MC-E 2S2P to 4S via switch?

[syc]

I have an MC-E coming in for another light, and was planning on building Martin's Circuit #10 (switch between voltage double and bridge rectifier).

But I was thinking the other night, and after sketching it out, I think it should be possible to set up a DPDT switch to change the wiring of an MC-E between 2s2p and 4s. And just front it with the basic bridge rectifier circuit. Would this give me much the same effect of circuit 10, minus the boost and smoothing that Martin's circuit provides?

If I understand things correctly, when the MC-E is wired 2s2p, it should have the same voltage and current requirements as 2 LED's in series, except that each LED will only see around 250ma, but the full voltage. So this should be no worse than what we see with 2 LEDs in series on a basic rectifier circuit. Once the speed picks up, you could switch it over to 4s, and get the full 500ma through each led. So that at all times, all LED's are in use.

Are there any drawbacks to this approach? Am I missing something? It seems that if you're using circuit 10, you are switching the LED's very briefly anyway as you go from doubler to rectifier, and the LED's tend to be more efficient in terms of light production when the current is lower.

Thanks for any advice,
Steve

 

[Eamon]

I don't think it makes any difference in terms of output. According to Znomit, circuit #10 gives 250ma in doubler and 500ma in rectifier. When #10 is in the doubler mode, emitters in 4s configuration are getting 250ma each at whatever voltage. Without the doubler, and emitters in 2s2p configuration, the emitters are getting 250ma each at whatever voltage. Assuming Vf, both configurations total to 3.2W.
I do think you would lose some of the low-speed output. The doubler increases the voltage at lower speeds, which a 2s2p config does not do.


Eamon

 

[znomit]

Are there any drawbacks to this approach? Am I missing something? It seems that if you're using circuit 10, you are switching the LED's very briefly anyway as you go from doubler to rectifier, and the LED's tend to be more efficient in terms of light production when the current is lower.

Martin points out a small drawback... from Martins page:
A word of caution: C1 should be mounted inside the headlight enclosure where it must be properly connected to the LED. Should the LED disconnect from the circuit, C1 charges to a rather high voltage (can be 100V when going fast). This could not only be dangerous to a person's health but upon reconnection to the LED, a dramatic peak current most likely decolors or destroys the LED. This is valid for most circuits on this page.

Regarding low speed performance, I'm running 5 leds on my road bike. I did a very hilly 420km ride finishing at 8am. Even though completely shagged and climbing decent hills I always had enough light in doubler mode. I do think three is a good number though.

I'll be wiring my MCE light like this with the first two LEDs lighting the top of the beam brighter. It behaves like 3 LEDs.

 

[syc]

Martin points out a small drawback... from Martins page:
A word of caution: C1 should be mounted inside the headlight enclosure where it must be properly connected to the LED. Should the LED disconnect from the circuit, C1 charges to a rather high voltage (can be 100V when going fast). This could not only be dangerous to a person's health but upon reconnection to the LED, a dramatic peak current most likely decolors or destroys the LED. This is valid for most circuits on this page.

Hmmm, for the particular bike in question, I doubt it will ever get to even 25 mph (it will be on my wife's around town bike). I'll have to figure out which caps I'd like to put on it - smoothing and/or standlight.

Regarding low speed performance, I'm running 5 leds on my road bike. I did a very hilly 420km ride finishing at 8am. Even though completely shagged and climbing decent hills I always had enough light in doubler mode. I do think three is a good number though.

These days, even sneaking out for a 100km training ride seems like fever dream - I may need to start taking vacation days to ride while the baby is in daycare.

I'll be wiring my MCE light like this with the first two LEDs lighting the top of the beam brighter. It behaves like 3 LEDs.

Eyeballing the efficiency charts, it looks like if you don't mind spending the money, using MC-E's in 4P config gives you the same power draw a 1 XR-E, but with higher efficiency. If you didn't want to soak up as many watts, you could put a bunch of 4P MC-E's together and maybe get 20%-25% more light output. Four of the 4P MC-E's would have as much light as your 5 LED headlight, though it would be hard to get a narrow optic.

Steve

 

[yellow]

Jürgen H's MOSFET --> WAY easier to build, nothing to switch - nothing to fail, better working (believe it or not, I am a convict)

PS: depending on Your dyno, might work even when all 4 in series.
But possibly no useable light when just going by foot with the bike
(a three in series already gives enough light to navigate, then)


PPS, as to that "better efficiency of led at lower currents"
whoever created that myth, ...
I have a triple Cree (Xr-E) on my town bike --> gets ~500-550 mA to the led in series.
Using that light and grabbing a handheld one with a single XR-E but this one runs on 900 mAh --> the single kicks the "low" current triple back into the woods. Like it got switched off.
CURRENT, CURRENT, CURRENT thats what counts ...

 

[znomit]

PPS, as to that "better efficiency of led at lower currents"
whoever created that myth, ...
I have a triple Cree (Xr-E) on my town bike --> gets ~500-550 mA to the led in series.
Using that light and grabbing a handheld one with a single XR-E but this one runs on 900 mAh --> the single kicks the "low" current triple back into the woods. Like it got switched off.
CURRENT, CURRENT, CURRENT thats what counts ...

3 LEDs at 500mA put out twice the light of one at 900mA.
See here.
To summarize you triple puts out only half what it should.