Efficient LED circuitry for Bike Dynamo

[ACharnley]

Some brilliant posts here. Let me explain the situation a little further. The 5V is after a buck / zener reg which takes up to 100V AC down to that voltage. 5V was chosen because at low speed it might be only 7V.

Multiple devices use the 5V source, hence a skhottky is needed to prevent the capacitor store from sending it's current to them. This wouldn't be required by using a lipo charger, but consider that the circuit must also be on while charging, i.e "passthrough" which is generally not advised with lithium batteries. I've looked for a small USB with passthrough but can't find one.

The capacitor charges at 4.8V but it's internal resistance means it'll be a few tenths of a voltage lower, and it discharges a few more quite quickly. This is enough for the red lights to dim right away (when using current limiting resistors).

There's a small boost circuit on aliexpress for usb which runs as low as 0.9v and gives 5V. I can use this for capacitance and 'power on' to ensure the voltage gets back upto 5V. The small buck circuits I have only start operating at 4.5V, hence the boost to 5V is required, and besides which the boost can't give 3.2V (white) if the voltage in is 4.8V.

With the figures from above, it looks like a 5F capacitor or thereabouts will easily cover my needs. I'll have a go and will report back!

Regards, Andrew

 

[Tony Stewart]

Keep in mind my previous opinions were for bike applications where you want a headlight with more power than the dynamo can provide, not simple 150mW rear Red flashing LEDs ( anything will do there).
If you want to see the road as well as a motorcycle and be able to remove the headlight from your helmet for other purposes, read again to understand what I have said so far.

If you get a good Shimano or Schmidt Maschinenbau dynamo , you can charge up your flashlights , while riding by day and use them by night while still charging, and have some electronic skills. The LiPo's used in torches are far better than an ultracap because the voltage drop for 50% of the power is so low. This also extends the lifetime.

Ultracaps will last as long as a filament light bulb in hot weather during the day >50'C in the sun while it is 40'C in the shade.

This is what I suggest for a torch
**broken link removed**

You would have to modify a LiPo chargers to run off the dynamo DC so can charge up all your batteries including your cell phone from peddle power.

But by having spare cheap LiPo 16850 cells for the torch and other applications, you can have the best battery charger , "head" light ( either frame / handlebar or helmet mounted or both ). I would use 1 for frame (like a motorcycle) and one for helmet for directional light.

Any questions? ( always have a spare)

 

[audioguru]

These lights:- ..... have a supercapacitor inside them, and they charge up in less than a minute from a dynamo, and run for 10 minutes or so after the bike stops. The light doesn't seem to dim when the bike stops, as long as it has run for a minutes or so before. When left for a long time, the light slowly fades out over several minutes.
The corresponding front light:- ..... also stays on when stationary, but dims when the bike stops. It seems to have two brightness levels, and I guess that the lower brightness when stopped is done to make the supercapacitor last longer.

Supercapacitor? Who says so, not the sales sheet. The prices of those lights are extraordinary!

I have a calculator that has embossed on its front, "dual power, solar cell and battery". Its battery died and it did not work even in sunlight so I looked inside and saw that the "solar cell" had no wires and was simply a photo of a solar cell.

 

[Tony Stewart]

The ratio of charge to discharge time tells what power ratio you have.

I can make assumptions based on experience with LEDs.

If we assume you have a 20W dynamo then you have a 2 W total load. For LEDs this represents a 0.5 Ohm load above 3V.


If you have a 0.2V leeway on Headlight from 3.1 to 2.9V with 0.5Ohm ESR and then max current is 0.2V/0.5Ohm = 400mA ( perhaps 300mA for headlight and 100mA for brake lights)

With a 10 minute time constant and the headlight current cutting in half, or 300mA then C=I*dt/dv= 0.3A*600s/0.2V= 600/0.06= 10,000 F
THis is the capacitance of a typical 18650 3.6V LiPo cell.

An ultracap in this voltage range for only 100F or 1% of the required capacity is $8 in volume.
https://www.digikey.com/product-detail/en/MAL219690101E3/4703PHBK-ND/5015887

Therefore , I doubt it is an ultracap.

 

[Diver300]

Well I opened it up and I've measured the discharge rates.

I was a bit over-generous about it not dimming. It does dim when power is lost, but it takes about a minute for the current to halve. The circuit has a 5.5 V 1.0 F capacitor, something like one of these:- https://uk.farnell.com/panasonic-electronic-components/eecs5r5h105/cap-double-1f-5-5v-rad/dp/9697497

Apart from that, there is a bridge rectifier, two SOT23 components, and a 20 Ω resistor and a 4k7 resistor, and the single 5 mm LED. I am fairly sure that the components are a current limit circuit, running at about 32 mA

The screen shot shows the discharge rate. The yellow is the capacitor voltage, the red is the voltage across the 20 Ω resistor. Having seen the capacitor data sheet, I would not expect such a big voltage drop, and I suspect that the capacitor, being 3 years old, has degraded. I might try a new one.

 

[ACharnley]

For the front, I have a headtorch that can recharge from the same usb. The front lights are just a bonus, or for when I can't be arsed to get the headtorch out.

I'm not surprised the German lights are crap. So are the dynamo rectifiers they sell.

 

[dr pepper]

Joes comments are good, I can add to that a little, if you do long trips reliability is an issue, you can improve that further by having modules, if you had a controller module plugged in to the system, you could make yourself a little patchlead that completely bypasses the controller and connects the dynamo direct to the lights, maybe via a simple rectifier, then carry this with you, it'll only be small and light, and will get you by if the lights pack up somewhere dark.
Also using lipo's is ok, but you might be better using standard batteries then you can get replacements while away if your stuck.