I’m making a design for a crank generator which will charge my phone and a battery at the same time. The generator will be connected to a bridge rectifier, then a buck (step down) converter, which will be connected to a usb. Is there any way the I’ll be able to use the power not drawn by the buck converter to charge a battery? And would I have to worry about to much current in the usb with or without drawing the extra power unused by the buck converter? (The generator power is alternating and will fluctuate in max volts because it will be power by my bike while riding). It would help if you could give me a layout for how it would work and how to regulate the current.
closed as too broad by brhans, winny, Dmitry Grigoryev, Bimpelrekkie, Brian Carlton Aug 26 '18 at 12:23
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This will not be too difficult. If you charge a capacitance with the output of the bridge rectifier, the voltage on the capacitor will decrease as loads are increased and increase as loads are decreased.
The buck converter will attempt to maintain a steady output voltage of 5V, and the more the phone draws, the more the voltage will tend to deviate from this. If it's rated at 5V, 2A, it should be able to hold a relatively steady 5V at a 2A load, although this does not mean it will work with adaptive fast charge or similar. Whatever load you put on the buck converter, it will just attempt to pull as much electricity as it needs to to maintain it's output voltage. If it draws more power than the generator can provide, the generator's output voltage will decrease until it is no longer able to run the buck converter, and depending on it's design, the buck converter will hopefully shut off, rather than operating in brownout. You can control the voltage at which this occurs. If you increase the voltage to the buck converter, on the other hand, the output will stay the same, just the buck converter will be operating at a different duty cycle, so extra power from the generator will not "come through" the buck converter in the way you are imagining, so if you want your loads to react to the generator output, you need to sense it on the input side of your voltage converter. Your battery bank does not have to be an over-the counter 5v input 5v output battery bank, and in fact using one is an extra, unnecessary voltage conversion, so it may be best to avoid wasting power this way, however, if you do want to, you can simply have a second buck converter/USB charger for the battery bank that turns on as described here.
The highest voltage that can be reached will be determined by the peak no load voltage coming out of the rectifier. Be aware that you can use MPPT(Maximum Power Point Tracking) to optimize the output voltage and current. If the minimum voltage for your buck converter to turn on is, say 6V, you can simply use a higher trigger point for the battery charger.
So if you're at a stop, the phone charger charges from the battery bank when the capacitor is at less than 6v. You start pedaling again, and when voltage hits 6v, the phone stops charging from the battery bank and is connected to charge off the buck converter. As voltage increases with your velocity and your phone charges(decreasing draw from 2A to ?), you will hopefully reach a point where you are producing more power than the 10W plus losses the phone charger can use, and when voltage goes above, say 7V, the battery charger comes online and charges the battery bank until the voltage on the output cap goes back below 7V.
If you use a multilevel setup like this, and your phone becomes completely charged, there will be no load to keep the voltage below 7v and it will automatically charge your battery bank with extra power. This may appear to be complicated, but it will allow you relatively optimal efficiency at a low skill level. You just need to learn enough to get over the hump. Keep reading and I'll see if I can provide you a block diagram a bit later.
You should start learning the most basic electrical math, Ohm's Law, Watt's Law, Kirchoff's current law. To do what I'm describing you'll have to use some voltage converters and op amps. If you can get your hands on a multimeter that will help you figure out your generator's max and minimum operating voltages. You need to figure out how you'll be attaching things as well and a bit of soldering might be necessary as well.
If your budget is going to be limited or you need to salvage parts, start looking for things to use as a generator and spinning them up to see how much power you can get from them. Start watching for cheap or secondhand automotive USB chargers. You can repurpose these if you want instead of buying a buck converter. Many of the ones I've tested work all the way from about 5.5 input volts to 18 or 20, and they can be had as cheaply as free. Decide what you want to use as a battery bank. Is it just a second USB device so you can carry it away or is it part of the bike/charger? If it's part of the bike/charger, you'll have to find a charger board for it that works over the voltage range of your generator. Once you get this figured out I can help you some more. Keep at it. If you learn this stuff at your age it can be a massive advantage for pursuing similar material later in life.
Generators have a current limit inherent in their design. this limit is due to the finite strength of the magnet.
Buck converters take more current at lower supply voltages, this property is called "Negative resistance" and it makes keeping them happy a bit tricky. If the current the buck converter wants exceeds the generator output current the generator voltage will kept below the buck converters operating voltage and nothing will happen, no matter how fast you crank it.
So you need to use a buck converter that has an input current limit and set that limit to less than the current limit of the generator. then the buck converter will not be able to force a brown, and will begin operating once the generator is turning fast enough.
Sharing this finite current between two loads is tricky. the best way is probably to arrange for the battery charger to only run when the generator output voltage is above, say, 8V , you need about 7V to run the buck converter for the 5V charging, but any extra voltage that shows up there is a sign that you can increase the load on the generator.
suitable buck converters modules for the first part can be had on aliexpress and ebay for a few dollars each, you need the ones with an input current limit they'll have three adjustment screws.
for the second part you'll need to customise the buck converter a bit.
current is not going to be the problem, but voltage could be. only speed limits the voltage from a generator so, if you're going down hill at speed, you'll need to arrange something to limit the voltage - you could feed the excess current to your lamp for instance, one you take enough current the generator output voltage will lower.
If you want to design from scratch instead of using pre-built voltage fed buck modules you should use a current-fed buck-converter circuit instead