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Old 07-05-12, 07:12 PM   #1
boomhauer
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LED lights are diodes. Right?

Why can't I just buy some CHEAP LED ligths (diodes) and run them in a bridge circuit from my Sanyo hub generator? seems like I could light my bike up like a christmas treee; for cheap. Should I stop my line of thinking right here before I waste a lot of research time?
thanks (in advance)
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Old 07-05-12, 09:23 PM   #2
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check the peak inverse voltage ratings on the LEDs you want to use. It can be done though.
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Old 07-05-12, 10:43 PM   #3
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I've done it before. The LEDs protect each other, so reverse voltage isn't a big issue. Be sure to get LEDs that can handle the current from the dynamo, which is 500ma. Regular 5mm LEDs won't cut it, since they are only rated for about 30ma continuous output. You can alternately ruin a bunch of low power LEDs in parallel, 15 or so per "quadrant" of the bridge. It'll be a nightmare of wiring, which is why it isn't usually done.
A simpler alternative would be to use two power LEDs wired in parallel with each other but with their polarities reversed, so only one is on at a time.
Neither of these two variants allow standlights or smoothing capacitors, and thus have pronounced flickering at walking speeds. Also, as a strategy for making a light you can see the road/trail with, you're better off using, more focused emitters than more small (and floody) ones. You could make a baller be-seen party light with a lot of small emitters all over your bike, but it won't be much use for getting around I'm the dark.
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Old 07-06-12, 04:37 PM   #4
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Be sure to get LEDs that can handle the current from the dynamo, which is 500ma.
If you're using high-powered LEDs directly in the bridge, you need twice as many as you would expect, since half of them are off half the time (and the other half is off the other half of the time). You could probably save money with a cheap diode bridge and half as many LEDs on the output.

Last edited by CaptainCool; 07-10-12 at 11:48 AM.
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Old 07-07-12, 07:49 PM   #5
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The important part of a bike light (particularly, a dynamo light, where the power available is low) is the optics. A home brew solution will not have suitable optics, and won't do a very good job of putting light where you want it, and not putting it where you don't.
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Old 07-07-12, 07:56 PM   #6
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The important part of a bike light (particularly, a dynamo light, where the power available is low) is the optics. A home brew solution will not have suitable optics, and won't do a very good job of putting light where you want it, and not putting it where you don't.
If your goal is to light your bike up like a christmas tree this isn't a problem.

Do you put an angel or a star on top though?
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Old 07-08-12, 12:29 AM   #7
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cheap diode bridge
While I parsed the OP's post as an attempt to make a really cool string of colored dots all over the bike, yes, diode bridges are in fact about 70 cents on Digi-Key. You're not saving paper money by cutting it out of your circuit.
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Old 07-08-12, 11:30 AM   #8
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Instead of going to a lot of trouble and expense, just go to the local Dollar Store and get strings of battery operated xmas lights. They have strings that will run a long, long time on two D cells and you can change flash patterns with the push of a button. Unless, of course, you are doing this as a learning experience. There is something to be said for that.

If you are stuck on the idea of running it off your generator then find out it's output voltage. I'm guessing the xmas lights operate at 3 volts (two D cells in series). Not certain how you would limit the output from your generator if it is higher than that. Maybe a resistor in series with the light string?
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Old 07-09-12, 06:03 AM   #9
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Each LED has a minimum forward voltage and a maximum allowed current. You have to design your circuit to take that into consideration.

If you use red LEDs, they have a fairly low forward voltage, probably around 1.8 volts each, and they usually want 20 or 30 milliamps. If you run the LEDs in series, if you put 5 of them in series you need 9 volts to get them to light (at minimum). However, you probably want to have more than that, because you also need a series resistor to limit the current, and the value of the resistor is calculated by the amount of voltage left over AFTER the voltage drop from the LEDs.

So if you want to run 5 red LEDs and have 12 volts, you need to have a resistor that's (12-9)/0.03 = 100 ohms, all the parts in series.

If you run different colors, they will have a different forward voltage; in general the farther up the spectrum the higher the voltage.

If you run multiple colors at once you really don't want to run them in series, because they're unlikely to have compatible specs and some will be much brighter than others, you'll want to run in parallel, each color string with their own current limiting resistor, otherwise only one will probably light.
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Old 07-09-12, 07:45 PM   #10
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Each LED has a minimum forward voltage and a maximum allowed current. You have to design your circuit to take that into consideration.

If you use red LEDs, they have a fairly low forward voltage, probably around 1.8 volts each, and they usually want 20 or 30 milliamps. If you run the LEDs in series, if you put 5 of them in series you need 9 volts to get them to light (at minimum). However, you probably want to have more than that, because you also need a series resistor to limit the current, and the value of the resistor is calculated by the amount of voltage left over AFTER the voltage drop from the LEDs.

So if you want to run 5 red LEDs and have 12 volts, you need to have a resistor that's (12-9)/0.03 = 100 ohms, all the parts in series.

If you run different colors, they will have a different forward voltage; in general the farther up the spectrum the higher the voltage.

If you run multiple colors at once you really don't want to run them in series, because they're unlikely to have compatible specs and some will be much brighter than others, you'll want to run in parallel, each color string with their own current limiting resistor, otherwise only one will probably light.
So, the xmas lights are probably wired in parallel and the led's are being overdriven?
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Old 07-09-12, 08:20 PM   #11
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So, the xmas lights are probably wired in parallel and the led's are being overdriven?
I'm sure they're wired in multiple sets of series/parallel arrangements. If I were doing it I'd wire series sets back to back so that half of the LEDs were on on half of the AC cycle, otherwise you're presenting a weird power factor to the mains. I wouldn't think they would be "overdriving" the LEDs at all (why would they, it would just burn out the LEDs). Just put all the LEDs of the same color in series, then connect each series set of LEDs in parallel with load limiting. If the strings have any kind of special features like flashing, then there's an IC to control that, and they may actually incorporate current limiting into the controller circuitry rather than using a simple resistor. Or not, it doesn't get much cheaper than resistors.
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Old 07-11-12, 11:09 PM   #12
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Couple of ideas to clarify. Basic Ohm's law for one: I = V/R.
What this says is that it's the voltage applied to a resistance that determines the amps flowed.
The higher the supply voltage, the higher the amps.
And the higher the resistance of the circuit, the lower the amperage flowed.
Doesn't matter if the dynamo can output 500ma or 500000000000000000000000000ma, it's the input voltage flowing across the diode's resistance that dictates actual current flow.

Curently, the Cree XM-L LED is the highest efficiency LED. Link to XML datasheet.pdf

It puts out 100-150 lumens/watt at the following power-ratings:

voltage amps watts LUMENS
2.90v 700ma 2.03w 300lm
3.31v 1500ma 4.97w 600lm
3.35v 3000ma 10w 1000lm

The trick here is how are you going to manage the voltage going into the LED and will your power-source be able to supply the necessary current without dropping too much voltage? This is where buck-boost regulators come in that can raise low and lower high voltages and keep it such that you have a steady current output. You'll also need a heat-sink for the LED as it get very hot. And add a reflector and case. This is A LOT of work to build a bike-light from components.

Or you can just buy the completed product for $46 Cree XM-L 3-mode bike-light , 1200 lumens. Come with battery-pack and charger. You can open the battery-pack and configure it to be charged by the bike-generator in parallel. Would be good idea to use bridge-rectifier and smoothing capacitor.

Or you can get a 50-100 lumen light and use the batteries as a regulator after the rectifier. Xingcheng Bicycle Head Light $4.80

After riding with +1000 lumens, I simply cannot go back to the lesser lights, no matter how inexpensive they are.

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Old 07-12-12, 08:14 AM   #13
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the voltage from a dyno is far from fixed, it will increase somewhat at no-load or low-load conditions. I don't think there are any dynos that will blow something like an XML just from current because the current rating is higher than the dyno will put out. Inverse voltage and heat will kill them though. Ohm's law doesn't really apply to a diode, generally people just assume they are a voltage drop, although even that doesn't really work in the case of a dyno. I haven't really tested dynos, but people say they are a current source. In a current source, the voltage adapts to whatever is required to flow that current. I'm sure a dyno isn't a perfect current source though.
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Old 07-13-12, 02:31 AM   #14
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the voltage from a dyno is far from fixed, it will increase somewhat at no-load or low-load conditions. I don't think there are any dynos that will blow something like an XML just from current because the current rating is higher than the dyno will put out. Inverse voltage and heat will kill them though. Ohm's law doesn't really apply to a diode, generally people just assume they are a voltage drop, although even that doesn't really work in the case of a dyno. I haven't really tested dynos, but people say they are a current source. In a current source, the voltage adapts to whatever is required to flow that current. I'm sure a dyno isn't a perfect current source though.
Diodes do have a resistance and voltage-drop. Just not perfectly linear. Forward-voltage (Vf) to amps flowed is in the XM-L datasheet and you can derive a resistance from that.

Regulators actually don't change voltage in order to maintain current. Linear regulators like the LM317T uses a voltage-divider and shunt-to-ground in order to provide a fixed current output. It takes whatever voltage is fed to it and splits it into two legs (Y). For example, with a 6v input:
LEG1 = 3.35v output to LED in order to flow 3000ma
LEG2 = 6-3.35v = 2.65v @ 3000ma is shunt to ground
The power-source in this case (batteries, transformers, etc.) will be outputting 6v @ 3000ma or 18w total. This is then split by the regulator to send 3.35v @3000ma to the LED (10w) and 2.65v@3000ma to ground (8w). Based upon the total consumption, 18w, and 10w actually used, you can consider this regulator to be 55% efficient.

More modern regulators, such as the AMC 7135 PIC, sends full-voltage across to the driven LED. Current is regulated with PWM, like a fuel-injector. There is no shunt-to-ground to dissipate the un-needed voltage that would've resulted in too much current flowing. A 13v battery can actually run a 3.35v Vf LED @3000ma by driving it at 25% duty-cycle. When battery-voltage drops near 3.35v, the regulator will use 100% duty-cycle (direct-drive) and connect the power-source directly to LED. In this case, you can consider this regulator to be 100% efficient.

If the 7135 is configured in a buck-boost configuration, it can even raise the supply voltage if it's less than the Vf of the LED. Common department-store flashlights (Husky from Home Depot), that uses 2x 1.5 AA or D batteries in series for 3v supply uses a boost circuit to send a higher 3.35v to 4.5v to the LED, like a P7.

Last edited by DannoXYZ; 07-13-12 at 03:14 AM.
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Old 07-13-12, 10:28 AM   #15
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I see no requirement to have a regulator in led circuit driven by a dyno because dynos never exceed the current ratings of available power LEDs. If you want to have a lot of low-power leds in an array, you can always parallel them to handle the current. A regulator would just waste power. I think the most active any modern generator light gets is switching the input circuit from a voltage doubler to a straight pass-through circuit. My Supernova does this in a fairly obvious way when cresting a hill. Haven't noticed it with the B&M Lyt, but I don't usually get going that fast with that bike. I have seen discussion of using an active circuit for improved low-speed use. It would also be nice to be able to dim the lights, but I don't know if I would actually do that in practice.
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Old 07-18-12, 01:47 PM   #16
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What about elwire?
http://www.instructables.com/tag/typ...?sort=FEATURED
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