Adjustable 3.2V to 9V at high amperage

Discussion in 'The Projects Forum' started by johncarr011, Dec 23, 2012.

  1. johncarr011

    Thread Starter New Member

    Dec 23, 2012
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    I plan to build a circuit that is able to output 3.2 to 9 volts of power, to a load of 1.1 to 5 Ohms (depending on a few outside factors). Battery around 3.7 volts

    Originally I was looking for a DC / DC step up (boost) regulator. I found plenty that were capapable of boosting adjustable voltage to my requirements with pretty simple schematics. But if the load is 1.1 Ohms, it would be drawing up to 8.1 Amps of current (at 9 Volts) correct? I cannot find a boosting regulator / controller that can handle that load.

    So unless I'm looking at things wrong, I may be looking at a higher voltage battery, and some sort of step down regulator. From average specs I've seen so far, they can handle higher current draw.

    Does anybody have any suggestions? In the end, I just want an adjustable voltage powersupply, from 3.2 to 9 volts, to drive a load of 1.1 Ohms. As small as possible, with simple wiring. If you can't tell, I'm very new to all this, but i'm trying to self teach, lol
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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  3. johncarr011

    Thread Starter New Member

    Dec 23, 2012
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    Ok, so I've been looking high and low for a simple converter that A - I believe I can figure out the application circuit in the datasheets, and B - Fully assembled, will take up very very little space. There are a thousand out there that seem to fit what I know my requirements are. (Don't know how imparative switching frequency is in my application, but I'm assuming not very.)

    What you've suggested Panic Mode, looks great at the "Simplified Schematic", but just like everything else I look at, once I get to the "Typical Application Circuit" portion of the datasheet, I start getting scared about space consumption (although I'm sure it's not as much as it looks like), and more so being able to assemble the circuit. It just seems like an awefull lot of external components for a chip to reduce and regulate voltage.

    When building my circuit, (following datasheet schematics) Can I just use the simplified schematic, as opposed to the big, 50 pc "Typical Application Schematic" as a reference?
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    You seem to have overlooked, or were perhaps unaware of, the power situation in any kind of power supply. The basic idea is that there is no free lunch, or alternatively power out will ALWAYS be less than power in. Sometimes much less.

    Back of the envelope calculation:
    9 Volts * 8.1 Amps = 72.9 Watts ; your output power
    Assuming your converter was 80% efficient
    72.9 Watts / 0.8 = 91.125 Watts ; your input power requirement
    From a 3.7 Volt Battery this will require
    91.125 / 3.7 = 24.63 Amps
    Wow! So a typical 1500 mAh Camera Battery would be exhausted in about 3.5 minutes.

    Ferite or Iron core inductors wound with wire of sufficient guage to handle those currents are not going to be small, cheap, or compact. I fear you have embarked on a fruitless quest.

    Choosing a battery of suitable capacity for your needs is also not going to be small or compact. As I see it you're hosed.
     
    #12 likes this.
  5. johncarr011

    Thread Starter New Member

    Dec 23, 2012
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    haha, great stuff again

    So how in the world are these circuits built? I guess if i have to come out and say it, (and maybe get laughed at) I'm trying to make a variable voltage e-cigarette. There are variable voltage e-cig's out there (maxing at 6 volts from a single 3.7 volt battery), but they are quite expensive in my eyes. So how do these things work? I was trying to mimic the idea of variable voltage e-cig to start, then after getting familiar with that working, add some unique features later on. I'm sure my best bet would be to buy one and do some reverse engineering, but I don't want to throw money away. Google has been pretty fruitless in the electrical workings of these devices, apart from some videos showing the basic assembly, but i still need more power than that for my final device.

    **Heating coil is idealy around 10 watts, maybe more, maybe less depending on taste**
    **My load resistance with a fixed voltage stock battery (3.7V 1000mAh) is 1.1Ohms +- .5

    I had assumed the existing designs used some sort of voltage boosting device inside, so as i researched boosting voltage, i was taught a jolt in voltage = proportional drop in amperage, so i thought the good idea was to buck a higher input voltage for more access to power.....

    I don't know what to believe anymore, I need to just give up huh?
     
    Last edited: Dec 29, 2012
  6. Papabravo

    Expert

    Feb 24, 2006
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    It's not entirely clear. The actual requirements may be different from the perceived requirements. The success of a design comes from a clear statement of the actual requirements. An example from a completely different perspective may be instructive.

    If you want to see out to the edge of the universe you need to grind the biggest possible mirror from a cylindrical piece of glass. If you go for the 12' mirror you will be doomed. On the other hand if you start with a 2' mirror, to develop your technique and proceed to build a 4', and an 8' mirror, you'll be ready to fabricate the 12' model. It turns out you'll get there faster than repeatedly trying to build the 12' mirror.

    Moral of the story; You'll want to try and figure out how may watts you can cram into a cubic inch; but you can't do it in one shot. Furthermore you don't really need to do it in one shot.
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
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    As papabravo pointed out it's hard to tell what you really want to do

    To control a heating element, you can use simple PWM starting with a higher voltage high capacity battery - you've seen that 1000mah won't cut it with the 1.1 ohm load. - Heating elements are power hungry...
     
    Last edited: Dec 29, 2012
  8. Papabravo

    Expert

    Feb 24, 2006
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    Last edited: Dec 29, 2012
  9. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Convert an ATX psu to variable that will give you all the power you need..
     
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Google "555 PWM circuit".

    With 12VDC available you can use a 555 pwm circuit, an N-Channel Mosfet to drive the load - easy, cheap and your done.
     
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