push out of this. However, when I connect my Powerfilm charger via a switching buck converter (8-24V to 5V USB) to the same e-load under the same lighting conditions, I get 0.5 Amp (2.5W) at CV 5V. This is in line with what my devices charge at when I plug them into this USB port. However, I can switch the e-load to CC 1 Amp and get the same 1A@5V output of this solar panel this way.Obviously my solar panel is capable of outputting more power than it is. Does anyone know what I could do to fix this? Would a MPPT charge controller do the trick (or is this just for ensuring against overcharge of lead-acid batteries)?Thanks!