Reading 0mA after running Solar Panel through buck converter.

Discussion in 'Power Electronics' started by sirchuck, Mar 16, 2019.

  1. sirchuck

    Thread Starter Member

    Feb 14, 2016
    131
    2
    If I run my rover with a 7.2V battery through a 5v buck converter I get a smooth 5v at about 130mA running through my meter when connected to the Raspberry Pi load.

    I have two 5v 500mA solar panels in series for 10v 500mA in good sun. I expect the amperage is much lower normally.

    When I check the volts coming off my Solar Panel Array it shows it is running at 8.60v. Great right? That's plenty to provide a smother 5v or so I thought.

    As soon as I plug in the same buck converter I used with the battery I drop to 3.56v on the solar panel side of the buck converter and 0mA, with or without a load. After the buck converter, my meter just goes dead, which I'd expect because I believe the buck converter won't output anything less than 5v.

    So any ideas why my buck converter is dropping 8.6v to 3.56v? I figure I = V/R may come in to play here but I really don't get it.

    I tried with linear 7805 regulators too, that had about 7v going in and 4.95 coming out which was great, but the mA output was only like 10 - 20mA, so much too low to even run my RaspberryPi. Sunlight wasn't great at that point though, or maybe the solar panel is cheap and really does not provide 500mA in normal conditions.

    My Buck Converter:
    https://www.ebay.com/itm/3A-DC-DC-B...rentrq:89298a701690a99c6049588bfff7d4df|iid:1

    My Solar Panel:
    https://www.amazon.com/AMX3d-500mAh...A&qid=1552784450&s=gateway&sr=8-1-spons&psc=1

    I've given up trying to make it work at this point, I'll just use a battery, but I'd love to know why such a huge drop in voltage when I used the buck converter.

    Thanks.
     
  2. wayneh

    Expert

    Sep 9, 2010
    16,102
    6,219
    You could try measuring the current draw of the converter when it’s pulling down the voltage of your panels. It sounds like it’s drawing enough to pull the voltage below where it can function. Then it gets stuck there just gobbling current.
     
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  3. oz93666

    Distinguished Member

    Sep 7, 2010
    645
    138
    Exactly .... you mean you've measured the open circuit voltage at 8.6 V from your panel ???... once the converter is connected it draws sufficient current for the voltage to drop below 7V , which the converter must see ...

    Another solar panel in series should fix the problem , then you should get charging even on cloudy days
     
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  4. sirchuck

    Thread Starter Member

    Feb 14, 2016
    131
    2
    I have no way of testing the current draw of the converter. When I attach my volt & ammeter to the solar panel side of the converter it shows the 3.56v at 0mA. However, I think you are correct, I just needed to hear it from someone else. In order to convert, my converter likely needs even more than my given 8.6v at low amperage properly work. I assume my lower voltage 7.2 battery worked because there was plenty of amperages to use.

    open circuit voltage - Yes, when I touch the + and - on my solar panel to my meter it shows me 8.6v under my lighting situation.

    :eek: Wow, ok I think I actually see what you are saying. With low amps my buck convert needs to draw more volts so it can give a clean 5 volts as output, and because I'm only giving it 8.6 at low amps it can never meet the required 7v for the converter to make it's 5v. :D

    I just paid for an MPPT, maybe that will also help me. I will try your 3rd solar panel idea though. It's funny because that's the reason I added the second panel thinking it would help guarantee me 5v longer at decent mA value. :)

    This is the MPPT I bought, did I waste my money? Is there a better 5v MPPT you would have told me to buy or could suggest?
    https://www.ebay.com/itm/5A-MPPT-Solar-Panel-Controller-Voltage-Step-down-Module-Constant-Current/232949162645?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649

    Thanks for your help, I'm going to try rigging up a third solar panel just to test for fun!
     
  5. pmd34

    Active Member

    Feb 22, 2014
    366
    154
    You need to see how much power your solar unit can deliver, put something like a 200 ohm resistor across it and then measure the voltage across this... this will give you and us a better idea of the power level you are seeing.

    Those sort of solar cells are actually I bit of a disappointment. I have one I was playing with at work.. it can juuuuuuust about light up a 5mm red LED!!
     
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  6. wayneh

    Expert

    Sep 9, 2010
    16,102
    6,219
    I don't understand that. I think we all know what your open circuit panel voltage is, about 8V, and how you measured it. Then you measured voltage with the converter attached and saw it had fallen below 4V. It takes a current draw to drop the voltage, but you're implying 0mA. That doesn't make sense. The meter needs to be in series with the panels and converter, and you may need to place the leads into a different jack on the meter to get a measurement. Shoot us a picture if you have doubts.
     
  7. sirchuck

    Thread Starter Member

    Feb 14, 2016
    131
    2
    Well, it seems it is using all available current from the solar panel to drop the voltage, but it wasn't enough so it turned some extra voltage into a current to compensate, so I read 0mA at 3 something volts, that's how I understand it anyway from the above commenter.

    In any case, I'm using a USB to USB ammeter & voltmeter there's really only one way to plug it in so I don't think I have any wires wrong, at least it works properly when I connect my battery to it.


    Yea, the ones I linked are not bad for little projects like that. With good sunlight, I may get near what it's rated at. I just hooked up my third solar panel, letting the hot glue dry now, then I'll test on the car and on an led circuit for us to see what we see.

    Meanwhile, here is a picture of my first rover, minus proper power for your viewing pleasure. Yes, I don't know what I'm doing and I'm sure it's not super stable with those dowl rods just superglued to the car, but I'll just be happy to see it work once. :) IMG_20190317_025710.jpg
     
  8. sirchuck

    Thread Starter Member

    Feb 14, 2016
    131
    2
    Ok, updated info:

    220ohm resistor and a white LED with 3 of these solar panels = 16.7v @ I want to say 40mA but that seems high. In any case, the three solar panels together in series in cloudy weather at 9:30am gave me 17ish volts and I saw up to 140mA of power :) That was more than enough to run the Pi!

    Running the motors didn't happen, but I am starting to believe this would do it if it were actually sunny out. I think you guys figured it out as far as my buck converter and that was a bit of a mystery to me.

    Perhaps, if I ever bother to attempt this again I'll go with a bigger car that can support 4 solar panels, two sets of in series and then connected in parallel for a max potential of 10v @ 1A and see how that goes.

    These panels are meant to give 5v, but like I said in cloudy weather they were putting out 17, so ya pretty good.

    Thanks for all the help, throwing the batteries on and stamping my project a failure. :D

    --------------

    Bonus for helping:
    One thing I did for this project was collect a bunch of random information around the web to lower the power needed for my Pi and various other things that are useful to set up a new Pi you may find useful at some point. For example, the Raspberry PI 0 W Idles around 130, but I got it down to around 70mA. Have a look, makes for a nice quick setup reference: (Rasbian Stretch)
    https://w2r.com/pi/
     
  9. wayneh

    Expert

    Sep 9, 2010
    16,102
    6,219
    Im glad you seem to be getting it under control but I hope you realize how confusing your statement is. As you load your panels, the voltage drops as the current increases. At the extreme, you can get maximum current by shorting the panel and taking the voltage to zero. If you were to plot that all out, you'd get the characteristic I-V curve for your panel. Sellers often quote the open-circuit voltage and the short-circuit current to make the panel sound powerful. Those two points are on the curve (they're not lying), but are not at the same point - you can't have both at the same time.

    Anyway, there's no way the panel voltage can be pulled down from 8V to 3V without some current flowing from the panels to the converter. So you're probably measuring something else if you see 0mA.
     
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