Help with a dynamo - Page 1
Forum section: electronics / beginners

author: trevorethier
author_rank: Newbie
timestamp: January 06, 2011, 07:07:25 pm
content: I was planning on building a bike light setup based around a hub dynamo.The exact hub is this:http://techdocs.shimano.com/media/techdocs/content/cycle/SI/HubDynamo/SI_2YH0A/SI_2YH0A_001/SI-2YH0A-001-ENG_v1_m56577569830673555.pdfSo i assumed that it would produce ac power between 0-6 Vpp.Upon getting the hub, i discovered that it will actually produce 0-100 vpp. As the load approaches closer to 3W it drops to around 6 Vpp.Getting to my question, the load is not constant (want to do different modes of light), how do i make a circuit that will keep the voltage steady around say 5-10 volts? keep in mind i wish to keep this as power effective as possible.Thanks for the suggestions.

author: Simon
author_rank: Unknown
timestamp: January 06, 2011, 07:14:42 pm
content: The simplest solution would be a zenner of say 8V 5W to cap the voltage. Although this is a wasteful method. Have you considered a battery setup ? Are you saying that if the dynamo is loaded the voltage comes down to 6V ? do you have a load/V plot/figures ? it might be a good idea to use a battery with a regulator, you could use a SMPS by rectifying the voltage with a doubling rectifier you would have about 12V so would still have the 6-7.2v needed for a lead acid battery

author: Floyo
author_rank: Regular Contributor
timestamp: January 06, 2011, 07:39:09 pm
content: What I would do is make an sepic type smps. This would ensure that you always have a stable output voltage and current, as long as your dynamo can handle the current. There are IC's available that handle all the switching for you and only need an external coil and a small hand full of components to make it work. Of course you would first rectify the AC voltage, possibly with Schottky diodes for the lowest voltage drop, and higher switching speeds (I think normal diodes will be just fine in this application though, it depends on the output frequency of the dynamo.)The rest of the solution depends a bit on what type of lights you want to use, I assume LED's. If so then you could opt for a converter IC witch has the ability to act as a constant current source, this way you ensure that you have the highest efficiency because there are no linear circuits wasting power, like drop resistors or linear regulators.http://en.wikipedia.org/wiki/Single-ended_primary-inductor_converter

author: trevorethier
author_rank: Newbie
timestamp: January 06, 2011, 09:01:35 pm
content: Zener , out of the question as only 3 watts total and i wish to maximize power going to the led's (want to do a nice headlamp).i was hoping to stay away from batteries and only use super caps with just low power led blinkers going when the hub is not moving and high power led on when it is rolling.and yes the dynamo goes down in voltage as the current draw goes up, there is no data sheet other then the link above, i can provide scope screen shots under different loads.i already have a circuit set up that will work with voltages from 2 to 15 volts getting the voltage to this range was the part i was unsure of, as i didnt think a SMPS would work for pulsating dc or not.Thanks for the help, will post the finished project in the project area when/if i make it work

author: RayJones
author_rank: Frequent Contributor
timestamp: January 06, 2011, 09:30:44 pm
content: Ahh, the good old hub dynamo.I've got a SON on one of my bikes and it has happily driven two CREE LED's in series for years, The only other component is a full wave rectifier.The secret?They essentially behave as constant current devices, typically limited to around 600mA, which works out to your 6V under full load for the nominal 3W rating.I say nominal because you can actually pull more power if you let the volts rise.Note that two CREE LED's in series are closer to 8V than 6V :-)The best solution is a shunt regulator, and this is the classic element in a light fitted with a bulb. Without it you can pop bulbs on fast downhill runs.A suitably rated zener will get the job done.Or as you already want to use LED's, simply use the LED's as the voltage limiters, and hang your taillight over the LED pair with the supercap and a blocking diode?.

author: Simon
author_rank: Unknown
timestamp: January 06, 2011, 10:00:46 pm
content: Quote from: trevorethier on January 06, 2011, 09:01:35 pmThe idea of a zenner was as last resort voltage protection, under normal conditions it will be out of the circuit, it will only come into play at high speeds and low current draw.The SMPS would work, all you need is input caps to smooth the pulsing out. not sure about your hub dynamo but all "dynamos" for bikes i ever had made AC despite the name, so you need to rectify and smooth it anyhow for any electronics processing of the power

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2011, 10:06:38 pm
content: There's no need for a zener, an SMPS or a resistor, as Ray said, it's a constant current source so just connect the LEDs to it via a bridge rectifier and it will be fine. If you want some light when you're stopped add a diode, a large capacitor, resistor and some little 5mm LEDs so car can still see you at junctions.By the way, it's rated for 6V RMS at full load which is 8.5V peak, not 6V peak.You could also try connecting a small fluorescent tube and starter to it without a ballast and it'll probably work.

author: trevorethier
author_rank: Newbie
timestamp: January 06, 2011, 10:29:50 pm
content: Yes it is ac(very very ugly sine, that is proportional in hz to the speed of the wheel) even though it says dynamo, i like the idea of zener to keep the voltage spikes out, i wasn't thinking of it like that at first.i am now thinking on having: dynamo -> bridge rectifier-> zener at max input of SMPS -> SMPS-> super caps-> Led drivers -> ledAnd for how im doing the Led's, im not just hooking up headlights that are always on.i plan on having flashing lights that are always running as long as there is power in the caps, bike in the city and visibility is necessary then i want to have a headlight that i can turn on and off at will using what ever extra power that is available.could it be as simple as hooking up the headlights straight to rectified dynamo with an on off switch and the blinker led's would get the correct amount of current no problem?I want the flashing led's as solids don't stand out as much during the day.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 06, 2011, 10:45:38 pm
content: FWIW, the B&M IQ Cyo's and the like use a "current doubler" circuit which is simply a buck SMPS running at ~50% duty cycle. Thus their single 3W LED is driven at ~1A.They have a caution not to exceed 7.2V if using a battery, ie the voltage is actually being regulated down, not the current, so if you exceed Vf of the LED, smoke is released.Yeah you could use a SMPS, but it is more things to go wrong on a dark rainy night, after all bikes are not renowned for their waterproofness :-).I honestly think if you want flashing you'd be better off with a pulsed low side mosfet to interrupt flow through your headlight LEDs, and a zener to catch the overvoltage when the load is removed.If the flasher circuit fails in that dark rainy night you can easily bypass the lot with a switch for constant on mode.You will find the dynamo starts producing usable light a little above walking pace, and noticeable flicker disappears above 10km/h.

author: trevorethier
author_rank: Newbie
timestamp: January 06, 2011, 11:11:43 pm
content: I was planning on putting the whole circuit, once completed in a water bottle sealed with silicone to keep the whole thing dry.I like the idea of a simpler circuit, can you further elaborate on the circuit, and parts suggested?But i want the headlights to run only when the hub is running, with an on off switch override.and the blinkers to run on whatever power is charged to the caps.im not familiar with pulsed low side mosfed, and how you would use those to blink the led's

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2011, 12:17:35 am
content: If you want flashing then you could just use a MOSFET switch to flip between charging the capacitor and powering the LED whist the dynamo is running, then changing to powering the LED from the super capacitor whilst stationary.I don't see any need for an SMPS, providing there's always a load on the dynamo there shouldn't be any need for it, the voltage should be low enough. The only thing I'd recommend is a zener if you're flipping from charging the capacitor to powering the LED but it could even be avoided if you implement zero crossing i.e. only switch when the AC voltage is zero.I dislike flashing bike lights and think they should be banned but if you do it at a high frequency it could be invisible and you could change the duty cycle as the speed changes to try to keep the LED a constant brightness.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 07, 2011, 02:03:38 am
content: The simple circuit I used was a full wave bridge rectifier to convert the AC to DC.The two CREE LED's were then in series across the rectifier's output.That was it, a solid light whenever the bike was in motion, I didn't even bother with a switch, not even in daytime.Now to add a pulsing mosfet, simply use a suitable mosfet at the negative end, source to negative, drain to cathode of bottom LED.First thought is an IRL540N - logic level mosfet.Something like a 555 could then drive the gate of the mosfet to produce the flash effect.You could replace the 555 with a PIC or similar if you want special flashing modes like a rapid flash, dwell, rapid etc.You will need a blocking diode and capacitor to tap off from the top of the LED string to derive power for the 555.The bypass switch would be across the source/drain of the mosfet - shorting it out when on.This still automatically makes the LEDs only run when the bike is in motion.Most important though is you must include some form of 8Vish shunt to protect against the HV swing when the LEDs are off.The zener is the most obvious choice, so long as it is 5W or more. (8V x 600mA)Hope this helps?P.S. sorry for the delayed response. I typed it up earlier by the time I finished the reply it seems the cloud vanished in a puff of smoke and EEVblog went AWOL for quite some time. :-(

author: trevorethier
author_rank: Newbie
timestamp: January 07, 2011, 02:35:48 am
content: thank you all for the replies (don't worry about timeliness really im in no rush and free advice is free advice) i think i have a good idea of how to do it now.As for,QuoteI know its annoying to drivers to a certain extent and i wish it wasn't, but i'm sick of being cut of or almost hit and having the driver say they didn't see me, i obey all rules out of my own self preservation, try to stay out of possible blind spots and my commute is only bike lanes and it still happens from time to time. i understand drivers sometimes don't expect some one to be there so i'm just trying to make myself more noticed. Chances are I will modify the circuit once i get it working to change from a state of on off blinky to a full power half power hopefully a suitable compromise.

author: Unknown
author_rank: Unknown
timestamp: January 07, 2011, 03:19:50 pm
content: Quote from: trevorethier on January 07, 2011, 02:35:48 amI agree that being visible is more important than not being annoying. One issue I notice with the popular blinking bike lights (which IMO blink because the batteries and LED's are too wimpy for proper continuous light) is that their fairly low frequency makes it hard to see motion from the corners of your eye. To determine direction and speed of a blinking light takes much more time than for a continuous light. So in my opinion, continuous lights are safer.But this may be different in your part of the world, depending on visibility, street lighting and drivers.

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2011, 08:55:18 pm
content: I think flashing gets more attention though.You could make it so it's continuous whilst the bike is moving and flashing when stationary. That way you have a bright light when it's important to see where you're going and flashing to get the attention of cars when you're stopped at a junction.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 07, 2011, 09:11:46 pm
content: Yeah, I've never rode with a flashing headlight in the dark and I suspect that it would drive me mad.However The way I read the OP, the flashing was for increased visibility of the bike in daylight, not night....I do however agree with depth perception issue of flashing lights at night.For this reason I use a constant on light in conjunction with the attention getting strobed version.

author: Simon
author_rank: Unknown
timestamp: January 07, 2011, 10:03:58 pm
content: I'd agree that flashing can be a problem making it more difficult for drivers to estimate the lights course. I'd suggest a constant light with a flasher.

author: trevorethier
author_rank: Newbie
timestamp: January 08, 2011, 07:21:52 pm
content: It is going to be solid headlights, flashing tail lights, with one solid in between the flashers.

author: sonicj
author_rank: Frequent Contributor
timestamp: January 10, 2011, 04:25:18 pm
content: if it were me, id get a planet bike super flash until the rear blinky is perfected. they run forever on 2 AAA batts and can easily be visible at around a mile. too many fools sending text messages while driving to take a chance!-sj