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I was looking around for a battery charger to charge the batteries in my home-made 36 volt electric skateboard. I haven't
found anything small and able to provide the current I'd need to charge in the amount of time I'd like. So I thought of
building my own. I came up with this idea.
The power
to our homes (here in the U.S.) comes in roughly 170 volts (peak to peak) The "Line" looks like a sine wave
relative to the "neutral" wire. If we rectify it, we'll have 85 volt peaks. It still looks like a sine wave but the neutral moves
down rather than the line voltage when it goes negative because now the current is only allowed to move one direction.
Relative to whichever's lowest, it looks like the absolute value of the sine wave. Put that through a large switching
transistor with supporting circuitry that'll only turn it on when the input is lower than a target voltage. Now if we set that
target voltage to 40 volts, the output would be like the following relative to whichever's lowest: It goes up from zero until it hits
40 volts and then immediately drops to zero. It stays at zero until the input voltage drops below 40 volts and it continues
down until zero and back up again on the second half of the wave to do the same thing. You then pass the output of this
through a final diode and into some capacitors and you're done. You've got a very efficient, but noisy, DC power supply.
From here you can make the target voltage variable by adjusting the values of the main transistor's supporting circuitry.
You can also put an isolating transformer before everything to keep the entire system ground from riding the negative
end of the input sine wave. (Or riding 40 volts below the high side, depending on whether you use P channel or N channel main
transistor.) You won't want to handle this thing bare handed or be any less than extremely careful without it.
The supporting circuitry could probably just be as simple as a not gate with a potentiometer doing voltage division on the
input.
Switched-mode Power Supplies
http://en.wikipedia...d-mode_power_supply
for reference [csea, Mar 07 2010]
[link]
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Is this similar to a switch mode power supply? |
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Efficiency will mostly have to do with switch resistance. Power FETs are extremely good these days, but overall efficiency isn't quite 100%. |
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You'll need some means of storing charge or current (capacitor or inductor) during the times when the source supply falls below the desired output level. |
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Your "not gate" sounds pretty much like a voltage comparator. There's a bit more to converting this signal to a useful means of power control. |
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Pretty much baked in modern switch-mode supplies. [link] |
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Also, depending on the chemistry of your batteries, you may want to adjust the charging current based on battery voltage and temperature. Lead-acid cells require different curves from AGM, Li-Ion, etc, for longest lifetime. |
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// 170 volts (peak to peak) The "Line" looks like a sine wave relative to the "neutral" wire. If we rectify it, we'll have 85 volt peaks. // |
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110V AC RMS; that means that the live line swings +/- 1.414 x 110 = 156V each side of zero on each cycle; full-wave rectified, this will give a noisy 160V-ish rail. |
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Yes, it can be chopped with a transistor into a low-ESR reservoir capacitor, but it's in the end just a very crude, noisy switch-mode design which can't be commercialised because of EMC issues. |
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Thanks, as an EE I was tempted to take him to school but you beat me to it. |
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I have a simple solution: find a car charger that will put out the juice you need. 10A? 20A? whatever. They are out there, Just make sure it's isolated. Buy 3 of them and wire the outputs in series. Problem solved. Efficiency will be "good enough" to charge some batteries.. A single switched mode that runs >95% in that range will be too expensive to justify buying for your skateboard. |
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Too bad you didn't make it 48V so that you could hit up industrial surplus for a cheap forklift charger. |
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This a straightforeward project though, and you can learn a lot by building your own power supply. It's a project that I do reccomend if you are up to it. Digikey.com is a good place to order the parts from, much cheaper and better selection than radio hack. Also you will need a scope or access to one, messing with switching circuits without one sucks. |
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"well baked" is such an understatement.. there is a whole industry founded on this. |
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Too bad the efficiency of batteries themselves is so
poor. |
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