LDO regulator OR buck converter

Discussion in 'General Electronics Chat' started by kaning, Jun 8, 2014.

  1. kaning

    Thread Starter Member

    Jun 2, 2014
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    im having all this trouble because i have 6v 3,7w solar panel. i want to feed usb charger (5v), nimh charger (5,6v to charge 4nimh's) and lipo charger (4,2v) up to 500mA each(max even because this solar panel cant give to much more). not at the same time ofcourse. is it worth all this trouble find adjustable low drop converter or a step down buck converter would the job better? they dont have voltage drop right? and so what happens to current? small waste or big? thank you

    buck converter like this http://ptrobotics.com/2647-lm2596-dc-dc-buck-converter-step-down-power-module-output-125v-35v.html
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Are you trying to charge the batteries directly with the output of this power supply or is it the input to battery chargers??
     
  3. kaning

    Thread Starter Member

    Jun 2, 2014
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    nevermind question about voltage drop :D
     
  4. kaning

    Thread Starter Member

    Jun 2, 2014
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    if i would use this step down buck converter the output would feed them directly. then with microcontroller i would make smart cut off system.
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    I suggest you measure the output of your solar panel with no load. If it is 7 volts or less then you can connect it directly to you USB charger without a regulator. Under load the voltage of your solar panel will drop.
    Your USB charger is a kind of regulator. A part designed to work at 5 volts input, normally has a max input range of 7 volts.

    Make measurements first. Just a suggestion.

    Mark
     
  6. kaning

    Thread Starter Member

    Jun 2, 2014
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    yes, my open circuit voltage on my panel is around 7v. my usb charger circuit is basically the regulator with a couple of capacitors. are you saying connecting solar panel directly to usb device with lets say a capacitor in parallel between them?
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think you are in for a hard time because of the solar panel. I think a shunt regulator might be your best bet.
     
  8. kaning

    Thread Starter Member

    Jun 2, 2014
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    wouldnt a buck boost converter do the job also??

    by now i think its my best option
     
  9. Lestraveled

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    May 19, 2014
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    Actually ronv suggested the safest approach. A shunt regulator. (Good idea ronv) A shunt regulator will load the solar panel and keep its output at 5 volts. As the charger draws current the shunt regulator un-loads the panel, keeping the voltage at 5V. If the charger demands all the current the panel can deliver then the shunt regulator essentially turns off.
     
  10. kaning

    Thread Starter Member

    Jun 2, 2014
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    after reading some things about shunt regulators i came up with this
    Vin - 6v
    Vout - 5v
    Iload - 500mA, but lets do math for 600 mA

    For in series resistor V=RI, 6/0,6= 10Ω
    for a 5,1v zener didoe, and since we want all the current possible that solar panel can draw zener diode should be P=VxI, 5.1x0.7(to be sure)= 3.7W

    i added those filter caps because i've been used to see them in theese kind of regulating circuit, filtering some oscilations. maybe i've never understood what values to use or to calculate them. maybe 0.1 uf would be enough?

    Anyway would this circuit work for the purpose? how to calculate losses, Vdrop?
     
    Last edited: Jun 9, 2014
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    Your circuit in post #10 may not be the best way. Here is why. When your battery charger is demanding max current the output of your solar panel will drop. You will also loose voltage through the 10 ohm resistor. A lot of voltage .5A X 10ohm = 5 volts. There goes all your voltage, so that 10 ohm resistor has got to go.

    Lets use the solar panels intrinsic "constant current" feature. Take your circuit, and remove/short the 10 ohm resistor. Get five 5.1V 1 watt zener diodes (1N4733A) and five 1 ohm 1/4 watt resistors. Put each zener in series with a resistor. Put the five zener/resistor assemblies in parallel. Now, replace the zener in your schematic with this assembly. What this does is:
    - The zener/resistors create a 5 watt shunt regulator That prevents the voltage from getting too high and damaging the charger.
    - The solar panel doesn't care that there is a zener diode across its output. It is a constant current device.
    - If the charger can draw enough current so that the panels output drops below 5.1 volts, then the zeners go high impedance (disappear) and you have a loss-less transfer of power between the panel and the charger.

    Mark
     
    kaning likes this.
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    There you go. Or if you want to be able to change the voltage for the different batteries you can use this. It is pretty much as Mark described except it uses a programmable shunt (zener) to control a large transistor that can handle your panels 3.6 watts. You can change the voltage with you micros D to A.
     
    kaning likes this.
  13. kaning

    Thread Starter Member

    Jun 2, 2014
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    thanks a lot for the tips! i'll definitely try this
     
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