Author Topic: Cheap efficient 5V voltage regulator for bike dynamo  (Read 675 times)

Seadog

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Cheap efficient 5V voltage regulator for bike dynamo
« on: August 17, 2018, 07:54:40 PM »
I've always dreamed about doing some long distance bike trekking, and would love to have a decent setup, but at mustachian prices. I picked up a great hybrid bike 3 years ago for $300 that retailed new for almost triple that, and *finally* found a great set of panniers at Value village after a year long search for $28 which retailed for about $200+ tax, and currently have the option to buy a neat shimano hub dynamo from my local bike donate/rebuilt/sell/reuse depot where I volunteer at for a fraction of new.

Unlimited free electrical power in addition to transport!

The main problem is the hub, basically being a generator outputs electricity in alternating form, with a frequency/voltage/current that's a function of the speed and tourque the hub is being spun at.

This would be fine for say a hard wired light, but the two issues are is that I don't want to get a new set of lights (current ones are USB rechargeable) and I would also ideally like to be able to use this to charge phone/GPS and other USB things while on the road.

Sensitive electronics however require a nice stable 5 V DC output. Is there a way to manufacture such a regulator cheaply?

There are commercially made ones such as this: https://www.spacycles.co.uk/m2b0s208p3467/CINQ-5-The-Plug-III  but at $200+ hardly good value IMO. There are cheaper ones, but nothing that reflects what I feel should be maybe $20 for some basic components.

I did come across others who built ones such as this: https://www.dr-lex.be/hardware/bikeled.html which consists of a basic rectifier bridge, and then a commercially available 5v regulatorwhich could be built for $10. The problem I read however is that these designs are notoriously inefficient at high speeds, and basically any energy above a threshold gets wasted, voltage gets regulated (lowered), but current doesn't get upped, rather dissipated as heat.

They say what I'm after is a switching regulator, which is also a transformer in addition to regulating, which seems like it's a good idea since the power is coming from me. Is there an easy to build, or similar bit of electronics that can be hijacked to accomplish this without too much trouble?

I guess it's a bit of a niche type of transformer, since at the 5v range most applications aren't concerned about 15W of wasted power. Someone modified a hand cranked radio/flashlight/usb thing and that's the sort of thing I'm looking for. 

FazJaxton

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Re: Cheap efficient 5V voltage regulator for bike dynamo
« Reply #1 on: August 31, 2018, 12:31:05 PM »
Yes, you can build something like this for cheap.  The wasting-type power supply is a linear regulator, which just burns off any extra power as heat.  You are correct that you want a switching power supply.  The electronic part is called a boost/buck converter (boost raises voltage, buck lowers voltage).  Something like this is likely what you're looking for:

https://www.digikey.com/product-detail/en/linear-technology-analog-devices/LT1107CS8-5-PBF/LT1107CS8-5-PBF-ND/889723

Other than that, the electronics you link to are pretty close to what you want: rectifying bridge, a shottky diode on the output, and lots of capacitance on both sides of the regulator.

If you are riding for a long time, this would probably be fine, but I don't know how hard it is on a lithium ion battery to be charged off and on for short intervals (i.e. stop and start on a bike).  Also power supply design is complicated, and driving LEDs is a lot simpler than charging a cell phone.  You might be better off charging a small lead acid battery, or maybe one of those travel-charger things, then using that to charge your phone.  If your custom electronics damage something, it is the cheaper middle-man, not your phone.

I'm a software guy, but understand basic electronics.  I'm glad to help you if you have more questions, but I am definitely not an expert in the area.  You might have better luck asking on hobby electronics forums like SparkFun or Adafruit.  Good luck!