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I bought a cheap, generic PWM solar charge controller to test some solar panel and wind turbine ideas and decided to pull it out of its case so I could look at the PCB.

I'd assumed the solar panel input would be connected to the microcontroller that handled the PWM charging which in turn would read the battery voltage and charge as necessary and/or connect the solar panel to the battery to keep it charged.

It seems like the positive terminals for the solar panel input, the battery and the optional load output are all connected via a common pour on the PCB.

Is anyone familiar enough with this circuit to comment on whether this is 'standard' and if it is - how would the micro stop the solar panel from over-charging the battery if the two are permanently connected or disconnect the load from the battery if the battery isn't properly charged?

PCB Bottom PCB Top

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  • \$\begingroup\$ Could you post a picture of the top so we get a better idea of the circuit? \$\endgroup\$
    – jonathanjo
    Commented Apr 4, 2023 at 14:02
  • \$\begingroup\$ @jonathanjo Done. \$\endgroup\$
    – formerlolz
    Commented Apr 4, 2023 at 14:14
  • \$\begingroup\$ Very nicely labelled, too. HY1707 is indeed an n-channel mosfet per winny's answer. \$\endgroup\$
    – jonathanjo
    Commented Apr 4, 2023 at 14:19

1 Answer 1

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N-channel MOSFETs are cheaper than P-channel counterparts so if you can get away with it, you design with common plus rail and do all switching on the low side, effectively switching ground instead of plus. Trace out the entire schematic from solar panel to battery with both plus and minus connection and you'll see.

Here is a crude approximation of what's probably going on:

schematic

simulate this circuit – Schematic created using CircuitLab

You will see the same thing on for example RGB LEDs, with common plus rail directly to the LED anode and a MOSFET to the cathode, switching on the low side to ground.

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  • \$\begingroup\$ "N-channel MOSFETs are cheaper than P-channel" Why is this? \$\endgroup\$
    – jonathanjo
    Commented Apr 4, 2023 at 14:20
  • \$\begingroup\$ @jonathanjo Hole mobility versus electron mobility IIRC. It takes about 3 times as much silicon area for a given Rdson and Vds to make a P-channel than an N-channel. \$\endgroup\$
    – winny
    Commented Apr 4, 2023 at 14:30
  • \$\begingroup\$ Thanks! That is in fact exactly what I just read elsewhere, and that holes have reduced mobility. I actually remember very clearly not understanding this well in my physics classes. \$\endgroup\$
    – jonathanjo
    Commented Apr 4, 2023 at 14:33

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