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DC/DC Converter (Miniature Remote Control Solar Car)

Discussion in 'General Electronics Chat' started by devonsc, Oct 16, 2004.

  1. devonsc

    devonsc New Member

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    Hi! I hope I didn't cross post any of the existing topics. Novice like me, desperately need help regarding a project that I'm working on at the moment.

    Chiefly, we're doing this project, called Miniature Remote Control Solar Car in a team of four. Where each person will take charge of one portion and the main portions are as follow:

    One: A workstation which will send the appropriate signals to manipulate the movement as well as the direction of the remote car.

    Two: Proper RF transmission between the remote control car and the workstation. Assures that all signals can be sent and receive reliably.

    Three: Manipulates the speed of the DC motor through the application of a microcontroller from the signals sent by the workstation.

    Four (which is my part): Charges a 12V DC battery by utilizing the Sun's energy from a solar panel. Due to the low voltage output, I'll be converting the output from the solar panel (output: 8-10V) to a level of 12V before charging up the battery through a DC/DC converter. This is the cheapest solar panel that we can find to compromise with our budget of this project. I would like to ask if there is any specific task that i can try to carry out to further expand my portion? Or any specific task that I can try to add on in this entire project?

    Btw, can I or am I suppose to connect directly the output of the DC/DC converter to the battery or I need an additional battery charging circuit? At this level, I found the DC/DC converter circuit but have not try it out. Was doing some reading regarding the battery charging circuitry at the moment.


    Desperately, need advice : :cry:
    Please? Thanks in advance.
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    A simple google search found lots of hits, you may find this one useful http://www.solorb.com/elect/solarcirc/scc2/index.html. I won't bother repeating my comments about available power, hopefully you will have taken them to heart by now :lol:

    I would also suggest you look at http://www.discovercircuits.com/S/solar-cell.htm for various projects.
     
  3. Russlk

    Russlk New Member

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    Allelectronics has a 60mm solar cell, 3 volts @ 40 mA, for $3.50 each. You only need 4 or 5 for a charger. Have you figured out how much charging current you need? How long is this car supposed to run? For maximum efficiency, you should connect the solar array direct to the battery (perhaps with a series diode to prevent any reverse current). I don't think you need to worry about overcharging the battery, but a 15 volt zener across the solar array will be all that is needed.

    How about posting your analysis so we can give some feedback?
     
  4. dave

    Dave New Member

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  5. williB

    williB New Member

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    you could monitor the battery voltage with a microcontroller
     
  6. devonsc

    devonsc New Member

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    First of all, I better apologize in advance for what I'm going to post to assure that no one gets mad.

    And of course, thanks for your concern on a newbie like me. :eek:

    Thanks to everyone and I might post some questions after I managed to test the solar panel. Btw, I'll be getting the solar panel from the retailer tomorrow.

    Thanks again...
     
  7. devonsc

    devonsc New Member

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    Hi! I've managed to test on the solar panel.

    Result:
    The output voltage is approximately 20V whereas the output current is approximately 80mA.

    I'm now studying the formula on how to obtain a maximum output power from solar panel.

    Any advice? Please? Thanks in advance!
     
  8. devonsc

    devonsc New Member

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    Hi! I now intend to extand my portion of project as explain in the following, any advice? please?

    Having the charing battery circuitry, I will now implement a PIC microcontroller to perform the following:

    a.) monitoring on the condition / lifetime of the battery using either DOT
    matrix display, seven segment display or LCD screen.
    b.) controlling of the "additional circuits".

    the additional circuits that i intend to add on would be:

    a.) economic mode or normal mode in utilizing the battery.
    b.) different charge-voltage, 6V, 12V, 24V.


    Any opinions on the matters stated above? Hope advice would be given. Anything impossible to be done above? Please advice? Please? Thanks..
     
  9. williB

    williB New Member

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    ehm...
    is the use of 12V battery "carved in stone"
    because
    1) for one thing, comverting from 12V to 5V wastes power.. a 6V battery would be a better choice..
     
  10. williB

    williB New Member

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    oh which micro comtroller are you going to use..???
    also better order your PICs from microchip soon ..because there is kind of a long leadtime ..before you can sucessfully get a program into the PIC..microchip offers free samples order at least 2 628A's they have an 8 bit A/D converter built in...
    and do you know anybody with a working PIC programmer.?
     
  11. spuffock

    spuffock Member

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    If you use a switching converter from a solar panel, and have the pic driving the switch, the charging current can be monitored by an analogue input. A useful algorithm is to increase the switch on time until the current falls, then decrease the on time until the current falls, and repeat , In this way the converter will always sit at the maximum charging current point, regardless of the illumination and battery voltage. You may as welll have the last milliamp. :wink:
     
  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The 628A doesn't have any A2D inputs, it has two analogue comparator inputs, which can be used to 'make' analogue inputs.

    Far better to order the 16F819, which has five proper 10 bit analogue converter inputs.
     
  13. williB

    williB New Member

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    sorry, i was wrong about the 628's ...684 is what i ment...
    i forgot that i ordered 2 628's for other stuff..
     
  14. devonsc

    devonsc New Member

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    Hi there! Real sorry for my late post.

    First of all, thanks again for you guys' concern about the problems that im currently facing.

    Thanks for the advice regarding the PIC microcontroller. The topic of this post seems to be off but hope you guys will not mind.

    At this point, I would like to ask for advice on how can I determine or calculate the maximum power output of my solar panel. The value of open circuit voltage that I obtained is approximately 20V while the value of short circuit current measured is approximately 80mA. I know this seem to be a crap question but really hope that you guys will not get mad. Can I just used P=IV to obtained the maxmum power output in this case? With reference to books regarding seimiconductor devices, it seems that this is not the actual way of calculating the output of the solar panel. Need advice. Thanks!

    Apart from that, I'm currently studying a circuit regarding the 6V/12V charging circuitry. If I face any problem, you guys dont mind me asking again, rite? Please? Thanks in advance.

    Btw, as attached is a simple block diagram. In the block diagram, the dotted lines indicates detachable.

    The charging circuitry, solar panel and battery will be mounted on the miniature solar car.

    However, the entire charging circuitry including the solar panel will be designed in such a way where it can be slotted into the miniature solar car and at the sametime, it can be removed from the solar car for other applications. This is the reason why there is a PIC microcontroller that will be used to monitor the condition of the battery when the entire charging circuitry and the solar panel is being used for other applications.

    Any opinions? Please? Thanks! :oops:
     

    Attached Files:

  15. spuffock

    spuffock Member

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    I'm not sure about calculating the peak power of a solar cell, the best I could do is attempt to measure it. Try a variety of loads, and measure the voltage and current in each case. Multiply them together and plot the result (power) against load on a graph. You will soon see where the peak is, and be able to take more readings closer in. I think the best load will vary with illumination. If you use a switching converter driven by a PIC it will automatically zero in on the best match for any load and illumination.
    Start with a 1 to 1 mark/space ratio, let it run for a few cycles and sample the current. Then increase the on time and reduce the off time by one step, wait a little while and sample the current again. If it has increased then increase the on time and reduce the off time another step and go again. If the current has dropped then reduce the on time. The rule is, keep stepping all the time, and if the current is less than the previous sample then reverse the direction of stepping. Good luck with it :)
     
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I would suggest you try loading the panel with different value resistors, and monitor the voltage across the resistor, and the current through it. You should be able to find an optimum value of resistance that gives the maximum power, using the P=IV formula you already know.

    Your current values work out at 1.6W, which is probably something like you may get from a smallish solar panel.

    For a start I should try connecting the panel directly to a 12V battery (through a reverse blocking diode) and monitor the charging current.

    Have you determined the power requirements of your car yet?, I suspect it's going to be a great many times more than your solar panel can supply!.
     
  17. devonsc

    devonsc New Member

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    I've already obtained a set of readings of voltage and current by using different value resistors to plot the IV characteristic graph. The readings are as shown in the following:

    V (open circuit) = 20.3V and I (short circuit) = 80mA

    Using 51ohm: 0.63V, 14.54mA
    Using 220ohm: 2.82V, 14.10mA
    Using 330ohm: 4.25V, 13.65mA
    Using 560ohm: 7.04V, 12.59mA
    Using 680ohm: 7.98V, 11.98mA
    Using 1kohm: 11.32V, 11.53mA
    Using 2.2kohm: 17.20V, 7.79mA
    Using 4.7kohm: 18.70V, 4.01mA

    I don't quite understand what you mean by the optimum value of resistance that gives a maximum output power. Through the P=IV, I obtain the power of only 133mW using the 2.2kohm. Does this mean that this is the maximum output power? :oops: Then, what does the 1.6W mean? :oops:

    I get what you mean, I'll perform this test as soon as the sun rise :( [/quote]
     
  18. devonsc

    devonsc New Member

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    Just to make sure that I will be doing the right thng when the sun rise later.

    Is this how I'm suppose to connnect it?

    => Solar panel (positive) to the diode (positive) to avoid reverse current?
    => Diode (negative) to the battery (positive)
    => Battery (negative) to the solar panel (negative)

    Sorry to put this in point form as I do not know how to come out with a diagram which can be viewed directly without downloading. :oops:

    :oops: Hopefully this is right.
     
  19. williB

    williB New Member

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  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That's exactly what I wanted you to do :D and it's something you can use in the write up of your project - good marks for things like that!.

    It looks like the maximum power is about 133mW into a 2K2 load, using it to charge a 12V battery should be pretty close to optimum, and give about 10mA charging current.

    The 1.6W I mentioned was just multiplying the open circuit voltage and short circuit current, the reason being that the maximum power was going to be less than that - considerably so.

    You notice how puny the power output is?, that's why I've been going on at you all this time - no point messing about with DC/DC converters if you don't have enough power to start with!.

    BTW, I've never played with solar panels, but I've always fancied getting one to experiment with - but I've always been aware of how large a panel you need for any reasonable output.
     
  21. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Wrong way! - at least as I understand positive and negative of a diode!.

    Positive of solar panel to anode of diode, cathode of diode to positive of battery - the cathode is the end with the stripe.
     

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