12 volt fog lamp as headlight

I don't think the chengine's alternator could supply the current to charge two batteries simultaneously. Someone over at the other forum is getting 12V but that is an exception.

Diode switches would allow the light to see 12V while the chengine sees only 6V, but the batteries will try to draw more than it can provide. It would be like hooking up a 6V car battery.

A current limiting resistor would probably eat up more current (as heat) than we can budget to it.

The 7ah battery is very small, but maybe I lied to you. Looks like the 12V is $19.99. The 6V 4.5ah is $8.99. Compare the batteries sizewise to the price tags on the shelves. The 6V is on end, the 12V is a side view.

Ted

I see what your saying Jason and I guess I would have to wire the switch for the light to break all connections for charging when in use, and break all the series wires when in charging mode. I wonder if I wil have to buy a industrial set of batteries or if I could just use 8 rechargeable c or d cells in two bays of 4(id need to look up whan amperage they can safely put out). Either way it would be a neat project and il let you guys know if and when I start it. Unfortunately it wont be for another 3 weeks 'casue im in school. One more reason to look forward to christmas break.

As for you XPosTech, I would take into account the heat losses and generator load by using a larger resistor, this would allow a small amount of current to flow through the circuit. According to my engines manual you can draw 3 watts (6 volts, .5 amps) from the engine without killing it. Because I only drive at night occasionally I would probably only need a fraction of this power to keep the batteries charged.

Thanks for replying guys

u could use a motorcycle or dirtbike generator like 15-20 amps engine should handle it and you'll have lots of power

After finding out that my engine put out 7.5 volts instead of 6, I went through my dads immense collection of light bulbs trying to find one designed for this voltage, the only 7.5 volt light bulb I found only draws .25 amps and is not even close to the brightness desired for a headlight. After visiting a local hardware store, NAPA, and radio shack I found that the bulb I had at home was the only one to be found rated for 7.5 volts.
With this information i decided that using a bulb from a 6 volt flashlight would be the closest thing to perfect for my bike. I took one apart and found tht the bulb was actually rated for 4.8 volts, if 4.8 can handle 6 I did not see why it couldn't handle 7.5, so I tried it. The engine cut out immediately after connection the first time, so I revved the engine slightly on the second try and found the light worked, though it flickered according to the rpms.
To remedy this I attached 2 capacitors from a tv circuit board in parallel with the light bulb, this got rid of most of the flickering and allowed the engine to idle while the light was turned on, though the idle had to be sped up slightly. After installing a switch and putting all of this into the water resistant housing of a battery powered bike headlight I now have a bright headlight that doesn't need replacement batteries.
Installing a few more capacitors should eliminate the flickering even further and allow the bike to idle at a lower speed. For now though, I am very pleased with the results had. Though I don't expect it to happen Il let you know if the bulb burns out due to the slightly higher voltage.

Id post pictures but my wiring is so catty wampus it wouldn't help explaining much.

so im trying to move to leds and i burned out the first set of 4. Has anyone put enough capacitors on the bike to actually measure the voltage coming off of a motor? i was hooking up two pairs series wired 3.5 volt leds, so unless i did something seriously wrong, the peak voltage should be greater than 8 volts according to the tolerance ranges. So is there something i don't know about leds or does my engine put out more than 7.5 volts like the manual sais?

the voltage is ac.sort of you can run a 12 volt led light for a taillight here is the name and part number sold at most auto parts stores and farm parts stores
Piranaha #V168XR led light comes with a chrome trim. about $8.00

Prasinos said:

so im trying to move to leds and i burned out the first set of 4. Has anyone put enough capacitors on the bike to actually measure the voltage coming off of a motor? i was hooking up two pairs series wired 3.5 volt leds, so unless i did something seriously wrong, the peak voltage should be greater than 8 volts according to the tolerance ranges. So is there something i don't know about leds or does my engine put out more than 7.5 volts like the manual sais?

Click to expand...

I used to be smart enough to tell you what you need. But like Norman said, the output is AC voltage, not DC. This is why lights flicker because it actually turns on and off very rapidly as voltage reverses polarity. And LEDs are polarity sensitive, and intolerant of overcurrent. 6 volts would be max capacity. You'd need a diode wired in series to prevent reverse polarity as well as a resistor in series to drop the voltage down. The exact resistor value is.....shoot, where is that formula?

Here is what I found on leds
LED Circuitry tutorial: how to hook up LEDs.
hope this helps

I didn't know that it was ac, that would explain why the capacitor hooked in parallel killed the engine and why my multimeter couldn't read the dc voltage. thanks a lot guys! I'll make a bridge rectifier and put it before the capacitor which should ensure smooth dc voltage. With that figured out I'll be on my way to lots of electric accessories in no time. I'm at school right now so i wont be able to do this for a while. Still though, does anyone know what the ac voltage is on these things?

Thanks again i'm mad exited to get home and work on the Azure Supperleggera.

Voltage is proportional to RPM, so if revved way up, voltage could climb and climb to......????

Obviously a graph would be ideal but I'm just looking for the maximum output, at or near redline, lets say.

Also, wouldn't it make sense that at higher rpms the engine would need to spark more often? And this higher voltage should cause a more powerful spark, which would basically "self regulate" the voltage supplied by the white wire. As long as the circuit using the white wire had enough resistance to prevent an overdraw of current.

Let me know if this is backwards thinking.

Prasinos said:

Obviously a graph would be ideal but I'm just looking for the maximum output, at or near redline, lets say.

Also, wouldn't it make sense that at higher rpms the engine would need to spark more often? And this higher voltage should cause a more powerful spark, which would basically "self regulate" the voltage supplied by the white wire. As long as the circuit using the white wire had enough resistance to prevent an overdraw of current.

Let me know if this is backwards thinking.

Click to expand...

It makes sense, but let me explain......... The voltage changes with RPM, so the amount of voltage is not too important, as long as there's enough there. It doesn't self-regulate, it just has a limit on how much power.

Here's some help......

Use your diodes or rectifier to make DC first. You can also add the capacitor to smooth out the voltage.

LED's are current sensitive, NOT voltage sensitive! LED's have a set voltage drop. You MUST use a resistor with all LED's to limit max current over a desired voltage range. For example, just 3.8V will burn out some 3.6V LED's without resistors!

Typical single 3.6V White LED uses approx 20mA of current. If you use a 150ohm resistor in series, you get approx 20mA from 6V. You could also try 100ohm for 30mA if the LED can take it (more brightness). Even if your voltage goes over 7V, the resistors will keep the LED's safe and protect from overcurrent.

Since you have almost 3 watts of power to use, I'd recommend using 2W or less of total LED's to allow some headroom to keep a strong spark.

Now there's an answer!

Thanks a lot for the help.

If the leds are not voltage sensitive that would mean they have negligible resistance? The way i thought of it if the led is designed for 20 ma 3.6 volts that would mean that it has a resistance of 180 ohms. do i take that into account or ignore it when choosing the correct resistance?

just get a 18vcordless tool battery and 1hour quick charger,weighs 1/2 pound or so.will power head and tail light for many hours.
dont forget inline fuse and toggle switch or dimmer.
price:2 12 volt car lights in series=24 volts =$12
battery and charger= $45
plus extra batteries are only like $20 and they last for 1 year or 2+

Prasinos said:

Thanks a lot for the help.

If the leds are not voltage sensitive that would mean they have negligible resistance? The way i thought of it if the led is designed for 20 ma 3.6 volts that would mean that it has a resistance of 180 ohms. do i take that into account or ignore it when choosing the correct resistance?

Click to expand...

Just take the voltage and mA into account. The LED actually does not have very much internal resistance. You can't calculate LED resistance like that because it's non-linear.

LED's are strange. It's because they are semiconductors, they don't have a linear resistance, that's why you have to add a regular resistor in series with the LED to run from a voltage source. For example, white LED conducts almost no electricity under 3V, but conducts well at 3.3V and over.

Because the LED is usually rated at 3.3V, it may use less then 5mA at 3V, but use over 80mA at 4V, because the LED takes a certain voltage to turn it on, but once over that turn-on voltage, the LED shows a very low resistance. The current rise curve is very rapid once the LED starts to conduct electricity above its turn-on voltage, so you need a resistor to limit the current properly.

Also because of that turn-on voltage, the voltage of an LED is pretty regulated, meaning that regardless of the input voltage you use with a resistor, the LED will always have about 3.3-3.6V across it. By changing the value of the resistor in series with the LED, you change the current, and as a result, the brightness of the LED.

so last question, how many ma can i force through the leds without damaging them. and if there is no voltage drop acros them how many could you put in series with a 7 volt power source?