Folks, Hopefully this is the right place to ask this. I'm a software engineer with VERY limited EE experience and am trying to learn. At the moment, I've got a project that I'm trying to power using a 6v 6w solar panel, a load sharing solar battery charger, and a battery - though for this issue the battery is not in the loop. I'm also using a low drop out linear voltage regulator since based on the sun, voltages from the panel could vary.

Solar Charger

LDO (3.3v)

When I plug the solar panel into the charger and take a voltmeter to the output leads, I get something nice and consistent around 5v (the panel is not in full sun at the moment). However as soon as I attach the voltage regulator (without any load) the voltage output from the charger goes pretty haywire jumping from 0 to 5. The output from the voltage regulator is around 1.8 or so. This definitely doesn't power my project since it needs 3.3v.

I'm using all purchased parts - didn't really design any circuit myself. The part list is above. Anybody know what is happening or how to get a consistent output? Thanks

  • \$\begingroup\$ First off, did you look at other solar related questions? Second off, you have to match the load of the source with a MPPT controller or you won't get maximum efficiency and source any reasonable amount of current. The more current you draw from the cell, the less voltage it will source \$\endgroup\$
    – Voltage Spike
    Sep 22, 2017 at 3:57
  • \$\begingroup\$ With limited EE experience, I would suggest you get yourself a readymade solar MPPT regulator with 3.3 V output. \$\endgroup\$
    – winny
    Sep 22, 2017 at 12:44
  • \$\begingroup\$ I did look at other solar questions. Nothing seems to fit my situation similarly. Also this is NOT an MPPT charger according to the manufacturer. \$\endgroup\$
    – BLowe
    Sep 22, 2017 at 13:50

2 Answers 2


The PV needs a minimum current to start the converter out in the sun, Indoors is like <1% of the current and insufficient to start it. It may need 5% of max to get started.

This has to do with the frequency range of the converter and the load impedance at a low frequency of the inductor. Once current source rises, it should be able to sustain the voltage until you apply a load that exceeds the supply. This is what a MPPT regulator will do match the load impedance between load and source to enable 80% of open circuit voltage. A simple LDO wont be able to capture the full power like a motor running a low RPM. You need a converter than runs at 80% full RPM or voltage in this case to convert to 4.2V.

Try measuring the following.

  • under a 300W lamp

    • Voc (open cct.)
    • Isc ( short circuit <1A range)
    • connect battery without regulator and measure charge current
  • then connect a large cap across PV , check voltage (1000uF)

  • then attach LDO and test.

then report results

Or simply add a large cap to PV and verify LDO connections.

  • \$\begingroup\$ You're saying that I need real sun and I need to draw something with some kind of load? \$\endgroup\$
    – BLowe
    Sep 22, 2017 at 2:47
  • 1
    \$\begingroup\$ yes because the PV is a current source starting at the open circuit voltage then drops with load current. \$\endgroup\$ Sep 22, 2017 at 2:52
  • \$\begingroup\$ I can do that (I think). Like I said, pretty new to this. PV is my photo voltaic panel right? When you say converter, do you mean the voltage regulator? The charger I'm using already has a huge capacitor on it though and it's not a MPPT controller if that's what you meant. I can't find anything on Google that's a MPTT? Let me know about these things and then I can test tomorrow. Thanks! \$\endgroup\$
    – BLowe
    Sep 22, 2017 at 3:25
  • \$\begingroup\$ i meant MPPT . the LDO should not draw much current with no load but needs an output cap/. \$\endgroup\$ Sep 22, 2017 at 3:27

I received notification from the manufacturer today that this is not normal behavior. I have a faulty part.

  • \$\begingroup\$ Please don't add "thank you" as an answer. Instead, accept the answer that you found most helpful. - From Review \$\endgroup\$
    – Voltage Spike
    Sep 26, 2017 at 3:48
  • \$\begingroup\$ I edited the answer to not thank everyone for help upon your request. \$\endgroup\$
    – BLowe
    Sep 26, 2017 at 4:33
  • \$\begingroup\$ Your heart is in the right place (re the thankyou), but stackexchange has fairly rigid definitions of and requirements for the three different things you can write; questions, answers, and comments. You'll find after a while that these constraints help to maintain order and usefulness. \$\endgroup\$
    – uhoh
    Sep 26, 2017 at 6:26

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