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Thread: Dynamo to 5V USB charger

  1. #1

    Default Dynamo to 5V USB charger

    Hi,

    I am looking to make a 5V USB battery charger for my SP Dynamo Hub. I will use the charged battery to charge my phone and other devices.
    I am planning to make this as basic as possible and just wanted to check if there are any concerns with my proposed design/circuit.

    From the dynamo (which says it is 6V 2.5W output) I will connect:

    2200uf 25V Capacitor -> Bridge Rectifier(1.5A 100V) -> Output Converter DC 7V-24V To 5V 3A Step-Down Buck with USB output.

    Should there be any issues with such a set up?

    Thanks

  2. #2
    Flashaholic* Steve K's Avatar
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    Default Re: Dynamo to 5V USB charger

    The first thing I notice is that the filter cap is placed at the dynamo output.. it should be moved to the output of the bridge rectifier.

    The only other thing that jumps out at me is the issue of the dynamo voltage when there is minimal load.
    The dynamo voltage is described correctly, and if there is always a 2.5 watt load, then the voltage should be fine. I anticipate that there could be a problem when the battery is approaching full charge and there is very little load on the dynamo. In that scenario, dynamo voltages can easily exceed the voltage rating of the capacitor and buck converter.

    This could be handled by selecting parts rated for higher voltages. Depending on how fast you will go, the voltage from an unloaded dynamo can be surprisingly high! I've measured 100V out of my SON dynamo at 50mph (going down a 14% grade). It's not easy to find a buck converter rated for high input voltages that is also happy running at 6 or 7V, so this solution might be hard to implement.

    An alternative is to use a shunt regulator to limit the dynamo voltage. Ideally, you would replace the buck regulator with a precise shunt regulator, but I suspect that you'll have trouble finding one. The next best alternative would be to place a high power bidirectional zener diode in parallel with the bridge rectifier input, or alternately, a unidirectional zener diode in parallel with the rectifier output. This would clamp the voltage to a safe level.

    The downside of this approach is that it will be a load on the dynamo. This means that you'll be generating 4 watts or more just to turn it into heat. Plus, you'll need to provide a way for the zener diode to get rid of the heat. If it overheats, it'll short out (most likely).

    I'd recommend something like the 1N5343 5 watt zener diode....
    https://www.digikey.com/products/en/...=0&pageSize=25

    The zener's cathode (the end with the stripe) should be connected to the positive output terminal of the bridge rectifier, and of course, the anode is connected to the negative terminal.

    You'll have to experiment a bit to see what it takes to keep the zener reasonably cool. To some degree, it might be adequate to simply solder the zener to a large area of copper plated circuit board. It might also be adequate to epoxy an aluminum heatsink to it. It'll take some tests to see what is easiest or more appropriate for your situation.

    That's all I've got for now. Good luck!

    edit/update: a similar discussion is evolving in the Supernova thread. See this post: http://www.candlepowerforums.com/vb/...=1#post5172438
    Last edited by Steve K; 01-11-2018 at 01:26 PM.

  3. #3

    Default Re: Dynamo to 5V USB charger

    Steve that is great info! Thanks heaps. I realized that i got the bridge rectifier and capacitor thinking over it last night. Over voltage damage is my main concern - I think the Zener diode is likely the best approach i have seen - heat will only be a problem when the voltage increased to much correct or is that it will always be loading the dynamo so the voltage does not increase?

    Does anyone know what the Sinewave Cycles Revolution unit uses to limit the voltage?

  4. #4
    Flashaholic* Steve K's Avatar
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    Default Re: Dynamo to 5V USB charger

    Quote Originally Posted by rjtedge View Post
    Steve that is great info! Thanks heaps. I realized that i got the bridge rectifier and capacitor thinking over it last night. Over voltage damage is my main concern - I think the Zener diode is likely the best approach i have seen - heat will only be a problem when the voltage increased to much correct or is that it will always be loading the dynamo so the voltage does not increase?
    Dynamos have a lot of source impedance (i.e. internal resistance and inductance), which is what limits the current to 500mA. If the load is drawing a lot of current, then the voltage drop across the internal impedance keeps the voltage low. As the current drawn by the load decreases, then there is less voltage drop across the internal impedance, resulting in higher voltage at the load itself.

    In the case of charging a battery, the voltage from the dynamo will be relatively low when the battery is drawing a lot of current. However, when the battery reaches full charge and draws very little current, then the voltage from the dynamo will be fairly high. Once that voltage reaches the level where the zener diode starts to conduct, the zener draw enough current to keep the dynamo voltage at the zener's rated voltage (with some tolerance).


    Quote Originally Posted by rjtedge View Post
    Does anyone know what the Sinewave Cycles Revolution unit uses to limit the voltage?
    their design doesn't appear to be intended to get rid of a few watts of heat, so I'm guessing that it is a buck converter (a.k.a. a switching regulator that acts as a series regulator). I'm curious as to whether they've used a standard buck converter, but used high voltage mosfets, or whether they've used a little microcontroller as the control chip, along with the usual buck converter components.

    By the way, I took a quick look at the Texas Instruments web site, looking for a buck converter than can handle 100V inputs. It turns out that they are selling one! The LM5161 is rated for input voltages of 100V, and can operate down to 4.5V. That could be very helpful! The package of this device is pretty small, so it's hard for a hobbyist to use. Seems like it would require a proper printed circuit board instead of some of the hacked up boards that I've made.
    Of course, for switching regulators, it's best to have a real ground plane and have the locations of the parts optimized, so a good 2 or 4 layer PCB is recommended.

    One of the interesting things about using buck converters is that if the load wants more power than the source (a.k.a. the dynamo) can deliver, then the converter may pull so much current from the source that the voltage drops too low for the converter to operate. In some designs, there is extra circuitry designed to limit how much power is delivered to the load, with the goal of optimizing the amount of power drawn from the source. This is where a microcontroller, or a really clever analog circuit, can be quite helpful!

    This problem of the load drawing too much power from the source is a bit of a concern with the shunt regulator too. The worst case is that the load will still draw the 0.4A or 0.5A that the dynamo can source, which isn't too bad.

    The best case arrangement is when the controller of a buck converter monitors when the load wants more power than the source can provide, and then adjusts the operation of the buck converter to draw the maximum amount of power from the source. This is not uncommon for converters designed to work with solar panels. I'm not sure if the Sinewave device does it or not. With a microcontroller, it should be possible... but then I'd also expect them to be advertising this capability. Not much point in adding the feature if you don't tell potential customers!

  5. #5
    Flashaholic angerdan's Avatar
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    Lightbulb Re: Dynamo to 5V USB charger

    Why do you want to make it yourself instead of buying a already available product like the B&M E-Werk/USB-Werk?
    bumm.de/en/products/stromversorgung.html
    Last edited by angerdan; 01-13-2018 at 06:01 AM.

  6. #6

    Default Re: Dynamo to 5V USB charger

    Quote Originally Posted by angerdan View Post
    Why do you want to make it yourself instead of buying a already available product like the B&M E-Werk/USB-Werk?
    bumm.de/en/products/stromversorgung.html
    Mostly because I like making things are knowing how they work. 160EUR is also no small amount of money.

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