A torchaholic's pushbike lighting system

A torchaholic\'s pushbike lighting system

I've read quite a few threads on pushbikes customised with 10's of Nichia cyan LED's and LED's on the wheels frightening the living daylights out of non-torchaholics. What I intend to build is a practical self-powered LED lighting system for my nwly aquired pushbike. The main criterion is ruggedness...conventional pushbike lights cannot survive our appaling roads.
The system is to be powered either by a dynohub or a "bottle" dynamo charging either a small SLA or a set of Nicads or NiMH cells. Each light will have 8 high brightness LED's in it. I've chosen to use an "add-on car reversing light" as the headlight and an "add-on" rear fog light as the back light. These normally take a 21W filament bulb. I'm thinking of increasing the amount of light at the front by use of an additional set of cyan LED's along with the marker white LED's so the effect will give a light output resembling the Halford's VISON series headlight bulbs.
I've thoght about mounting the LED's through holes drilled in the lamp lenses; the c iurved surface will then allow you to have a nice beam spread.
The lighting system would be controlled via a motorbike-style handlebar-mounted thumbswitch.
What type of electrical output have bottle dynamos got... is it AC or DC? The AC (if it is AC) would be of a varying frequency depending on the speed of the bike. The Dyno Hub generator would generate a low frequency AC whilst the bottle dynamo with its high rotatiomnal speed will produce waveforms of several kHz.
To produc the charging circuit I would need first to convert the dynamo's AC into DC then incorporate some kind of regulator circuit to limit the voltage across the system. Some kind of regulator circuit needs to regulate the power so as to charge the battery up and not blow the LED's. Possibly the best and cheapest circuit limiter would be a zener diode. Alternatively I would use an IC regulator (LM317 comes to mnd from my distant dabblings with PSU electronics.....!
The charging circuit and the SLA/NiCd battery will be located in a diecast metal box attached to either the seat tube or slung under the cross bar.
What I should end up with is a self powered bike light system that unlike filament dynamo operated lights would be maintenance free, and he lights are still powered even when the bike is stationary or if the bottle dynamo is disengaged from the wheel.
If you have any circuit suggestions and advice please post to this thread... your collective torchaholic knowledge would be very useful.
Thanks

Matt (EMPOWERTORCH)

Re: A torchaholic\'s pushbike lighting system

Can you explain to us Yanks what a pushbike is? I'm assuming a standard bicycle that you pedal?

Re: A torchaholic\'s pushbike lighting system

Yanks??? More like we are Colonial Philistines. hehehe

If the 'generator' has AC output, you would certainly need a diode rectifier (not zener). Zeners act like resistors: add in series, and divide in parallel. I had a Heathkit 10 Mhz Ocilliscope that I built, and had to replace a zener in it. Very expensive from Heathkit, but I found 2 in series would make the correct voltage and much cheaper than buying a not otherwise available one from the company. If you use a zener, make sure it can handle the needed current.

Re: A torchaholic\'s pushbike lighting system

Originally posted by Charles Bradshaw:
Zeners act like resistors: add in series, and divide in parallel.

Click to expand...

<font size="2" face="Verdana, Arial">er, do they? I thought Zener diodes were a special class of diodes, that maintain a fixed voltage across them. This property allows them to be used as a crude voltage regulator. Add in series would be true I guess, but I'm not sure about in parallel. From what I (think I) know, two equal valued Zeners in parallel would maintain the same voltage across them as just one, but with better regulation.

About AC rectifiers, you have a choice of a simple single (?) diode rectifier which "wastes" the current when the AC voltage goes negative, or a more efficient full wave rectifier bridge using four diodes. The cirucit for that is pretty simple; a quick google search would probably be a lot of help

Re: A torchaholic\'s pushbike lighting system

I'm showing my age here... As I remember the little "bottle" dynamos/generators that are driven by the side of the tire produce about 6 to 8 VAC, maybe half an amp to one amp. (that was about 40 years ago). So you'll need to rectify the output for DC voltage , a small full wave bridge should do the job. (Alternatively, you could use a small DC PM motor as a DC generator).

Not sure of what dynohubs are, presumably a generator that is in the hub of the wheel, rather than driven off the tire's surface.

If you need higher voltage, a simple doubling circuit with two rectifiers and two electrolytic caps will offer higher voltage, but at the loss
of half the current. The hi-tech solution is with a "boost" switcher IC regulator, I'll leave the description of that to those more knowledgable in that department.

Bateries are a good idea so you can have light when going slowly or standing still.

/ed brown in NH

Re: A torchaholic\'s pushbike lighting system

Bicycle dynamos produce AC output. The frequency changes with speed. The maximum (open circuit) output _voltage_ will also change with speed; essentially the maximum output voltage will go up linearly with speed, and can be quite high when you are going down hill.

The dynamo is designed with a rather interesting combination of internal resistance and inductance, so that the maximum output _current_ will remain relatively constant.

What the variable maximum voltage and maximum current means is that with a load on the system (say a lamp), as you speed up, the following happens: The voltage starts rising and the current flowing through the load increases. As you get faster, the voltage gets higher, and reaches the point where it pushes the maximum current (as limited by the dynamo) through the load. At this point, when you go faster, the dynamo output voltage stops going up, since it is delivering its maximum current. However the higher voltage is _available_ should the load be interrupted in any way.

Because of the natural current limiting, you can use a dynamo to run a sufficient number of LEDs _directly_, using a simple rectifier (or if you double the number of LEDs and put the back to back, the LEDs themselves would act as the rectifier). You just need enough LEDs to carry the full current.

You can also use the voltage/current aspect to get more power output of a dyno; if you connect a couple of lamps in _series_ to the dyno, then (once your speed is high enough) full current flows through both lamps and they both light up. Two 6V 3W lamps in series will cause the dyno to operate at 12V and produce 6W, but only if you go fast enough. You can buy systems with 2 3W lamps in series, along with a switch so that you can have 1 lamp on at low speed, and both on at higher speed.

For battery charging, you need a simple regulator circuit, but it has to be a shunt regulator. A common voltage regulator is a series regulator, and it acts to block excess current from the load. But with the constant current aspect of the dyno, the output voltage will go up and up until something breaks.

You need a shunt regulator which will send the excess current around the load to ground; this is where a zener diode will come in; a zener diode is a diode that doesn't conduct until a sufficiently high voltage is applied, and then it will conduct. You need to select a zener diode which can carry the maximum dyno current; this will be a pretty hefty device depending upon your design voltage.

-Jon

Re: A torchaholic\'s pushbike lighting system

Thanks guys!
It looks like it could be quite a simple job to do then! With the amount of power avaiilable from the dynamo, it may be necessary only to use the dynamo for some of the time to keep the battery refreshed. SLA or nicads, that is the question...
Looking at the current available I reckon that I couled just about power a 5W Luxeon for the front light. f I were to get a luxeon I ould mount it facing backwards towards the reflector of a car spot light. With two corrctly placed Luxeons you could feasably have a dipping beam!
However, I'm trying to keep this system as simple and as cheap as possible, and I'll use some of Mr Paul Flannery's fine LED's in my front light as planned. The main reason for sticking with cheaper technology is that here you don't want to make a pushbike or motorbike too appealing to thieves!

Re: A torchaholic\'s pushbike lighting system

sorry about typos folks... I don't know whether its me or my web box keyboard's batteries thats at fault!