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generator keeps blowing led lights

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wolfie

Experienced Member
Joined
Jul 1, 2009
Messages
187
Location
ontario
i have a generator for my bike and i recently decided to install it on the bike that i am currently using. i haven't seen the origanal lights that came with the generator for over 2 year so they are probably long gone or destroyed, so i went down to the dollar store and they had this rear light with 5 red led's that runs off 2 AA. i hooked it up in series with a front light from another bike i had and it worked great at first. i've probably gone about 20km with them on and only 1 of the led's are still working. do you think they would keep blowing because of the generator or could the led's be that cheap? do you think adding a capacitor might help? i also remember trying leds in the past with the same problem. they alway blow. i am not sure what voltage the generator is but is has not blown my front light which is only 2.4volt bulb.
 
You're overdriving the LEDs--there really is no such thing as a "cheap LED". If the rear light was rated for 3V, then you need to make sure that the voltage isn't exceeded or the LEDs will self-destruct. You can make up a cheap voltage limiter by putting 4 silicon rectifers in series across the taillight--1N4002s should be fine. The forward voltage of a silicon rectifier is about 0.7V, so 4 of them would start conducting at about 2.8-3V.
 
i just thought that if i was overloading them they would all blow at once and not 1 by 1. all 5 leds are wired in parallel and that parallel circuit is wired in series with the front light because i thought the generator was 6volt but i have not blown my front light bulb which is 2.4volts which makes me think that it is a 3 volt generator.
 
wolfie said:
i just thought that if i was overloading them they would all blow at once and not 1 by 1. all 5 leds are wired in parallel and that parallel circuit is wired in series with the front light because i thought the generator was 6volt but i have not blown my front light bulb which is 2.4volts which makes me think that it is a 3 volt generator.
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It's not quite that simple...
With nothing connected, the generator is probably capable of putting out more than 3V, but when you connect the light bulb the voltage gets pulled down to (presumably) 2.4V. When you put two items in series, the voltage is not necessarily divided equally; suppose that with no load the generator puts out 5V and you have the bulb and LEDs in series. The 5V might well be divided with 4.5 V across the LEDs and .5V across the bulb; the higher resistance of the LEDs will keep the bulb from lighting, the voltage won't drop very much, and the LEDs (and the sequencing electronics if any) will probably blow. Another factor to consider is that the generator is quite probably putting out AC, and the LED light may not be able to handle that.

A meter on the generator would help...
 
If the generator is putting out AC (which is quite likely), a diode in series will also help. But these little generators rarely put out anything that remotely resembles being well-regulated voltage. The voltage can rise fairly high. Also remember that if the generator (really a magneto) is rated at a nomial 3V output, that's RMS, not peak-to-peak. It's the peak voltages that will do semiconductors in.
 
the generator does seem to be AC. i didn't know that. would the silicon rectifers still work? i find that the generator seem to have a limit. it seem to not put out any more power whether i'm going 2km/h or 10km/h.
 
Interesting problem, here. A single diode will waste a half-cycle of the AC output and headlight output will suffer. You could use a 4-diode bridge to provide DC to everything and then limit the voltage across the LEDs with either a zener diode (3.9V would probably be okay) or 4 rectifiers as we already mentioned.

However, if the generator really is rated for 3V output, I'd run the headlight straight from it and then use a diode in series with the taillight and 4 silicon rectifiers or a single 3.9v zener diode across the LEDs to limit any overvoltage. Or you could use a 3.3V LDO, such as a 78L33 in series with the LED taillight and diode and forget about the 4 rectifiers.

Lots of choices on this one.
 
i have been doing alot of thinking and research on google and now have an idea. if i use 4-diode bridge could i use the DC output to charge 2 1.2v rechargable batteries? i was also wondering if it would be a good idea to use a capacitor?
 
I set up a similar arrangement using lead-acid batteries on our tandem. Put a sealed-beam headlamp on the front and used the generator on long downhills during the day. It was very nice to have on a long tour. Rechargeables can be pretty finicky about charging conditions, which is why I elected to use lead-acid and not NiCd batteries. But you can probably get away just fine with NiCd or NiMH cells. I would not use Lithium-ion, however--they're too easy to damage.

No capacitor necessary--and you get the bonus of having light when you're stopped.
 
Chuck(G) said:
I set up a similar arrangement using lead-acid batteries on our tandem.
<snip>
Click to expand...
A tandem??? Got any pictures?

@wolfie:
Sounds like a plan. As Chuck says, no capacitor needed; just put rechargeable AAs in your new taillight and hook it and the bridge in parallel with the generator (alternator) and headlight.

It would be nice to know what the alternator is actually putting out; remember that you'll lose almost 1 volt (0.7) in the bridge.
 
Thanks Chuck for all the info. i just finished making the diode bridge and there is still enough power to light an LED light so i'm happy. now i just have to get or find 4 silicon rectifers.
@Mike:
i plan on using 2 NiCd C type batteries in the front headlight. i would also like to know the voltage that it is putting out but i have no idea where my digital multimeter is.:(

the next generator i plan on installing is a 12volt generator that i can use as an electric motor:). i got one for my bike and one for my dads bike.
 
wolfie said:
Thanks Chuck for all the info. i just finished making the diode bridge and there is still enough power to light an LED light so i'm happy. now i just have to get or find 4 silicon rectifers.
<snip>
Click to expand...
?? If you've made the bridge then what do you need 4 silicon rectifiers for?

And I'm assuming that you're using another battery-powered taillight which will already have current-limiting and/or blinking circuitry; keep in mind that putting an ordinary LED directly across a couple of batteries will probably blow it.

I had an electric bicycle, but before I could put new batteries in it somebody stole it. :cry:
 
MikeS said:
?? If you've made the bridge then what do you need 4 silicon rectifiers for?
Click to expand...

I think he wants to use the 4 silicon rectifiers as a clamp/overvoltage protection for the LEDs. 4x0.7 = 2.8v.

Scavenging rectifiers is pretty easy if you have a bunch of deal CFLs around. Don't break the glass and just use a hacksaw to cut open the base. There's a little PCB in there with all sorts of interesting parts, usually including several silicon rectifiers. Dispose of the glass part the way you would normally.
 
yes. i don't want to be killing any more LED lights. i will be using the old lights case and just be replacing the LED's.

if someone wants to try stealing mine they can try but it is bolted to my bike. i could spend $1 to get a new one if i wanted but i am cheap when i can get away with it.
 
Chuck(G) said:
I think he wants to use the 4 silicon rectifiers as a clamp/overvoltage protection for the LEDs. 4x0.7 = 2.8v.

Scavenging rectifiers is pretty easy if you have a bunch of deal CFLs around. Don't break the glass and just use a hacksaw to cut open the base. There's a little PCB in there with all sorts of interesting parts, usually including several silicon rectifiers. Dispose of the glass part the way you would normally.
Click to expand...
D'oh again; I think I'll just sit on the sidelines from now on ;-)
Yeah, I scavenge those CFL bases too; amazing what they manage to squeeze in there.
 
sorry to bring this up again. i found a sealed 6V 4Ah / 20HR LEAD-ACID BATTERY. i was wondering if i can just hook it up to the output of the diode bridge? my generator is 6 volt 3watt. i found a picture of what it looked like when i first got it.
DSC_0205s.jpg
 
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