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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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    Thank you message

    Thank you very much :)

    And I hope I answered your question regarding the physical size of the panel. Sorry if I didn't.
     
  2. devonsc

    devonsc New Member

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    PIC

    Hope you guys dont mind me posting this here. It's about PIC, it is part of my project.

    Would like to ask what and where should I start in learning about PIC, apart from getting the PIC units ready?

    I've downloaded the datasheet for 16F628A, going through the datasheet as well as a PIC cookbook at the moment.

    I intend to use the PIC to get feedback from the battery as well as the load to read the following:

    a.) Read current current-load.
    b.) Present the approximate life-time of the battery at that level with that drawn current.
    c.) Present the battery level in terms of percentage.

    Any direct help or advice or comment is very much needed. Thanks...
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Re: PIC

    I would suggest using the 16F819 instead, this has A2D inputs, which will make life much easier - you will also need a precision voltage reference to give an accurate reference.

    You will need to convert the current reading to a voltage, so the PIC A2D can read it (a simple opamp circuit will do this). In order to give life-time and battery level readings you will have to plot the performance of the battery system, you can then incorporate the details in the PIC program, possibly as a simple lookup table.

    I would suggest you have a look at my tutorials, they may give you some ideas - particularly the analogue one.
     
  4. dave

    Dave New Member

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  5. devonsc

    devonsc New Member

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    Hi there,

    I'm working on the PIC at the moment. Thanks for that hint on using 16F819.

    Would like to ask something simple. Confused :?
    Recently, I posted my idea in a discussion room regarding the boost DC/DC converter that I intend to built after some simulation done. However, I was told that I shouldn't be using the 555 timer but a PWM to control the ON/OFF switching of my MOSFET to increase efficiency.

    :?: I thought in actual fact, PWM is actually the implementation of a 555 timer? Blur here...

    Apart from that, can I ask for some advice on how to make a proper selection of zener diode? Apart from the working voltage, how about the power dissipation? Should I just select the zener diode I need with the specific working voltage and select the lowest power dissipation available for that working voltage?

    Thanks.
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The 555 can be used to make a crude PWM system, but it's not terribly good at it, it was never designed for that use - there are many specific switchmode PSU chips available - Maxim make them, and so do most other manufacturers.

    You select it based on the power it's going to dissipate, so you need to either calculate it, or measure it.
     
  7. devonsc

    devonsc New Member

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    Thanks for the in regarding the PWM IC. I found SG3524, still reading the datasheet.

    For zener dioide:
    Would like to ask you again regarding the measurement or the calculation of the zener diode. Does it mean that I can measure the output voltage as well as the output load current, then by using P=IV to obtain the output power? Then, from here, I select a higher range power as the appropriate zener diode to use? Thanks.

    Lots to read... :cry:
     
  8. Oznog

    Oznog Active Member

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    I thought you'd gone another path already...

    Now be sure to look at the data sheet for how much power a PIC consumes. You may need to run it at a fairly high frequency to make your inductor work well, and this means higher power. So you may be looking at 10 or 20mA to run it, check the spec.

    To do this efficiently, you might want to replace the diode with a MOSFET, the diode consumes 0.7v, around 6% of the power for 12V. There are PIC chips with an "Enhanced CCP" module that can do this better since they provide a second PWM signal delayed a fixed amount so both switches aren't on at the exact same time.

    Now here's the key: you've already seen how critical it is to load that solar cell with the right current. If you miss the "maximum power point" either high or low, you get less power overall. Your PIC can be set up to measure the current going to the battery and try making slight adjustments to the duty cycle either up or down to find the max power point. It will change as lighting conditions change so it requires continuous adjustment. The current is all that matters, but the PWM won't know if higher or lower duty will make it higher until it tries it out. The battery voltage could also be used since it will always be higher voltage when the current is higher, but it is not only a smaller change but actually lags behind the current change significantly so it's not as easy.

    Max power point tracking can definitely get more juice out of a solar cell. Now don't get carried away, it's limited. Might only be 10%. So I'm not sure it will even make up for the losses in the converter.

    On a practical level, you're still making a mistake by not matching the cell with the battery. This complexity will take up space, weight, and (probably) efficiency. But, this sounds like an educational project so I think you're learning truckloads more than if you'd just connected a panel to a battery and called it "done". On that level, you're a friggin genius.
     
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You need the current through the zener diode, you already know the voltage (it's the voltage of the zener). Yes, P=VI is the correct formula.

    I'm not sure what circuit you're refering to with the zener?, is it one you've already posted?.
     
  10. devonsc

    devonsc New Member

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    :oops: I think it's going to take me sometime to understand this further. As I go along, reading the datasheet in detail :? A little confused at this moment. Sorry. There are a few matters which are way too "high", its going to take me sometime to understand :cry:

    A real beginner here...Thanks for your reply. Thank you very much! Hope you don't mind to guide me through my project.
     
  11. devonsc

    devonsc New Member

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    Does it mean that all I have to do is to measure the current drawn by the load, say approximately 5mA. Then, I multiply with the 15V. From here, I obtain 75mW. Does this mean that I should purchase one with 15V and closest higher power?

    Yup, it is the same circuit I posted earlier. Mind to comment? Thanks.
     
  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Do you mean the one labelled ZD1 13.6V, fed from a 220 ohm?.
     
  13. devonsc

    devonsc New Member

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    Sorry, the schematic diagram is suppose to be on Page 7, entitled Boost Converter. However, there isnt any zener diode drawn. Sorry as I thought I did. I intend to have the zener diode at the end of the circuit.

    Does it mean that I just have to measure the load current and multiply with 15V for the power selection?
     
  14. devonsc

    devonsc New Member

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    About the PIC 16F819, does it mean that I will only be using the Port A for my entire project? Since I'm only using the ADC converter. :oops:

    Would like ask if the only ports that I should be using would be Port A, from AN0 to AN4?
     
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    No, the current THROUGH the zener diode, as I've no idea how it's connected I can't suggest what that might be?.
     
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It depends what you are doing, presumably you will be requiring some kind of output from the PIC? - having analogue inputs and no outputs isn't going to achieve very much - although writing the program would be easy :lol:
     
  17. devonsc

    devonsc New Member

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    Sorry, I was thinking to contruct the zener diode in this manner. I tried simulating without the zener diode but I couldnt get a constant output, so I was thinking to add on the zener diode.
     

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  18. devonsc

    devonsc New Member

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    Thanks :) Sorry for my nonsense again. By the way, I'll try to come out the entire circuit connection by .... as soon as possible. Hopefully you don't mind helping me to correct it if there is something wrong with it.

    Thanks in advance
     
  19. devonsc

    devonsc New Member

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    Need help again. Sorry.

    :?: Would like to ask for advice on how should I supply the 555 timer or the PWM IC (at this stage, still undecided which to use for the boost DC/DC converter). Any advice on how should I operate this device? From the solar panel? If its from the solar panel, won't I get a varying operating value? Issit okay to operate it under this condition?

    Need advice, thanks in advance.

    :?: Apart from that, any suggestion on what op-amp can I use for my current to voltage converter? Will 741 opamp do? I've not read up more about it. Real sorry, was thinking that you don't mind giving me some exact hint. Too much to read, have not finish understanding the PIC. :cry:

    Thanks in advance.

    :?: Would like to ask regarding MOSFET. Mind to briefly explain the purpose of the diode found in the MOSFET? :oops:

    Thanks again.
     

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  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    This isn't a good idea! - the whole point of the switchmode supply is to make the most of the limited available power, this zener on the output is going to waste most of the power that the panel provides!. To regulate a switchmode supply you apply feedback to the switching circuitry, this then regulates in an efficient manner.
     
  21. devonsc

    devonsc New Member

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    Thanks for "saving" me. Thanks a lot. But I tried simulating the boost DC/DC converter without the zener diode and I couldnt obtain a constant or steady output. Is this simply because of my wrong selection of capacitor and inductor? Mind to give some hints on how should I determine the capacitor as well as the inductor value?

    Thanks a lot
     

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