Motorbike LED blinkers

[namtip]

Hello all,

First of all I'd like to say hi as this is my first post (go easy on me please...) and secondly that I have read the LED sticky and need some more application-specific info.

I've been scratching my head for a while on this one and thought it was time I could use some help. I want to convert my bike's blinkers, tail lamp, stop lamp, and front position lights to LEDs to save wattage I'll need for other accessories (don't ask... ).

I have bought 250 white diffused LEDs and have begun with making up a blinker. This panel (which will be housed later) is made out of veroboard with an array of 24 LEDs (arranged in a 6 x 4 rectangle approx 1.5" x 2.5"). They are wired in the following way: 3 LEDs in series x 8 in parallel (hope that makes sense). I have not used any resistors at all yet because I'm not sure how to proceed in the most efficient way of powering it:

The problem is the unstable voltage source of my bike, measured to be as low as 10 V on a cold winter morning and as high as 15V at full throttle on a summer's day . Now I understand LEDs need a constant current source, and I've experimented with a variable voltage supply and a calibrated current meter and have found that 120 mA is the target current I want the array to run at - not pushing them, but making them pretty bright. I've even played around with a LM317 wired in line in the constant current configuration and using a 30 ohm feedback resistor to some degree of success. The only problem is, that the LM317 may be good at providing approximately the correct constant current at a constant 12 V input - but if I vary the input voltage between 10 and 15 V, the current drawn by my array swings between 45 mA and 150 mA respectively. Not good!

My question to you is this: will I have to use both a 12 V regulator (e.g. LM 7812) in line with an LM317 constant current source to do this for each blinker? Or is there a simpler way that I'm not yet seeing?

Thanks for your replies. And yes, I understand I'll need to make another flasher unit - I know about the loading theories etc - maybe this will be a future post...

 

[john monks]

You do not need a 7812 regulator. One regulator will do the job. And you may not need a regulator.
You may start by figuring a maximum voltage of 15 volts, using an appropriate resistor for a super bright led or led in series.
Surface mount super bright white LEDs probably run at 150 milliamps At 3.5 volts. So two in series is 7 volts. So the resistor would be around 53 ohms. 50 ohms is probably good enough. So that would take care of one side.
Will your turn signal relay work at that low of current? This will not work it most cars.
When you say the voltage goes up to 15 volts that tends to indicate that your bike does not have a voltage regulator. Lots of bikes only use a magneto with no regulator. With no regulator what you are doing makes no sense. You will be overcharging your battery and end up saving nothing. Need to check. If this works well you might set up a LM317 to current limit at 150 milliamps.
What is the make and model of your bike? This might change the answer.

 

[CocaCola]

Or is there a simpler way that I'm not yet seeing?

You say that that the battery swings from 10 - 15 Volts, this is unlikely as a 12 Volt battery would be deceased at 10 Volts... The swing is likely more like about 12.6 to about 14.5 Volts... It might be 12 Volts on a bad day, this would indicate that it is nearly deceased...

The quick and dirty pick the appropriate resistor to balance the series of 3 LEDs for 15 Volts... In doing so you will have your worst case covered, and in all likelihood most of the time you are using the lights and riding the battery will be between 14 and 15 volts... The only time the battery should fall bellow say about 13.5 volts is when the bike is not running...

Doing it this way you will have 'full' brightness when the bike is running and a slight decrease in brightness when the bike is off, this is true even for conventional bulbs and lights on a bike...

*Edit I see John beat me to the punch on some of the same aspects, the glory of having a toddler and baby both erupting into need, the minute I start to type something...

 

[namtip]

John Monks:

I've already bought the LEDs and put a lot of time into making one of the blinkers. I don't have the facilities for surface mount stuff, so I want as much as possible to stick with what I have already and work a solution around it. I will remake the turn signal relay so that it's not dependant on current. My bike is a British Armstrong MT500 built for the army -so the regulator is not a precision piece of electronics, more likely just a rectifier. My battery has seen better days it's true. I left it out all over winter and it is pretty much dead.

Coca-Cola:

As above, my battery pretty much is deceased! I know I should probably get a new battery and an uprated regulator, but I don't have £££s to spend! I like the idea of catering for the voltage swing to make the project as versatile and rugged as possible (I like to throw my bike and myself through muddy fields and rivers as much as possible...)

Using a 9V household battery connected straight to the array makes them light pretty bright - I just want them to draw the same current for voltages anywhere between 10 and 15V. How hard can it be? (really hard? )

 

[CocaCola]

As above, my battery pretty much is deceased!

There is no pretty much about it, a 12 Volt battery at 10 Volts is toast, AKA damaged... A traditional lead acid 12 Volt battery is considered deceased at about 11.75 Volts anything less and damage starts to occur...

I just want them to draw the same current for voltages anywhere between 10 and 15V. How hard can it be? (really hard? )

Why is this so important to you? If it's all about keeping the LEDs at "full" brightness then get a low drop out 9 volt regulator, and assuming your LEDs are happy at 3 Volts forward wire it up... I would still likely put a 1 Ohm resistor in series just because, but it's technically not needed as long as you input voltage is regulated tightly...

The reason I said low drop out is that most regulators require + 2 volts, and since you insist you battery if falling to 10, a standard 9 Volt regulator won't be able to regulate 9 volts with that 10 input...

 

[john monks]

Very good. I checked and the MT500 does have a voltage regulator.
I'm assuming it has a kick starter so the battery should no longer be an issue.
So I'm assuming the LEDs are through hole devices rated for around 30 milliamps.
If this is so a LM317 and a 39 ohm resistor should make a good series regulator.
You can look at the LM317 data sheet for a good schematic.

 

[namtip]

Thanks John, I did find that a 30 ohm resistor with an LM317 gave more or less what I wanted. I've been doing a lot of thinking today thanks to your and CocaCola's input. I've decided that I will buy a new battery (I'm going to need it sooner or later anyway) and an uprated rectifier/regulator 1) because it may (but not necessarily) give me a more stable voltage, and 2) the standard unit only makes use of two of the three coils in the generator - the other coil being grounded and not used. I'd like to make use of that third coil.

In any case I was playing around again today with my variable voltage supply and current meter and different resistors and I found that a simple 47 ohm resistor in series with the display gives a decent brightness to the LEDs at anywhere between 10 and 15V without pulling over 120 mA (my theoretical 'safe ceiling' based on how my LEDs are wired and what the manufacturer says). I think this is what I'll use for now unless you can persuade me for some reason to stick with the LM317 idea (I do have a bunch of these kicking around). I think with blinkers thermal runaway is not going to be an issue anyway...

This issue will arise when I come to designing the front running lights which will be switched on all the time - then the LM 317 may come in handy...

Thanks again guys