Mosfet rectifier and boost cap?

[syc]

I was looking through some old threads and came across Martin's comparison of the mosfet rectifier circuit that yellow always recommends, and the various combinations of diode/capacitor circuits that Martin has published.



The boost cap seems to provide a significant advantage for getting the light output ramped up earlier, in a very simple circuit.

A drawback to the mosfet rectifier, is that it can't handle a cap in series immediately following it. But I was wondering if you could put a bipolar boost cap on the dynamo output before the mosfets, like a mosfet version of Martin's circuit 5, leaving out C1, and replacing the D1...D2 with the mosfets?



Could you for example put the C2 cap onto one of the dynamo leads before it goes into 1 or 2 in K1 in this circuit?



On a bottle dynamo, the dynamo frequency is high enough so that I haven't seen a need for the smoothing cap, and I thought it might be handy to get that .6v voltage drop back from the diode bridge. But of course, I have no idea if this would work or not.

Thanks,
Steve

[Steve K]

short answer: yes.

long answer: yes, probably..... I'm not going to guarantee anything without a lab test, but I don't see any problem. The mosfet rectifier can't block reverse current, so you can't use a smoothing cap, but it rectifies power just fine. Adding the series cap won't affect that.

and now the part where I wander away from the question that was asked and into a tangential issue: If low speed performance is important enough that you want to add 3 capacitors that are bulky and have relatively high failure rates, why would you use 3 LEDs in series? Just use two LEDs in series. Or add a switch to allow you to short out one or two LEDs at low speeds. This is just a pet peeve of mine. In a former life, I was taught to never use aluminum electrolytic caps because their reliability wasn't good enough for our type of product.

best wishes,

Steve K.
(try to find high reliability versions of the caps... ones that can handle the ripple current)

[syc]

and now the part where I wander away from the question that was asked and into a tangential issue: If low speed performance is important enough that you want to add 3 capacitors that are bulky and have relatively high failure rates, why would you use 3 LEDs in series? Just use two LEDs in series. Or add a switch to allow you to short out one or two LEDs at low speeds. This is just a pet peeve of mine. In a former life, I was taught to never use aluminum electrolytic caps because their reliability wasn't good enough for our type of product.

That's a legitimate question, and the general answer is that I'm just trying to explore the space of possible solutions.

The more specific answer is that my commuter got stolen, along with the dynamo light on it, so now I'm looking to build up another fairly simple light for a replacement commuter. And I was thinking of using the mosfet and dual crees, but with a smaller bipolar cap (like maybe 15uf) to get a boost in current at commuting speeds. The theory was that the light would come on a little earlier and also get a boost in brightness around commuting speeds. Martin's site said that a 220uf bipolar cap was right for low end boost on a bottle dynamo, whereas a 1500uf cap would be appropriate for a hub dynamo. Elsewhere Ktronik mentioned that he used a 100uf bipolar cap on his hub dynamo light to hit 910ma@27.5kph. Assuming that there's some proportionality to the sizing of a cap for the bottle dynamo, maybe you could get away with a 63V cap in the range of 4.7uf to 22uf, which are from 6mmx11mm to 10mmx16mm, which is pretty small, and they are cheap enough to buy a few in order to try.

I had also looked at the darlington/zener switch that yellow had on his light, and considered having an additional led come on as the voltage increased, but it seemed like too much trouble for this light. For commuting, 2 bright leds with the right optics seems good enough for me - especially if you can arrange a boost at the high end of your commuting speed.

On the other hand, it makes me wonder if the Supernova E3 used this kind of setup, or if they just went digital, with a PIC microcontroller and some power mosfets. I would think that the digital approach would work better because you don't necessarily want the extra led to come on just when the voltage reaches the minimum threshold, but when the dynamo frequency indicates a speed where an extra light may come in handy.

[unterhausen]

I think a high quality electrolytic cap will last 10 lifetimes of a circuit like this. If someone were to build with 5 year old Chinese electrolytics they got for pennies on ebay, there might be a problem. The complexity of a switching circuit may not be worth it for many of us.

[markus_i]

.
Steve K.
(try to find high reliability versions of the caps... ones that can handle the ripple current)

Look at professional electronics suppliers (e.g. Farnell, Digikey, RS Components,...). Usually, they'll provide datat sheets for the caps. and these data sheets should state the acceptable ripple current. Just don't buy any caps without these informations. Usually, they are a bit larger, and you might look at those with one or two steps higher voltage ratings.

Bye
Markus

[Steve K]

That's a legitimate question, and the general answer is that I'm just trying to explore the space of possible solutions.

The more specific answer is that my commuter got stolen, along with the dynamo light on it, so now I'm looking to build up another fairly simple light for a replacement commuter. And I was thinking of using the mosfet and dual crees, but with a smaller bipolar cap (like maybe 15uf) to get a boost in current at commuting speeds. The theory was that the light would come on a little earlier and also get a boost in brightness around commuting speeds. Martin's site said that a 220uf bipolar cap was right for low end boost on a bottle dynamo, whereas a 1500uf cap would be appropriate for a hub dynamo. Elsewhere Ktronik mentioned that he used a 100uf bipolar cap on his hub dynamo light to hit 910ma@27.5kph. Assuming that there's some proportionality to the sizing of a cap for the bottle dynamo, maybe you could get away with a 63V cap in the range of 4.7uf to 22uf, which are from 6mmx11mm to 10mmx16mm, which is pretty small, and they are cheap enough to buy a few in order to try.

It might be worth playing around and seeing what size cap works for the results that you want. Get some cheap caps for the experiments, and upgrade when you find a size that you like.

Keep an eye on the current coming out of the dynamo, though. Make sure that you don't exceed the LEDs limits. Also, be sure to accout for the change in capacitance that will occur over temperature, and check that the max capacitance won't overcurrent the LEDs.

Has anyone shown that high voltage caps are needed for this application? I don't recall what the frequency of the SON is at low speeds, but let's just say that it is 100Hz. Also, lets assume a cap of 100uF. At this speed, the cap has an impedance of 16 ohms, which at 0.5A will only see 8v. That's not much, and at higher speeds, the voltage drop will go down as the impedance drops. Seems like there's not much need for a 65v cap.
Note: a good guideline for derating electrolytic caps is to use a cap with double the voltage rating of the actual voltage encountered. or is it just a 50% margin??

I had also looked at the darlington/zener switch that yellow had on his light, and considered having an additional led come on as the voltage increased, but it seemed like too much trouble for this light. For commuting, 2 bright leds with the right optics seems good enough for me - especially if you can arrange a boost at the high end of your commuting speed.

On the other hand, it makes me wonder if the Supernova E3 used this kind of setup, or if they just went digital, with a PIC microcontroller and some power mosfets. I would think that the digital approach would work better because you don't necessarily want the extra led to come on just when the voltage reaches the minimum threshold, but when the dynamo frequency indicates a speed where an extra light may come in handy.

I've kicked around the idea of using voltage as an indicator of when to switch in a second load in series, but the frequency is a much more reliable indicator. A buddy has built such a circuit using dual one-shots, and it's not too complicated. I'm in the process of putting together one myself, although I'll admit that just two Cree XR-Es are pretty darned good by themselves. I've already built the light with 4 XR-E's so I might as well finish up the circuit.
Besides, I need a good excuse to build a board and play with the scope. :-)

oh... regarding finding good caps... I always go to the manufacturer's web sites, such as Kemet's:
http://www.kemet.com/kemet/web/homep...weben/products
Their literature will tell you everything you could want to know about what a good cap is.

regards,
Steve K.

[syc]

Keep an eye on the current coming out of the dynamo, though. Make sure that you don't exceed the LEDs limits. Also, be sure to accout for the change in capacitance that will occur over temperature, and check that the max capacitance won't overcurrent the LEDs.

Here in the temperate SF Bay area, temperatures range from say 40 to around 100 (fahrenheit). Do I need to worry about capacitance changes within those ranges? This light is going on a recumbent, and I don't think a recumbent has enough agility (or at least, I don't have enough agility on a recumbent) to ride it on anything even remotely icy.

Has anyone shown that high voltage caps are needed for this application? I don't recall what the frequency of the SON is at low speeds, but let's just say that it is 100Hz. Also, lets assume a cap of 100uF. At this speed, the cap has an impedance of 16 ohms, which at 0.5A will only see 8v. That's not much, and at higher speeds, the voltage drop will go down as the impedance drops. Seems like there's not much need for a 65v cap.
Note: a good guideline for derating electrolytic caps is to use a cap with double the voltage rating of the actual voltage encountered. or is it just a 50% margin??

I emailed Martin and asked him about the voltage ratings on the caps for circuit 10, and hopefully he won't mind if I excerpt his email here:

The caps C2, C3 have that high voltage rating because I want them to have a certain physical size to be able to dissipate some heat. They get warm because of the AC running through them. Electrolytics belong to the most lossy capacitor types out there.
Beyond this, you find that at the resonance frequency (resonance of dynamo inductance with C2,C3 capacitance) , there's a rather high voltage across the C2,C3. It's just one spot frequency somewhere in the lower third of the speed range, but the capacitors have to withstand that. When riding at high speed, the caps are safe.
You can use lower-voltage capacitors and your chance of having them blow or dry up increases, particuarly if you ride for a long time at that specific speed that has the resonance frequency. It doesn't mean something blows rightaway, it depends on too many parameters to predict when this happens. Catastrophic failure is not guaranteed, more often the caps lose some of their capacitance and gradually become more lossy, so you light gets dim and dimmer. I personally try to stay far away from any marginal situation with my values.

C1 doesn't carry much AC ripple, so I don't add much margin there.

It sounds like the high voltage rating is just to basically heatsink the caps at a resonance frequency. But if you are only using a single bipolar cap, instead of a pair of caps, would this resonance still arise? If the resonance does arise, and it coincides (maybe causes?) the current peak, then maybe I do need a big cap, because I would hope to get the current peak around typical commuting speed.

I've kicked around the idea of using voltage as an indicator of when to switch in a second load in series, but the frequency is a much more reliable indicator. A buddy has built such a circuit using dual one-shots, and it's not too complicated. I'm in the process of putting together one myself, although I'll admit that just two Cree XR-Es are pretty darned good by themselves.

I took my modded Dinotte Ultra 3 out the other night for an errand, and the ~200 lumens coming out of it was more than adequate for around town commuting, so I think dual crees at about 2/3 the current should be fine as well. Though when I can get back to long distance events, it would be good to have more lights in case I'm on a timed event - so one of Martin autoswitching circuits will go on my "event" bike.

A friend who's a hardware guy tells me that its really easy to build up an 8 bit microcontroller board with some mosfet switches for controlling the lights. Plus, I understand C and assembler a lot better than I understand capacitance, resonance and voltage, so there's more interesting things I could play around with.

But mostly I think its just an overactive engineering mind overengineering the problem for entertainment. Boost cap and a switch that changes the wiring from 2p2s to 4s would probably be the simplest solution to almost any riding I might reasonably do.

But where's the fun in that?

I've already built the light with 4 XR-E's so I might as well finish up the circuit.
Besides, I need a good excuse to build a board and play with the scope. :-)

I know the feeling (even if I don't have a scope...)

[unterhausen]

It sounds like the high voltage rating is just to basically heatsink the caps at a resonance frequency. But if you are only using a single bipolar cap, instead of a pair of caps, would this resonance still arise? If the resonance does arise, and it coincides (maybe causes?) the current peak, then maybe I do need a big cap, because I would hope to get the current peak around typical commuting speed.

...

A friend who's a hardware guy tells me that its really easy to build up an 8 bit microcontroller board with some mosfet switches for controlling the lights. Plus, I understand C and assembler a lot better than I understand capacitance, resonance and voltage, so there's more interesting things I could play around with.

...

I know the feeling (even if I don't have a scope...)

A bipolar cap is just two caps back to back, they have the same properties as two single electrolytic caps.

I have also considered using one of the tiny microprocessors for this. I have some of the TI MSP430 boards that are tiny. I am also fairly comfortable with analog electronics. I'm a little reluctant to use a microprocessor based circuit with my dynamo though, it seems like it's a complex problem that may actually be better solved with discrete electronics.

I'm still waiting for the pics of someone riding around with their scope -- I have a front rack but my battery powered scope is on the fritz.

[syc]

A bipolar cap is just two caps back to back, they have the same properties as two single electrolytic caps.

Thanks, so I can't lower the voltage rating based on that. Does Steve K's comment about the lower impedance at higher frequencies mean that a lower voltage rating is okay?

The particular dynamo I have is the AXA HR, and its been tested on the enhydralutris site. It looks like if I'm commuting at 15mph, it should be around 265hz, 6.6v and between .7 to .8 amps.
I think that with the mosfet rectifier, I won't need to disable the zeners to use all the current available, but if I use the boost cap, the zeners will have to go.

I have also considered using one of the tiny microprocessors for this. I have some of the TI MSP430 boards that are tiny. I am also fairly comfortable with analog electronics. I'm a little reluctant to use a microprocessor based circuit with my dynamo though, it seems like it's a complex problem that may actually be better solved with discrete electronics.

I'm not a hardware guy, so for me, its kind of an internet treasure hunt and then asking people who know better whether something would work. It would be nice (for me at least) to feel a little more like the master of my lighting destiny.

I'm still waiting for the pics of someone riding around with their scope -- I have a front rack but my battery powered scope is on the fritz.

The easier way to do it is to just put your bike on a stand, or put the wheel on a truing stand and wire the dynamo to your breadboard.

[unterhausen]

The easier way to do it is to just put your bike on a stand, or put the wheel on a truing stand and wire the dynamo to your breadboard.

What fun is that? Actually, I've threatened to put my bike on my rollers.

[Resqueline]

The ripple current rating of 'lytics is a factor of both capacitance & voltage. Increase the voltage rating and you get a higher current rating. The current rating is actually only dependent of can size - in a particular model series.
But there are huge differences between different makes & models and how much current they can pass in the same can size, so you might get away with a lower voltage rating and a smaller can. Shop around for the ones with the lowest ESR and hence the highest ripple current rating.
The resonance voltage will still rear its ugly head at the capacitor at a specific speed though.

[ktronik]

I am no expert, like the rest of the people on this tread, but I can say its come along way since I first told 'martin' what I had found, with his circuit & 3 luxeon stars...must be 5 years now since I plugged it in for the first time...of course he would not believe it & rang me up from Germany to confirm my findings...I did not believe it...I just keep staring @ the multimeter strapped to my bar..."how could this be so??" its only a 3w dynamo, I must have done something wrong...so I re-check my work... many times... but each time, the same result...to this day I have not had any break downs, not one with the most simplest of martins circuits, I have also done a lot of real world testing, & found some caps with high ripple & low ESR...

I have not had a problem with one bi-polar cap or 2/4 (non bi-polar)caps & I use these circuits every day...

ON the simple cap circuit I have used 4 /220uf, high ripple current caps, to give me a 100uf bi-polar config... for extra reliability with a higher voltage rating than I needed...these are a over kill as the reg bi-polar have been working no probs...

I have be using these component values:

+ jaycar cat # for people in OZ

#RE6348: 220uf 63v low ESR

FEATURES:

1. Stable, low impedance characteristics
2. High ripple current
3. Low D.C. leakage current
4. Extended life - high reliability withstanding 2,000 to 4,000 hours load life at 105°C
5. 105°C
6. Radial leads on standard pitch

* Impedance: max at 20°C 100kHz

** Ripple current measured at 105°C 100kHz

Size (Dia x Lmm): 10x20

Impedance: 0.060

Ripple Current mA rms: 1220


#ZR1023 1n5822 Schottky 40v / 3A


Fullwave, 220uF, 3 LEDs (off-road)

Read as Speed V I P
4.0 km/h 7.59 V 2 mA 0.01 W
5.0 km/h 8.12 V 23 mA 0.19 W
7.0 km/h 8.71 V 100 mA 0.87 W
9.0 km/h 9.18 V 180 mA 1.65 W
12.0 km/h 9.63 V 310 mA 2.99 W
15.0 km/h 10.05 V 480 mA 4.82 W
18.0 km/h 10.30 V 660 mA 6.80 W
19.0 km/h 10.36 V 670 mA 6.94 W
20.0 km/h 10.28 V 660 mA 6.78 W
24.0 km/h 10.16 V 600 mA 6.10 W
29.0 km/h 10.09 V 570 mA 5.75 W
35.0 km/h 10.04 V 550 mA 5.52 W
53.0 km/h 10.04 V 530 mA 5.32 W




Doubler, 220uF, 3 LEDs (off-road)
Speed V I P
4.0 km/h 8.30 V 41 mA 0.34 W
5.0 km/h 8.58 V 70 mA 0.60 W
7.0 km/h 8.92 V 120 mA 1.07 W
9.0 km/h 9.30 V 200 mA 1.86 W
12.0 km/h 9.76 V 350 mA 3.42 W
13.0 km/h 9.82 V 390 mA 3.83 W
14.0 km/h 9.74 V 370 mA 3.60 W
16.0 km/h 9.60 V 320 mA 3.07 W
18.0 km/h 9.55 V 300 mA 2.87 W
20.0 km/h 9.51 V 290 mA 2.76 W
23.0 km/h 9.48 V 280 mA 2.65 W
26.0 km/h 9.45 V 280 mA 2.65 W
31.0 km/h 9.43 V 270 mA 2.55 W
54.0 km/h 9.40 V 260 mA 2.44 W



Or for ON-road I use 100uf...


Fullwave, 100uF, 3 LEDs
Speed V I P
4.0 km/h 7.69 V 3 mA 0.03 W
5.0 km/h 8.14 V 26 mA 0.21 W
7.0 km/h 8.47 V 50 mA 0.42 W
9.0 km/h 8.79 V 100 mA 0.88 W
12.0 km/h 9.16 V 170 mA 1.56 W
15.0 km/h 9.44 V 250 mA 2.36 W
18.0 km/h 9.75 V 350 mA 3.41 W
21.0 km/h 10.04 V 490 mA 4.92 W
23.0 km/h 10.24 V 610 mA 6.25 W
25.0 km/h 10.42 V 740 mA 7.71 W
26.0 km/h 10.50 V 820 mA 8.61 W
27.0 km/h 10.58 V 900 mA 9.52 W
27.5 km/h 10.62 V 910 mA 9.66 W
28.0 km/h 10.55 V 900 mA 9.50 W
30.0 km/h 10.34 V 740 mA 7.65 W
31.0 km/h 10.33 V 710 mA 7.33 W
37.0 km/h 10.14 V 630 mA 6.39 W
43.0 km/h 10.06 V 600 mA 6.04 W
54.0 km/h 10.00 V 570 mA 5.70 W


Doubler, 100uF, 3 LEDs
Speed V I P
4.0 km/h 8.10 V 24 mA 0.19 W
5.0 km/h 8.30 V 42 mA 0.35 W
7.0 km/h 8.57 V 70 mA 0.60 W
9.0 km/h 8.79 V 100 mA 0.88 W
12.0 km/h 9.09 V 170 mA 1.55 W
15.0 km/h 9.49 V 270 mA 2.56 W
18.0 km/h 9.89 V 430 mA 4.25 W
19.0 km/h 9.91 V 470 mA 4.66 W
19.5 km/h 9.93 V 490 mA 4.87 W
20.0 km/h 10.00 V 500 mA 5.00 W
20.4 km/h 9.70 V 400 mA 3.88 W
20.5 km/h 9.68 V 390 mA 3.78 W
21.0 km/h 9.64 V 370 mA 3.57 W
21.7 km/h 9.63 V 360 mA 3.47 W
24.0 km/h 9.57 V 330 mA 3.16 W
28.0 km/h 9.49 V 310 mA 2.94 W
36.0 km/h 9.42 V 290 mA 2.73 W
55.0 km/h 9.37 V 270 mA 2.53 W



if you are really really fast or ebike you could use 47uf for MAX power output, but real peaky curve...

Fullwave, 47uF, 3 LEDs
Speed V I P
4.0 km/h 7.60 V 2 mA 0.02 W
5.0 km/h 7.83 V 9 mA 0.07 W
7.0 km/h 8.14 V 29 mA 0.24 W
9.0 km/h 8.44 V 61 mA 0.51 W
12.0 km/h 8.75 V 109 mA 0.95 W
15.0 km/h 9.03 V 160 mA 1.44 W
18.0 km/h 9.20 V 210 mA 1.93 W
21.0 km/h 9.42 V 270 mA 2.54 W
24.0 km/h 9.60 V 330 mA 3.17 W
27.0 km/h 9.81 V 420 mA 4.12 W
30.0 km/h 10.04 V 530 mA 5.32 W
33.0 km/h 10.28 V 680 mA 6.99 W
36.0 km/h 10.52 V 860 mA 9.05 W
37.0 km/h 10.53 V 920 mA 9.69 W
39.0 km/h 10.75 V 1050 mA 11.29 W
40.5 km/h 10.85 V 1150 mA 12.48 W
42.0 km/h 10.24 V 750 mA 7.68 W
44.0 km/h 10.26 V 720 mA 7.39 W
53.0 km/h 10.16 V 640 mA 6.50 W



Doubler, 47uF, 3 LEDs
Speed V I P
4.0 km/h 7.91 V 13 mA 0.10 W
5.0 km/h 8.05 V 21 mA 0.17 W
7.0 km/h 8.29 V 42 mA 0.35 W
9.0 km/h 8.47 V 63 mA 0.53 W
12.0 km/h 8.73 V 102 mA 0.89 W
15.0 km/h 8.96 V 140 mA 1.25 W
18.0 km/h 9.19 V 200 mA 1.84 W
21.0 km/h 9.46 V 280 mA 2.65 W
24.0 km/h 9.71 V 380 mA 3.69 W
27.0 km/h 10.00 V 510 mA 5.10 W
29.5 km/h 10.16 V 610 mA 6.20 W
30.0 km/h 9.70 V 380 mA 3.69 W
31.0 km/h 9.65 V 370 mA 3.57 W
32.0 km/h 9.62 V 360 mA 3.46 W
35.0 km/h 9.55 V 330 mA 3.15 W
54.0 km/h 9.40 V 290 mA 2.73 W


I have also cracked the 1000lm mark with these basic circuits!!!

http://forums.mtbr.com/showthread.php?t=481299

K

[syc]

Hi ktronik,

Thanks for posting your measurements, they are really interesting!

A wrinkle that has come up now that I actually go to look for parts is that even a 22uf bipolar cap seems to have ripple currents around 125mA. If the generator is putting out from 700-800mA then I suppose this ripple current rating is too low?

[ktronik]

for me simpler is better...so I love these basic circuit for dyno stuff...

Q: why a 22uf cap?? 47uf is way too peaky a curve let alone 22uf...(in the basic load matched circuit)

If you really have to use 22uf & one single bi-polar cap that has low ripple current (not recommended) is not enough, just make a 22uf (or any cap size) with higher ripple from non bi-polar caps...again I am not the expert here... but if you put, say 2 / 220uf 63v caps in series (1220ma MAX ripple) you would have a 110uf 126v cap...then top & tail the same caps & you have the bi-polar version...

do this to make what ever cap you want with high ripple...for extra points add some 'blead resistors' to keep it all balanced... linky


K

[Bandgap]

I'm a little reluctant to use a microprocessor based circuit with my dynamo though, it seems like it's a complex problem that may actually be better solved with discrete electronics.

I have built several different dynamo controllers , some with microcontrollers, some with analogue electronics, and some with digital electronics.

The MPU versions allow SO much more to be done with far fewer components.

If you have the time, go MPU.

Steve

[syc]

Q: why a 22uf cap?? 47uf is way too peaky a curve let alone 22uf...(in the basic load matched circuit)

This is for a bottle dynamo. On Martin's page, he notes that for a bipolar cap, a 220uf cap works for a single LED, and he doubles the capacitance when he breaks it into a pair of normal caps. For 2 leds, he specs a pair of 220uf caps instead of a bipolar cap, which seems to translate into a 100uf bipolar cap.
If a 100uf bipolar cap is what you would use for bottom end boost, then to scale it for boost at the road and offroad speeds, I figured I'd try using capacitance around .1 and .2 of the low end boost values. So, try 10uf for road and 22uf for offroad.

If you really have to use 22uf & one single bi-polar cap that has low ripple current (not recommended) is not enough, just make a 22uf (or any cap size) with higher ripple from non bi-polar caps...again I am not the expert here... but if you put, say 2 / 220uf 63v caps in series (1220ma MAX ripple) you would have a 110uf 126v cap...then top & tail the same caps & you have the bi-polar version...

So, do you spec the caps at 4x of the bipolar uf rating when you break it out into 4 caps? Or do you spec it at 2x of the bipolar uf rating?

do this to make what ever cap you want with high ripple...for extra points add some 'blead resistors' to keep it all balanced... linky

Yeah, I was wondering about the bleed resistor and how necessary that is. Since I was going to try the mosfet rectifier, I think I'd have to bleed the caps into the circuit after the first LED.

What I was really looking for was the most efficiency with the lowest parts count and very small size. Wiring up 4 caps and 2 resistors instead of a single bipolar cap starts to kill it for me.

At this point, I think I'm just going to do a 2 LED light with the mosfet rectifier, a single bipolar cap for low end boost (since its easy to drop in) and the easy standlight with the supercap, voltage clamp diode and resistor. That should give me a lot of light, quickly, out of a simple bottle dynamo with only 10 parts to wire up.

[ktronik]

I have built several different dynamo controllers , some with microcontrollers, some with analogue electronics, and some with digital electronics.

The MPU versions allow SO much more to be done with far fewer components.

If you have the time, go MPU.

Steve

MPU you say... please enlighten us with these low part count circuits... as you said, so much more can be done with microcontrollers... please do tell...

K

[Calina]

MPU you say... please enlighten us with these low part count circuits... as you said, so much more can be done with microcontrollers... please do tell...

K

+1 , please, please.

[syc]

+1 , please, please.

Lets see if we can prompt him into telling us his design by indulging in some brainstorming? This is just brainstorming of course...

Here's a thread where a voltage regulator + pic controller + fet gives you a 6 component PWM LED lamp.

Starting with that, the direction I would brainstorm it would be to replace the LiPo batteries with boosted rectifier of the type we're discussing in this thread (or just use the original diode + cap design), keep the existing fet used for PWM, and replace the 6V bulb with a string of 4 LED's that are wired 4S, but by default are shorted to 2P2S by a pair of power mosfets controlled by the PIC. Use a PIC that has a frequency counter, and wire that into the circuit so that you can use the frequency to switch the LED's from 2P2S to 4S, as well as maybe use the PWM fet to control the actual brightness of the LED's.

At low speeds, the boosted rectifier would power the LED's in 2P2S mode, and above some programmed frequency, the PIC would unshort the LED string back to 4S when there's enough speed to power them all. Instead of switching between a doubler and a full wave rectifier, you switch the LEDs between parallel and serial.

Parts count:
6 for the original circuit
3 for a mosfet rectifier + bipolar cap
2 power mosfets for switching the LEDs from 2P2S to 4S (maybe only 1 if you can use the same chip as the mosfet rectifier)
1 Cree MC-E with directly addressable dies

So, maybe a dozen components for a 4 LED autoswitched dynamo light with good low end performance, and PWM controlled output? Is that do-able?