Trail bike light mod

[funkstrong]

Hi guys, I currently tool around trails and camp sites on an old 1973 Honta ST90. Cool little trail bike with pretty poor stator output. Runs on a 6 volt system. Puts out 6.5 volts and maybe an amp, no more. I've been looking at a cheap way to get more light out of it vs the old stock headlight. The ST90 uses a sealed beam and I like the old stock look. So here is my plan:

Order a low price flashlight that uses the CREE xml t6 led. Like this

Create a small voltage regulator using this

Use a small driver that will accept the 6 volt input.

Bypass the battery in the flashlight and hard-wire it to a disconnect on the bike.

My only worry is what I should drive the T6 at, and how much heat the small flashlight can dissipate. I will have the empty battery tube to use for a heatsink if need be. Any ideas, input? If this works I'd like to also try it out on my bigger dual sport bike.

 

[Alaric Darconville]

If this works I'd like to also try it out on my bigger dual sport bike.

Don't even think about doing that to the "dual sport" bike if it's one such that is sold as a road-legal bike, with the regulated motor vehicle equipment expected of such a vehicle, as in headlamps, tail lamps, and turn signals. Replacing that bike's headlamp with a flashlight is unsafe, ineffective, and illegal.

I wouldn't even recommend this with the ST90-- more light isn't necessarily better if the beam pattern is poor.

 

[funkstrong]

I probably phrased that wrong. My goal isn't to replace the headlight, its to supplement it while driving off-road. It would be especially helpful on the little ST when I'm driving all over the place on little woods trails. I would probably replace the reflector in the flashlight to the pattern that would work best on the trails. As far as the dual sport bike, my goal wouldn't be to replace the headlight, that would be absurd. I'm just thinking of a low expense alternative to something like this.

 

[Hamilton Felix]

Hmmm.... now you've gotten me thinking. I have a 1976 Honda CT90 in a shed; it really should be in use. Since this "two wheeled Jeep" would be crawling around trails in low range - and has a very tiny and primitive 6 volt electrical system - it could use some help. It occurs to me that there is a great selection of bicycle lights available today. I am not sure about prices, but some of them might serve as low speed offroad lights. With some decent rechatgeable batteries and a charger, this could be a completely separate add-on to an otherwise stock bike.

Funny, but I have recently been thinking about flashlights as bicycle lights. Right now I have in my pocket (OK, a cargo pocket) a Nitecore EA4W that can put out from 65 to 860 lumens, all on 4 AA cells. Two sets of Eneloops, a small Sanyo charger, and I'm all set up with a pocket searchlight. Just make a handlebar clamp to hold the flashlight and don't worry about modifying anything else.

 

[funkstrong]

Yeah that also crossed my mind about just strapping the flashlight onto the bars and calling it a day. I could even tap the stator output to charge a spare battery. It was just so cheap and easy for the voltage regulator I decided why not hard-wire it. That of course led me down the "more power" path haha. All I would need is a driver that requires a minimum of say 5 volts and I'd be in business.

 

[DIWdiver]

The LM7806 would require well over 6.5V input to generate a 6V output. You'd need a low-dropout regulator. I did a quick search and didn't find anythng in the TO-220 package. I did find the LM2940 would be a good choice, but it's 5V (which I think is fine). If you were to get a nice regulated voltage, you could use a simple resistor for the driver. Nothing fancier is needed.

Also, if you have less than 1A to power the thing, you can consider lights using other LEDs Most of the Cree high power ones can handle 1A. They may be less expensive than XML lights, or offer some better options.

You could consider a driver like http://dx.com/p/18v-5w-cree-circuit-board-for-flashlights-16-8mm-5-5mm-26110 as it wouldn't need the regulator in front of it. Only a nice surge suppressor to protect it from spikes. Because it's a switching regulator, the input current is less than the output current. As delivered, it would probably draw 0.8-0.9A when driving a Cree LED.

If you need to reduce the input current, you can by swapping out the sense resistor (it's the black rectangle marked R22 in the picture).

If you do something like that, I can help you pick the parts you need. They would only be a few bucks.

By the way, the XML-T6 at max current of 3A only puts out 900+ lm. If they are claiming 1600, one might wonder what else they are stretching the truth about.

 

[funkstrong]

Hi DIWdiver, thanks for all of the input. This is my first foray into leds, so I've got quite a bit to learn.

That makes sense with the 7806. I think it was mainly used for 12 volt conversions so the dropoff didn't matter as much, but with only around 6.5 to spare, it could become a problem. I thought that a driver might require stable input, but with the driver you suggested, that doesn't seem to matter. What would be used as a surge suppressor? In the wiring schematic for the ST90, the battery acts as the voltage regulator and surge suppressor. If you have any suggestions for parts I'm open haha. The flashlight seems fairly well made despite the exaggerated lumen claim. I took it apart and it does appear to be an xml-t6. I'd be open to swapping emitters though so I can take more things apart.

 

[DIWdiver]

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LED drivers generally don't require a fixed input voltage. In general, LED drivers are designed to take a range of input voltages, and create a regulated current output to drive the LED(s). I picked that one because the input voltage range and output current seemed to match your desires.

There are two things that can happen in vehicles that require surge or transient suppression. The first is fast transients. When you switch on or off something like your starter, headlights, or any other electrical load, you cause a sudden change in current flow. There's an electrical phenomenon that's very much like momentum (imagine water flowing in a pipe, having momentum). It's called inductance. When you suddenly shut off the flow, it causes a rise in voltage. Just like when you suddenly shut a valve, you cause a pressure spike in a pipe. In plumbing it's called 'water hammer' and can damage valves and other plumbing parts. In electronics it's called a spike, a surge, a transient, noise, and other things and it can damage your delicate electronics. Since it's generated IN the wiring, the battery cannot protect you from it unless you are wired directly to the battery (and even then it's not perfect). They are called 'fast' because the duration is typically measured in microseconds. But the voltage can be in the hundreds or thousands of volts.

The second thing that can happen in a vehicle is called load dump. AFAIK it only happens in systems with an alternator, not a generator. While a generator creates a voltage, and you can (at least in an ideal world) take whatever current you want from the generator, and the voltage doesn't change, the alternator doesn't work that way. It wants to create a certain current, and you can take whatever voltage you want. Since your vehicle wants a constant voltage, they added a voltage regulator. The voltage regulator controls the amount of current the alternator creates, attempting to keep the voltage constant. When you turn your lights on, the regulator sees a voltage drop, decides more current is needed, and adjusts the alternator to create more current. When you turn the lights off, the opposite happens - the voltage rises, and the regulator adjusts the alternator to create less current. However, the voltage regulator takes some time to react, in the tens to hundreds of milliseconds. Normally your battery will handle the increase or decrease in current, and it will not allow the voltage to change much, while the regulator reacts. But as the battery ages, it's ability to do this gets compormised. Or you could have a loose connection somewhere. If you dump a huge load, like the starter, and have a severly compromised system, you get a large rise in voltage. I've seen various numbers for 12V automotive systems, but the one I believe most is 65V for 400 milliseconds. That can easily destroy electronics designed for 12-15V!

FYI, generators may seem better, and may actually be better in some respects. The reason we use alternators instead is that they are much cheaper to build.

Since your bike has by your own admission a rather anemic electrical system, I don't think you need anything like the kind of protection that's designed for load dump in modern cars with 200A alternators. The automotive LED driver I make can handle 85V indefinitely and will absorb transients above that. But I think for your bike and the driver I suggested, I'd suggest a small 12-15V transient supressor like this. If you feel rich and over-cautious, you could spend an extra $1.50 (FOUR times as much!) and buy a huge one like this. Both of these are unidirectional, or polarity sensitive (you mustn't install them backwards). There are bidirectional ones available that look and act very similarly but aren't polarity sensitive. I would NOT recommend you use bidirectional ones. The unidirectional ones have one end (the cathode) marked with a band or some other obvious (or sometimes not obvious) marking. This is the end you connect to the HIGHER voltage side.

For best protection, the suppressor should be mounted close to the driver (or whatever you are trying to protect), rather than at the battery or at the alternator. If you use a driver with a lower maximum input voltage, you want to be careful with your selection of transient suppressor. The suppressor must not conduct at the maximum voltage in the bike, but must not allow the voltage to rise above the maximum input voltage of the driver.

If you install an appropriate transient suppressor close to your driver, you should be good to go.

 

[DIWdiver]

My only worry is what I should drive the T6 at, and how much heat the small flashlight can dissipate. I will have the empty battery tube to use for a heatsink if need be. Any ideas, input? If this works I'd like to also try it out on my bigger dual sport bike.

I think the bike is going to limit you more than anything else. That light housing should handle a few watts with no problem, and it sounds like you don't have more than a few watts to put into the light.

If this works I'd like to also try it out on my bigger dual sport bike.

Please, don't even THINK about putting this on a bike that's road-legal.

 

[funkstrong]

Thanks again for the help, I could read about this all day. These old 6 volt bikes are known to blow a fuse now and then because of the spikes from turning on the headlight so that diode would be a nice cheap way to protect the driver. Rest assured this is all for off-road use, will never be turned on when driving on roads. I think I'm going to go for it with my cheap flashlight to see the results and if it works well I will later try to find a small waterproof housing so I can mount it in a more permanent way with a better reflector to control the beam pattern and output. Did you say you designed an automotive led driver? Have any information about what you use it for? haha

 

[DIWdiver]

Check it out for yourself.

http://www.candlepowerforums.com/vb...Automotive-linear-13-5A-LED-driver-adjustable

Unfortunately, it won't work at 6V.

 

[funkstrong]

I'm back, been a bit sidetracked. I've got a selection of drivers that will work for my project but I started looking more into the transient suppressors you suggested. I don't fully understand the standoff voltage. Wikipedia isn't very helpful, it says this is "the voltage below which no significant conduction occurs". Does this mean that if the reverse standoff voltage is 12v, no conduction occurs below 12v? Not that I don't trust your suggestions, I'd just like to learn a little while I'm at it, haha.