Shimano dynamo hub 5V converter - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: April 26, 2014, 12:41:55 am
content: POST REMOVED AT USERS REQUEST

author: IanB
author_rank: Super Contributor
timestamp: April 26, 2014, 12:53:26 am
content: One possibility to consider is a standard mains USB adapter. Perhaps it is possible to modify one of these to accept the lower voltage from the dynamo instead of the expected ~120 V AC from the mains?(Note: Wouldn't a dynamo produce DC and an alternator produce AC?)

author: tom66
author_rank: Super Contributor
timestamp: April 26, 2014, 12:10:15 pm
content: You likely want a switching converter with a wide input range.The problem is most converters I am aware of work up to about 80~100V for the Vcc with an internal regulator for the 5V/12V gate drive.So to work around this, you need to have a high voltage regulator providing the Vcc for a controller IC, then use one with a high voltage MOSFET (with a separate gate driver.)Example HV regulator:http://uk.rs-online.com/web/p/low-dropout-voltage-regulators/7924461/

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 26, 2014, 12:32:48 pm
content: How bout you wind a small toroidal transformer, say a T-137-3 to do 1:3, get the voltage down to a useable level and the current up; rectify it with a LT4320 and some FETs, then a standard switcher.

author: IanB
author_rank: Super Contributor
timestamp: April 26, 2014, 04:49:33 pm
content: Quickly checking on Shimano dynohubs, the nominal output is 6 V 3 W AC, rising to maybe 10 V at high speed. However, to obtain these voltages the dynamo must have the bulb connected. If you leave the dynamo open circuit the unloaded voltage will go much higher.Therefore it looks like the key to using these dynamos to run an electronic load is to make the circuit look like a resistive load to the dynamo in order to damp out the voltage spikes. You are never going to see voltages like 80 V when applying a reasonable load.

author: IanB
author_rank: Super Contributor
timestamp: April 26, 2014, 07:40:57 pm
content: I don't think you need to have the resistor there all the time.What is really needed is a two step system.In step 1 the voltage is rectified, regulated and stabilized.In step 2 the stable voltage is used to feed the USB converter or other device.To regulate the voltage a shunt regulator could be used that bypasses current around the storage capacitor when the voltage gets above a threshold of 12 V or so. This would be like a variable resistor that only turns on when necessary. The shunt could perhaps be implemented with a zener voltage reference and a pass transistor on a heat sink.Once you have a stable voltage, it might be that an automotive 5 V USB adapter could work? You would have to test it with a variable power supply and see how low the input voltage could go before the output drops out.It's adding weight of course, but a battery could be a valuable part of such a system instead of a capacitor. It will help to smooth the voltage and will provide power when the bike is stationary. But perhaps this is more complexity than you want, since now you need to protect the battery from overcharging.

author: IanB
author_rank: Super Contributor
timestamp: April 26, 2014, 11:34:13 pm
content: Quote from: pereczes on April 26, 2014, 11:19:30 pmFair enough, but the regulation bit is what is importantQuoteWhen you are considering power demands in the order of 2-3 W then ordinary capacitors do not store energy to any useful extent. What they do is smooth things out over time periods of milliseconds.Secondly, it is not the capacitor you need to protect from overcharging, it is the USB power supply that must be protected from excess voltage inputs. This is where the regulation comes in as mentioned above: the regulation prevents you from blowing up your electronics.The capacitor, in fact, is hardly critical in its size or selection. You seem to be concentrating on the wrong thing.

author: IanB
author_rank: Super Contributor
timestamp: April 27, 2014, 01:20:10 am
content: Quote from: pereczes on April 27, 2014, 01:12:25 amSorry, should have previewed before posting. Fixed now.

author: IanB
author_rank: Super Contributor
timestamp: April 27, 2014, 01:38:27 am
content: Does the dynamo produce AC? I don't know. Let's consider that a "dynamo", by definition, produces DC. If it produces AC it is not a dynamo (unless the marketing people are being loose with their words). A machine that produces AC is an alternator.Secondly, let's dispense with this idea that a 1000 µF capacitor can store electricity. It really can't. When you say "after a while" or "in a few seconds" you are mistaken. The time involved is a small fraction of a second. Basically in an instant.Let's find out why.Energy storage can be measured in joules. For a little AA size 1.5 V alkaline battery, the energy stored is approximately 10,000 J or 10 kJ (round numbers).Suppose we charge a 1000 µF capacitor to 100 V, the energy stored is 5 J, or 0.005 kJ. If we charge the capacitor to 12 V (more likely from our 6 V dynamo) the energy stored is going to be 0.072 J or 0.000072 kJ.So let's compare 10 000 joules with 0.072 joules. Which do you think is more useful, an AA alkaline battery or your 1000 µF capacitor?The answer is, forget comparing capacitors to batteries. They don't compare.

author: IanB
author_rank: Super Contributor
timestamp: April 27, 2014, 02:28:41 am
content: Right, it produces AC. Then let's call it a generator and don't mention dynamos. It might seem pedantic, but it helps to avoid any confusion.I think you will find that batteries beat super capacitors in terms of size, of weight, and in the convenience of their voltage output. If you want to use it in the night to check emails and watch a movie, you need a battery. In that video you linked to, you saw how big those ultra capacitors were? They were as big as a water bottle. Can you imagine strapping four of those to your bike?

author: IanB
author_rank: Super Contributor
timestamp: April 27, 2014, 11:54:11 am
content: OK, some formulas.Energy storage is measured in joules. One joule is one watt for one second. In other words, 1 J = 1 Ws. It follows that 1 Wh = 3600 Ws = 3600 J. We could also say that 1 mWh = 3.6 J.Let's take a simple 1.2 V AA size NiMH battery (I pick this because it is simple, robust, safe, and easy to charge). It has a nominal capacity of 2000 mAh. Since it discharges at a more or less constant 1.2 V we can estimate its energy storage as 2000 mAh x 1.2 V = 2400 mWh = 8600 J.Let's now try charging the 2600 F capacitor to 2.7 V. Its energy storage is given by 0.5 x 2600 x 2.72= 9500 J = 2600 mWh.So that big capacitor is comparable to an AA size rechargeable battery in energy storage.Let's now look at charging rate.The hub generator produces a nominal 3 W at 6 V. For the sake of argument, let's put four of our AA rechargeable batteries in series (makes roughly a 6 V battery) and try charging them at 3 W. This would be a current of approximately 0.5 A. We measure this charging current as a "C" rate, relative to the battery capacity. Since the battery capacity is 2000 mAh and we are charging at 500 mA, this is a four hour charge rate or 0.25C. For an AA size NiMH cell the allowable charging rate is anything less than 1C, so we are far below the maximum possible rate and the battery will not be stressed.One reason I picked NiMH here is because these batteries are very forgiving to charge. Basically you can just dump current into them and they will be happy. If you over charge them a little it won't hurt them. You could basically just connect the battery up to the generator through a rectifier and that's it. What I would recommend for charge control is to put a temperature sensor inside the battery and stop charging when the batteries start to get warm. That's all you have to do.