led rectifier for slow motion

I see a lot of interest in mosfet rectifiers so the lights can turn on at slow speeds(ie while walking the bike). I haven't tried a mosfet rectifier yet, but I just tried wiring up my MC-E as a rectifier(having 4 dies was just begging for it). The results were rather disappointing, I still couldn't turn on the lights while walking. Have to be rolling around 10km/h before lights turn on...though they seem to stay on while I slow to about 7.
I haven't measured AC voltage coming out of dynamo at low speeds recently, but I seem to recall getting 3volts at 5km/h (this is a shimano 3d72 dynamo).

Can someone explain what is going on? I understand that when wired as a rectifier only half of the leds turn on at a time resulting in less light at higher speeds. However I don't understand why leds don't turn on sooner.

I don't see a mosfet rectifier performing any better with an MC-E. Are people here using leds that turn on sooner(ie at lower voltage)? Is that determined by Vf in the datasheet?

The only option I see at this point is to use a supercap for a standlight, then use a microcontroller to shut the light off before the cap is drained and then turn the standlight on while dynamo voltage is too low to turn on leds. Surely there is a better, simpler, more analog way to do this.

I'd guess somehow your wiring must have a flaw. Using the MC-E as a rectifier will make each half-wave light a two-chip series. This would, in my understanding, have to be equivalent as far as start-up speed to a two-LED light with low-drop rectifier. I can say that my triple starts up at speeds well below 10km/h.

panicmechanic said:

I'd guess somehow your wiring must have a flaw. Using the MC-E as a rectifier will make each half-wave light a two-chip series. This would, in my understanding, have to be equivalent as far as start-up speed to a two-LED light with low-drop rectifier. I can say that my triple starts up at speeds well below 10km/h.

Click to expand...

Thanks. I forgot to put a load on the rectifier(starting projects past midnight is not the best idea). I added another led for load, circuit works as expected now. Going to try to see if I can make this into a real circuit with a standlight.

update: Actually driving an led is a bad idea since high voltage drop from this rectifier means the led will be limiting current. I shorted the rectifier..and holy crap is it bright. Now to figure out how to hook up an ultra cap to a couple of the leds to add a standlight. Suggestions?

Zener, ultra-cap, LDO, standlight LEDs, in that order.

Put them all in parallel, use a Shottkey bridge, and put a huge capacitor across them?
You can't do that with a mosfet bridge because they conduct backwards, so the cap will discharge into the dynamo winding between cycles.

You can use two Shottkeys and two mosfets to lose 0.4V drop.

if you can stand a little more speed before it all works, use two series pairs of led die and the same circuit and make it brighter.

Personally, the flashing drives me nuts.

Anti-parallel combinations of leds across a dynamo are a bit hard to de-flash (for me anyway - I seek a good answer if anyone has one

Steve

Some of the circuits on http://pilom.com/BicycleElectronics/DynamoCircuits.htm indicate how to maximize low speed performance. e.g circuit 8. My twin XP-G circuit 7 is a bit flickery at walking speeds but is very nice at about 5 going on to 6 mph. I wonder if a single XP-G (or XM-L) would be even better, not that I'm too bothered about performance at such low speeds, but can see it might be desirable under certain circumstances.

...... also, I guess you've probably read this back to front

http://www.candlepowerforums.com/vb/showthread.php?220699-Mosfet-rectifier-and-boost-cap

wait.... which problem are you trying to solve?

To get light at low speed, you need to have a low voltage drop across the load and rectifier. For a low drop across the rectifier, schottky diodes are simple, cheap and popular. For essentially no voltage drop across the rectifier, mosfets are the way to go, but are not cheap, not as simple, and honestly, not really required in most cases.
To get a low voltage drop across the load (i.e. LED), just use one LED series. Or use a LED with a lower voltage drop, such as an amber LED. Since the dynamo is only putting out 0.5A, the light output will only be 200 lumens or perhaps much less.

If you wired up the four LEDs in the MC-E as a full wave rectifier, then you have two LEDs in series instead of one, and it won't operate at the very lowest speeds.

If you want light at slow speeds and more light at high speeds, then you need to get a bit fancier. Either use the tuning cap such as shown in the pilom circuits by Martin, or switch in extra LEDs in series when riding faster.

Personally, I use a mosfet bridge rectifier with two LEDs in series at low speeds (with a Schmidt dynamo). The LEDs are Cree XR-E. I think the newest Cree products have a Vf that is 0.1 or 0.2v lower. This design satisfies me while riding at 3.5 mph up a very steep hill on my commute.

regards,
Steve K.