low voltage dc- high voltage dc converter needed

Discussion in 'General Electronics Chat' started by fabieville, Sep 19, 2010.

  1. fabieville

    Thread Starter Member

    May 25, 2009
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    I need a circuit that can take a low voltage dc at about 2V and step it up to about 18V or higher with a lot of current at the output. 10amp or more because at the input the current will be very high so I need good amount of current after the conversion process.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    If you want to step-up 2 Volts to 18 Volts at 10 Ampere output,
    you will at least need 90 Amperes at the input.
    I say at least as it is dependend on the efficiency of the converter.

    Bertus
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You may also find that it's rare to find an "input" voltage relating to a power source as much below 3V due to the inherent voltage drop in semiconductor junctions. It just isn't an efficient voltage to work with.

    How did you end up with such a small voltage to work with and what are you trying to power?
     
  4. timrobbins

    Active Member

    Aug 29, 2009
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    16
    Marshall, I think fabie doesn't quite understand alternators.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    The OP has posted almost the same question here http://forum.allaboutcircuits.com/showthread.php?t=43067. He created a new thread because he was not happy with the answers he got. I think Fabieville at least need a crash course in generators before he proceed with the plans. If not I am afraid he will destroy valuable equipment
     
    Last edited: Sep 19, 2010
  6. fabieville

    Thread Starter Member

    May 25, 2009
    26
    0
    the think is that i have a motor driven alternator that creates a lot of current but the voltage is very low so what i want to do is to boost the voltage output so i can implement it in a wind turbine application. So it creates charging voltage for a 12V battery at a much lower rpm. The thing is that it creates 2VDC at about 50 amps short circuit when the motor is at about 200rpm and it creates 24VDC at 120Amps short circuit at 2500RPM so what i want to do is to boost that 2v to 18V and hence i can charge the battery when the motor reaches 200rpm which would be perfect for a wind turbine application and then judging by the amps i would have a decent amount of amps when the battery starts charging at 200RPM.

    Please give me some feedback on this if its possible or worthwhile doing.
     
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The voltage is low because it is not being used as it was DESIGNED. The rated current and voltage are what they are because the generator was made to function PROPERLY at those voltages and currents. Using it in a different way(with very low RPM) will not do what you think it will. I already told you the current output at that low of an RPM will be terrible. Trying to draw a large current from the generator at such a low voltage will cause the output from the generator to be loaded down and the voltage will disappear completely.(or it will jump around between nothing and a few volts very randomly)
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    If you really want to use it at a low RPM then you must redesign the windings. Low RPM generators were large in diameter(for high peripheral speed at the outer rim) and had multiple magnet poles on the rotor(usually 6 to 10). The alternator you have is probably not physically designed on the inside in a way that will allow you to rewind it for multiple poles and it is positively not big enough around to give a high ft per sec. speed at the outer edge of the rotor at such low RPM's

    You should look up(google) homemade axial flux wind generator. There are several websites showing how to build a proper battery charging/wind driven generator.
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Yea, I remember reading through that and it got kind of confusing.

    Alternators have always been a major problem when building a wind power harnessing device, they all have minimum shaft speed which must be attained before they start putting out a useable amount of power and due to the output frequency being 100% dependent on the shaft speed they really need to be rectified into DC to serve any useful purpose.

    A good example can be observed with almost any car at night. When you're driving along at a normal speed they're quite happy but when you come to a stoplight and your engine finally drops to its low idle point the headlights usually dim to some extent as you're right on the edge and the battery is helping to make up for the excess draw. Left in this condition long enough and the battery will slowly run down. It's not as noticeable in newer cars because the computer will eventually try to make up for the low output by increasing the idle speed.

    If it were simple half of everyone in the country would hook an old car alternator up to a windmill through some belts and pulleys to increase the speed. They still do, but they accept the fact that they'll only get intermittent charging of the battery so they only depend on it for occasional duty loads. More often seen is the inline 6 cylinder engine from a long ago rusted out pickup truck hooked up to several alternators which makes a fairly useable emergency backup if you set the engine's idle speed high enough.

    We've come out with specialized alternator designs which work better under variable wind speed conditions and we've come up with some specialized transmissions that can help keep an alternator running in its efficient range but both solutions are quite expensive.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Long post coming - hold on.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Being able to put out 24 VDC tells us this isn't your normal alternator or the regulator circuit isn't working.

    I'd say let's give this the benefit of the doubt and verify your ouput in a little more scientific way as I'm not aware of how you're measuring "short circuit" voltage.

    2V, 50 amps at 200 rpm means you should be able to get 100W out of it so, in theory, it should be able to drive a 100W load while maintaining that 2V.

    A 0.04 ohm load will draw 50A from a 2V source. I'd verify this by spending a few $$ before I started putting a lot more time and money into the project.

    Here's a link to a 0.02 ohm 50W resistor, two of them in series would provide the proper load but we'll need a proper heat sink.

    http://www.mouser.com/ProductDetail...=sGAEpiMZZMvhlCB8CTbT5Puz7dMW2dIaonKLOziO4Vg=

    According to the data sheet you've got to keep the heat sink at or below 50*C before you start derating the power. They claim a Thermal Resistance – Rthj - of 2.1*C/W so you're going to need a fairly decent amount of heat sink. (and contact between it and the device)

    They'll set you back about $20 including the shipping.

    Now you've got a proper load you can put an ammeter in series with but I've left no margin for safety in the event you drive this thing past a 50A output.

    If you can verify that the next problem is going to be another exercise in fun. To get that 2V up to 18 it would have to be modulated. There are some reasonably priced MOSFETs with an on resistance of less than 0.001 ohms @ 10V even when driven by a logic level input but here's where a new problem arises.

    A boost circuit is pretty well out due to capacitor size and diode losses leaving you with having to custom design a transformer and your modulation is going to need to be sine wave in nature. That would force you to operate the MOSFETS in their linear region most of the time so they'd eat up all your power.

    In other words all of the above is nice theory but pretty welll proves it to be impractical.

    A possible solution? If this truly is an alternator that puts out DC then there are internal diodes somewhere. Get rid of those, figure out a way to energize the field coil and you should have an AC output to work with. Your chances of actually making this into a viable project have become a reality again.
     
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