Tiny/Cheap Solar clipping circuit

I'm building a small solar charger for my iPhone.  Ive done some tests and it seems happy to use voltage 4-6.2v
My panel is 6.8 volts Voc
The iPhone does do some weird pulsing of drawing the power, and wont start until it sees a lower voltage.
So I want to keep this really simple, no switches or converters -  just clip off the top of the voltage.
Ive tried a 6.2 volt zener across +/-  but it gets too hot.
Any ideas?

Comments

  • mvasmvas Registered Users Posts: 384 ✭✭✭
    You need to add a Current Limiting resistor between the Zener Diode & the Solar Panel
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    mvas said:
    You need to add a Current Limiting resistor between the Zener Diode & the Solar Panel
    That will limit power available for the load. If the solar panel is close to the power the phone draws, that could be a problem.

    I'd suggest a bigger zener.  Digi-Key has a 10W zener for $30; that will work for a panel up to 10W.  You can probably find one on Ebay for less.
  • SimonSolar2CSimonSolar2C Registered Users Posts: 4
    Hi - my panel is 2 watts.  I need a simple elegant solution.  Was thinking a zener controlling a transistor which is essentially a linear regulator clipping the top of the voltage but hard on pass through when the phone is connected drawing power ?
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    edited December 2016 #5
    Hi - my panel is 2 watts.  I need a simple elegant solution.  Was thinking a zener controlling a transistor which is essentially a linear regulator clipping the top of the voltage but hard on pass through when the phone is connected drawing power ?
    Then just get a 5 watt 6.2V zener and use it in shunt.  Less than 50 cents.  Clip a heatsink on it if it gets too hot. 

    Or if you want to get fancier, use a 7805 linear regulator (along with the recommended capacitors.)  If you want lower dropout try a LM2940-5.0 (5 volts.)  If you want an adjustable voltage AND low dropout try an LMS8117.
  • SimonSolar2CSimonSolar2C Registered Users Posts: 4
    I found that a simple shunt zener caused too much heat, so that it might burn the person.
    Is there a way to keep the voltage low without short circuit shunt?
    Ideally the solar panel would sit at 7v when there is no load, but the voltage presented to a load would look like 6.2v, or lower.
    When current gets drawn, the solar panel voltage comes down as the current draw increases.
    Someone suggested an NPN transisitor would do that, but Im not convinced.
    Here are two circuit options:
    Is the NPN redundant here, or is it behaving like I want ( the load will 'see' 6.2 volts but panel sits at 7v so there is no big current being dissipated when no load is connected ) :

  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    I found that a simple shunt zener caused too much heat, so that it might burn the person.

    So get a bigger heat sink.

    "Is there a way to keep the voltage low without short circuit shunt?"
    There are two ways to do linear regulation with a solar panel.
    1) Shunt (i.e. a zener.)  With this you dissipate maximum power at no load.
    2) Series (i.e. a regulator.)  With this you dissipate maximum power during loaded operation.
  • SimonSolar2CSimonSolar2C Registered Users Posts: 4
    Hi Bill, I tried using a low drop out regulator (http://www.farnell.com/datasheets/808935.pdf?_ga=2.108242309.680706486.1494294481-1373478196.1490314122) hooking it up in a standard manner.
    Sadly whilst this produces 5v at the output, it does not pull the solar panel down at all, into its MPP, so little to no current is generated.
    I only need to regulate the voltage when no current is being drawn, once the phone starts pulling power, I suppose I want to bypass the LDO regulator.
    Im surprised I cant find a simple solution for this.
    The problem with option 1) above, is that it seems to result in the 'system' settling at the zener voltage, and not transferring all the power available.
  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    >  it does not pull the solar panel down at all, into its MPP, so little to no current is generated.

    Pulling the panel voltage down will not increase the current output.  But a larger solar panel (same voltage) will.

    I am available for custom hardware/firmware development

  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Hi Bill, I tried using a low drop out regulator (http://www.farnell.com/datasheets/808935.pdf?_ga=2.108242309.680706486.1494294481-1373478196.1490314122) hooking it up in a standard manner.
    Sadly whilst this produces 5v at the output, it does not pull the solar panel down at all, into its MPP, so little to no current is generated.
    That's a 300ma regulator; is that really all you need?

    If the input is 8 volts and the output is 5 volts and you're not getting enough current before the regulator folds back, get a larger regulator.

    If the input is 5.3 volts and the output is 5 volts and you're not getting enough current, get a panel with a higher voltage.

    The problem with option 1) above, is that it seems to result in the 'system' settling at the zener voltage, and not transferring all the power available.

    Let's say you use a 5.6 volt zener.  If you are drawing so much current that the voltage drops to 5V then you are getting all the power the panel is capable of at that voltage; the zener is out of the circuit at that point.

  • BB.BB. Super Moderators, Administrators Posts: 32,598 admin
    If you can use a USB charger--This one supports 12/24 Volt battery input (it claims):

    https://www.amazon.com/Anker174-Dual-Port-PowerIQx2122-Technology-Motorola/dp/B00B8M4IMK

    Should work with an Vmp~18 volt solar panel (assuming enough sun and not too much USB load):
    12 / 24 volt input compatible with all standard cigarette-lighter sockets.

    Since it is a switching power supply--Should be pretty efficient and not run hot (like a shunt regulator would).

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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