dc - dc converter?

Discussion in 'The Projects Forum' started by Sharpen047, Aug 27, 2012.

  1. Sharpen047

    Thread Starter Member

    Apr 19, 2012
    48
    0
    hey guys, I am looking to power an led (30 Vf) with a 12v power supply or car battery charged by solar panels.
    i found one on ebay for about $8, but dont want to wait a month to get it from china now or when it fails.
    http://www.ebay.com/itm/120W-DC-Con...705?pt=LH_DefaultDomain_0&hash=item4169d97af1

    So im looking for an adjustable converter but 30v should be fine even with 28-29 volt and under power the led.
    i have a very basic understanding of inductance but wouldnt mind learning more while im at it and im sure i will spend more than $8 making my own but might as well learn while im at it!
    i found this http://www.daycounter.com/LabBook/BoostConverter/Boost-Converter-Equations.phtml
    Hoping for 120 watt capable, ill probably use about 80w with a 100w led at 30v.
    ive looked on google and only found lower than input voltages or merging 12v psus which gets costly quickly for me destroying them basically.
    I apologize for my poor understanding of the calculations and terms, it just looks so simple i thought id ask to see if i could make one or two.
     
    Last edited: Aug 27, 2012
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,664
    634
    What a bargain! I have to recommend that at that price, you buy two from China (so you have a spare); they don't cost much more than the raw parts would cost you.
     
  3. Sharpen047

    Thread Starter Member

    Apr 19, 2012
    48
    0
    i definitely agree! id rather just have the ability to put one together out of the parts box. once i know what the ics are ill have a much better understanding of how it works.
    I also have a cnc i just completed and want to start making some circuit boards to see what tweaks i need to make to get everything accurate.
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,173
    1,797
    The chips are the least of your problems. The art and skill in constructing an SMPS (Switch Mode Power Supply aka DC-DC converter) is the passive component selection. Starting with your requirements you will need an inductor that can handle high peak currents without saturating and you need stable low ESR capacitors. Next you will need to select a switch and last you need a controller. These things are not, repeat NOT, thrown together from a junk box unless you enjoy letting the magic smoke out of the things you build. At your current, apparent, level of knowledge and experience I strongly recommend buy versus build.

    To gain the knowledge you desire, start with modest requirements and be prepared to throw the first one away because you'll end up doing that in any case.
     
  5. Sharpen047

    Thread Starter Member

    Apr 19, 2012
    48
    0
    alright thanks. guess ill get to reading since i know smps designs are prohibited.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,173
    1,797
    Prohibited? No I don't think so. They are just difficult and challenging to get right. Difficult and challenging does not mean impossible, especially if you have a network of Dutch Uncles.

    In case you meant prohibitive, as in more trouble than you are willing to expend, then I can agree with you.
     
  7. Sharpen047

    Thread Starter Member

    Apr 19, 2012
    48
    0
    im willing to spend some time to learn how to make the converter. just a matter of what i should do to start. ill start by trying the volumes on this site to get started. i know i wont wake up tomorrow and know how to make one. took me 4 years to rebuild my car, 4 months to build a cnc from some remnant metal from the metal shop and i am set on making one of these for many reasons. Just not sure ill understand everything by reading it. as with a 555 led dimmer i got tons of help on before, i built it and it took about 4 timers for me to get the timer wiring correct then 6 transistors to realize how they work together to form a working transistor signal and on/off switch using a capacitor to calculate how fast it was switched on and off. ive only just started learning how to work with electricity and circuits but ill get there one day. ill start today.. well a couple of months ago!
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,173
    1,797
    If I were you I would start with the basics of inductors, especially toroidal ferite cores, and capacitors with respect to temperature stability and ESR (Equivalent Series Resistance). Buy some cores and some magnet wire and learn how to wind and characterize inductors. Buy some COG or X7R capacitors with low ESR and figure out a way to measure those things. This will put you head and shoulders above most of the posers.
     
  9. Sharpen047

    Thread Starter Member

    Apr 19, 2012
    48
    0
    ok so just checking to see if i am right so far.
    a henry is the measurement of change of one ampere per second at one volt through the inductor?
    inductor types are mainly categorized by the frequency levels they are used at? eg laminated vs ferrite? ferrite being used for higher frequency applications.
    inductors can only be used with AC like transformers.
    You can make an inductor use power from dc by using a sine wave converter made of 555 timers?
    ESR is the combined resistance between the inductor and capacitor in series?
    COG and X7R are low loss types of ceramic capacitors that are much more accurate and have a near linear temp coefficient?
    inductors limit current and not voltage.
    am i looking to find the ESR or types of inductors used with types of capacitors?
    i see there is a lot of math involved and havent worked on that yet but i also dont feel comfortable ordering any cores yet until i know about what i should be looking for.
    thanks again for the help and direction Papabravo
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,018
    1,537
    Also have a look at Texas Instruments "switcher pro". Lets you chose, design and simulate without actually having to buy anything. http://www.ti.com/tool/switcherpro
     
  11. Papabravo

    Expert

    Feb 24, 2006
    10,173
    1,797
    You are beginning to get the hang of it and you are very close.
    Generally speaking we don't consider the resistance of the magnet wire used to wind an inductor. We consider the current carrying capacity of the wire by guage and how many turns we can get on the core to get the inductance we need. As an extreme example let us consider what would happen if we wound an inductor with nichrome wire. Here the resistance is enough to generate considerable heat, burn your fingers, and basically raise the temperature of the toroidal core. You don't want to do that.

    With capacitors and ESR you need to know that this is not a "real" resistance. It is a deviation from ideality that you cannot measure with a DC ohmeter. It just means that all capacitors stop looking like capacitors in some frequency ranges. What you want in capacitors that you plan to use in a SMPS is a minimum distribution of values of ESR over the frequency range of interest.

    Speaking of frequency ranges. If I have a 40 kHz. square wave with a 50% duty cycle and 7 ns. rise and fall times, what is the "frequency range of interest"?

    PS Using a simulator is a good way to check your understanding without inuring great expense, BUT there is no substitute for building a circuit for real.
     
  12. Sharpen047

    Thread Starter Member

    Apr 19, 2012
    48
    0
    i honestly dont know where to start with "frequency range of interest"
    What do you mean by minimum distribution of values? That you want to get them all to operate within the same frequency ranges so they "look like capacitors" through the entire range of input/output, while taking the ESR into account?
     
  13. Papabravo

    Expert

    Feb 24, 2006
    10,173
    1,797
    If you a have a pure sine wave at a frequency ω, then you have a narrow "frequency range of interest". If you consider a square wave as being composed of a sine wave of a fundamental frequency plus and number of even and odd harmonics then the "frequency range of interest" expands toward positive ∞. My point here is that with a square wave a capacitor needs to stay a capacitor to much higher frequencies.

    The "distribution of values" means a maximum and minimum or something more complicated with a mean and a standard deviation. If you buy 100, quarter watt, 5% 1KΩ resistors you will find "distribution of values" when you measure them. That is the idea.
     
  14. Sharpen047

    Thread Starter Member

    Apr 19, 2012
    48
    0
    alright so i looked up the pattern for a sine wave circuit output and found that the pattern of the output can be at a very high frequency then lowers slightly then becomes high again then starts over agian in the negative.
    This is why you need a higher frequency capacitor right?

    I am pretty used to calculating leds voltage drop, and they differ even from the same mfu and color etc. This is probably what you are talking about, tolerance from the rated value of resistors caps and leds.
    now to find out how to take all of this and put it together into something i can use or calculate...
     
Loading...