How to make a rectifier for LED bike light?

Okay, here's what I have :
- a 6V/3W dynamo (Dymotec S6 which gives a constant output of 6V from a speed as low as 10km/h=6mph)
- a white Cree XR-E LED for front light (measured 3.18V at 350 mA)
- a red Cree XR LED for tail light (measured 2.08V at 350 mA)

I want to drive both LEDs in series (they can be overdriven since they are both capable of being driven at upto 700 mA). Now I need a rectifier to convert the dynamos alternating current (AC) to direct current (DC) but I don't know much about making a rectifier. So how do I have to construct an efficient rectifier and which parts do I need and where can I get them?

Thanks for your help!

Cheers,
Erasmus.

Simplest way is to use four diodes.
The common 1N4001 (also 1n4002.......1n4007) will do the job easily and is available at just about every electronics store. They are rated at 1A and each drops (wastes) around 0.8V in operaion.

Slightly more efficient (very slightly more) are Schottky diodes which only waste 0.5V each.
The 1N5819 is an exampe of these. Any 1A device will do

All are avaiable from Maplins inthe UK, don't know about other countries - maybe Radio Shack n the US. Hmmm Belgium - not sure their.

I have no way to draw diagrams here - so bear with me here.

A ha here you go ---- http://en.wikipedia.org/wiki/Diode_bridge

Solder or use a screw connector bloc to connect two diodes in series, nose to tail.
And do the same again with the other two.

The mid points of the two strings are the ac inputs from the dynamo.

Then tie the plus ends (with a printed band) of the two strings together - this is your positive connection to the leds.

Tie the negative ends together and this is your -ve connection.

Don't forget, the frame connection of the dynamo is likely to be one of the ac outputs, so the leds and thie connctions have to be isolated from the frame.

You don't need a switch as I think the S6 comes off of the wheel to turn the lights off.

If you have a dynamo that needs a switch - like a hub dynamo - put it in one of the dynamo leads, not between the rectifier and the leds unless your diodes are rated at 200V or more.

There are ways to connect bipolar transistors and mosfets to make a more efficient 'active' rectifier (mosfets approach 99.9 pr cent efficiency) but I doubt you will be able to fell the difference in padalling effort anb the cut-in speed will only be about 1mph different. - although with the S6 you may have to save eery millivolt (see below)
I have used all four of the above methods with success.

I think the S6 has a clever voltage regulator in it, and if your total rectifier and led drop is more than this voltage, you may problems.

The more conventional type of dynamo has a really good characterisic for driving two power leds in series.

Steve

Hi Erasmus,

the easy way would be to get a bridge rectifier module. 4 pins, two marked ~ for the AC, one marked + and one marked -. Not (easily) available as Schottky, but if they're matched for voltage and operating current, Silicon diodes aren't that much worse than Schottkys. The problem might be that the S6 is voltage regulated (and has an internal setup completely different from an 'ordinary' bike generator). I don't know
- if the output of the S6 will provide enough voltage for your setup (3,xx V front, 2,xx V rear, 1,xx V Rectifier)
- if the S6 retains a sufficiently low current limit

I.e. you have the chance that your LEDs won't light up as well as the chance that they'll get fried. If it works out, be sure to tell us. If it doesn't, sell the S6 (e.g. on ebay) and get a SON (ok, basically any other hub generator will work almost as well, at least for the first few years...).

Bye
Markus

In the states, you can purchase a "full wave bridge rectifier" IC for a couple USD. I have confirmed that this will convert the AC of a Shimano Nexus dynamo hub, with somewhere around a 1V drop, to DC.

As far as I know, the shimano LED light that is sold for use with the hub, seems not to use any sort of rectification at all. In fact, it doesn't even employ a capacitor. At low speeds the LED will flash quite conspicuously. Shimano effectively employ the LED as a half wave rectifier by dumping the AC right into the LED, probably through a resistor. At more than a mile per hour or more, the flickering disappears due to the frequency of the AC power being speed dependent.

While you effectively lose all the current half the time, you don't suffer voltage loss from one or more additional diodes.

There was an english guy and his wife who rode across europe and asia who made a neat device for his Schmidt dyno hub. It could charge his laptop during the day, run lights at night and charge his cellphone among other interesting things. Can't seem to find a link, but will post one later if I run across it.

One thing that did occour to me was that an LED is a diode.
Why not make up the rectifier using the "diodeness" of the LED.
Half the LEDs will light on one cycle of the AC and the other half will light on the reverse cycle of the AC.
This would dampen the flickering effect even more.

Nick

Your setting is very close to a system I built couple of years ago. In addition to your system I used 5 nimhs to keep the leds alive when not driving the bicycle. When moving, the dynamo charged the nimhs and provided electricity for the leds.

The challenging part of the project is the fact that you have so low voltage (6 volts). Let's assume you lose 2*0,75=1,5 volts in the bridge rectifier. It means that the dynamo has to provide 7,5 volts to keep the rectified voltage at 6 volts. However, because the dynamo has quite high internal impedance, its output current will decrease (compare it to a battery with high internal resistance: in order to get higher voltage out of it you have to lower the load current). All this means that quite low voltage drop in the rectifier leads to a big loss in power. Remember, power=voltage*current, and in your application both will drop; voltage due to rectifying and current because of the internal impedance of the dynamo.

This problem will be greatly diminished if you use 12 volt dynamo. That's the easy solution. I decided to take the hard way and built an active rectifier. Basically it uses 4 logic-level mosfets (2 p-types and 2 n-types) in bridge configuration instead of diodes. The gates of the mosfets are driven with four operational amplifiers (LM358, IIRC). The input current for the op-amps is taken from 8 capacitors which were charged through diodes by the input AC during each half-cycle (I did not want to use small coin batteries for this purpose...).

Should I rebuild the circuit now I might add emitter followers between op-amps and mosfet gates. Now my op-amps have quite hard time driving the gates that have clearly too high gate-source capacitance, some 100 times greater than the maximum load capacitance suggested by the op-amp datasheet! But since the frequency is quite low the circuit works fine anyway.

Last, I strongly advice you to seek as much information as you can find. Do not try to re-invent the wheel, there are hundreds of people who have carried out similar projects. You can find info on active rectifier by googling, using e.g. words "active rectifier", "ideal diode". Here is some good general info on bike electricity: http://www.myra-simon.com/bike/tips.html .

And I wish you all the best with your project! :thumbsup:

-N

Doh!Nut said:

One thing that did occour to me was that an LED is a diode.
Why not make up the rectifier using the "diodeness" of the LED.
Half the LEDs will light on one cycle of the AC and the other half will light on the reverse cycle of the AC.
This would dampen the flickering effect even more.

Nick

Click to expand...

You have to check what is the maximum reverse voltage for the leds. IIRC, it is some 5 volts for luxeon III.

-N

Erasmus said:

I want to drive both LEDs in series (they can be overdriven since they are both capable of being driven at upto 700 mA.

Click to expand...

I don't think you have any risk of overdrive. No matter how fast you spin a standard dynamo it can not provide more than 500-600 mA. The coils inside the dynamo get saturated and simply can't create more current. It is the voltage that goes up when increasing speed.

-N

Talk about reinventing the wheel, I think I just about reinvented this thread:

Bicycle dynamo powered Fenix P1D-CE or other Cree XR-E light

I'm still trying to figure this whole thing out myself, but here are a few useful links that I have found so far:

http://www.c-realevents.demon.co.uk/inprogress/dyntorch/dynotorch.htm
http://www.pilom.com/BicycleElectronics/DynamoCircuits.htm
http://www.ktverkko.fi/~msmakela/electronics/led/index.en.html
http://www.mouldy.org/projects/High-Power-LED-MTB-Light
http://lists.topica.com/lists/bikecurrent/read
http://nordicgroup.us/s78/dynamo.html
(That last link is thumbs-down on dynamo lights, but it still has some useful information down at the bottom.)

Pkease continue to add information as you make progress. I'll do the same.

Thanks!

MOSFET - rectifier rulez

If You want all the power possible, there is a much more "intelligent" way (read: more complicated than possibly necessary)


the MOSFET-rectifier. Sorry, I only have this link and there is the circuit added as .pdf at the end
I cut some of the needed boards with a Dremel, but some guy sells them around here for just some bucks.
photo if the thingy


two or three Luxeons already light with this thing, when the ordinary rectifier or a schottky does not even show a glimmer (at low speed).
BUT it is not possible to have a Gold Cap lighting up all the leds when the bike stands still, one led at least has to flicker without any condenser attached.
(of course You have to get rid of possible z-diodes inside the Dynamo to power up more than 2 Led in series)

then there is the real thing (which I am working on):
automatic switching of a given number of led. 2 on low, 3 on high speed (maybe even more, depends on dynamo)
again a link
might work if C1 and Dz get connected after the uppest led (?)

You might want to check out this post...

https://www.candlepowerforums.com/threads/111773

I get around 14w from the hub dynamo... its a little different now & peaks @ around 1A into 3 LEds...with cree XR-E that is like 720lm from a dynamo, best light I have ever used (other than my L&M HID)


Ktronik

Thanks for the additional links! That is awesome, very inspiring news! I definitely need to get my mitts on a few of these units for my family and friends. With that kind of brightness available, dynamos gain a whole new lease on life among cyclists -- although it is also true that powerful LEDs and lithium batteries make it equally possible, for some at least, to just go with a battery-powered rig instead. I'd go with both -- compact battery powered unit on the helmet, and dynamo powered unit on the bike. This is really a godsend for bicycle tourists and commuters alike.

If you are a DIYer, I will have CNC heads out soon for you to fill & can make you the dynamo circuit I used to put in it...



This is a Dynamo unit here, & when we put cree's in it it will be close to HID bright!!

Ktronik

Beautiful! I'm game for that!

Cool CNC project! I plan on buying myself a Schmidt SON hub soon. Why can't I run four Schottky diodes into:
+___xre____xre
I I
-____xre____xre
Granted I have limited experence with dynamo hubs, but I see everyone using 3 3w instead of 4 so what gives? I really like this guy's TL082 current sense circuit:dynamo circuit and Ktronik's CNC Head for Cree's would rock together, but I want as much light as possible
If I'm totally overestimating the output(6-8VAC 500mA-1A) of this hub tell me

ktronik said:

If you are a DIYer, I will have CNC heads out soon for you to fill & can make you the dynamo circuit I used to put in it...

This is a Dynamo unit here, & when we put cree's in it it will be close to HID bright!!

Ktronik

Click to expand...

Ktronik-
Any chance you'd sell just the dynamo circuit? I have a tri-lux III that I'd love to run off my Shimano dynohub. Currently, the light is driven off an nflex and a 14.8 li-ion pack...

Thanks,
baker

If you have a look @ the circuit it is only a few componts... for fun you should have a go at making it youself... if you get stuck PM me...

Best

Ktronik

crash82 said:

Cool CNC project! I plan on buying myself a Schmidt SON hub soon. Why can't I run four Schottky diodes into:
+___xre____xre
I I
-____xre____xre
Granted I have limited experence with dynamo hubs, but I see everyone using 3 3w instead of 4 so what gives? I really like this guy's TL082 current sense circuit:dynamo circuit and Ktronik's CNC Head for Cree's would rock together, but I want as much light as possible
If I'm totally overestimating the output(6-8VAC 500mA-1A) of this hub tell me

Click to expand...

Its the caps that do the magic, not the diodes...all the info on how & why it works is on his site... please have a good read first, it will come to you...

ktronik said:

If you have a look @ the circuit it is only a few componts... for fun you should have a go at making it youself... if you get stuck PM me...

Best

Ktronik

Click to expand...

I was afraid you'd say that...electrical science is the only class I've ever had to take twice! The same laziness that cursed me in that class has prevented me from doing a dyno version of my tri-lux. Let me review the schematic and check it out...

baker

Nereus said:

I decided to take the hard way and built an active rectifier. Basically it uses 4 logic-level mosfets (2 p-types and 2 n-types) in bridge configuration instead of diodes. The gates of the mosfets are driven with four operational amplifiers (LM358, IIRC). The input current for the op-amps is taken from 8 capacitors which were charged through diodes by the input AC during each half-cycle (I did not want to use small coin batteries for this purpose...).
-N

Click to expand...

You don't need the op-amps or capacitors, only the four mosfets - with cross-couple the gates.

I have been using a 4xmosfet active rectifier in my Luxeon bike light for a few years and it has been reliable - my system separately switches between 6V (2xLuxeon) and 12V (4xLuxeon) so the fets get used at both voltages.
Note - It will not work if there is a big capacitor or a battery on the output as it does not reverse-block.

I apologise if this is what Yellow's website said, I could not get on to it.

You can also use four cross-connected bipolar transistors with base resistors, although you waste the base current. I have used this reliably as well. You have to reverse emitter and collector connections to get past the inherent base-emitter Zener. The Zetex ZTX and equivalent sirface mount devices are ideal for this as they saturate well even with a forced gain of 200.

I did use op-amps when I had to build a 3-phase active rectifier.

Ktronik - I do like that machined housing!!! Are they collimators or reflectors?

The inverse-parallel connection using two leds to rectify each other gives less light than with a rectifier and both leds connected in parallel.
The reason is that doubled current in the back-to-back arancements cuts led efficiceny by about 40 per cent.

There is also significantly more flicker at low speed when using a Schmidt although i am not sure why.

Steve