Brake light.

[Calvin_Skinn]

I am trying to build a led brake light for my motorcycle and I don't know how exactly to manage resistors.
LED specs:
1 watt red
2.4v
.35amps

It is a 10v circuit.

My plan is to use 8 of them, 4 pares (of two LEDs wired in parallel) wired in series. How do I manage my resistors?

[sdg]

My plan is to use 8 of them, 4 pares (of two LEDs wired in parallel) wired in series.

One LED fails, the other in the pair instantly gets twice the current, goes poof and voilà!, no more brake light...

[georges80]

High power LEDs should have a current regulated LED driver (DC : DC converter) to feed them. The driver also needs to withstand all the nasty spikes/surges etc of an alternator based charging system + inductive loads (starter motor) etc.

Beyond that (even though most won't care...), for street use there is typically some DOT requirement for all external lights on vehicles.

cheers,
george.

[Calvin_Skinn]

So say a buck converter? the rectifier isn't enough?

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[Calvin_Skinn]

My plan is to use 8 of them, 4 pares (of two LEDs wired in parallel) wired in series.

One LED fails, the other in the pair instantly gets twice the current, goes poof and voilà!, no more brake light...

I was thinking one resistor for each led.

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[bills]

My plan is to use 8 of them, 4 pares (of two LEDs wired in parallel) wired in series.

One LED fails, the other in the pair instantly gets twice the current, goes poof and voilà!, no more brake light...

look here! http://led.linear1.org/led.wiz

[ConKbot]

4 red leds in series isn't the best, in an automotive system as the system voltage changes, since the current limiting resistor is relatively small in value, the brightness will change more. 

I've done a led "upgrade" for a friend who got an eBay aftermarket light that was shitty.  3 in series with a bigger limiting resistor was much more consistent in terms of brightness vs Vin. Though that was just little 50mA piranha leds.

The best would be to put them in series and use a boost driver. Georges80's website (www.taskled.com) is a good place, off the top if my head, the hyperboost driver should suit you, if it's still in production. No affiliation, just a happy customer.

[Calvin_Skinn]

4 red leds in series isn't the best, in an automotive system as the system voltage changes, since the current limiting resistor is relatively small in value, the brightness will change more. 

I've done a led "upgrade" for a friend who got an eBay aftermarket light that was shitty.  3 in series with a bigger limiting resistor was much more consistent in terms of brightness vs Vin. Though that was just little 50mA piranha leds.

The best would be to put them in series and use a boost driver. Georges80's website (www.taskled.com) is a good place, off the top if my head, the hyperboost driver should suit you, if it's still in production. No affiliation, just a happy customer.

Thank you.

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[Calvin_Skinn]

4 red leds in series isn't the best, in an automotive system as the system voltage changes, since the current limiting resistor is relatively small in value, the brightness will change more. 

I've done a led "upgrade" for a friend who got an eBay aftermarket light that was shitty.  3 in series with a bigger limiting resistor was much more consistent in terms of brightness vs Vin. Though that was just little 50mA piranha leds.

The best would be to put them in series and use a boost driver. Georges80's website (www.taskled.com) is a good place, off the top if my head, the hyperboost driver should suit you, if it's still in production. No affiliation, just a happy customer.

[Calvin_Skinn]

Now why does this happen. In series 8 x 2.4v = 19.2v
8x0.35amps=2.8amps
19.2v x 2.8amps = 53.76 watts !?

2.4v x .35amps = 0.84watts
0.84watts x 8 = 6.72watts ?

Why aren't they the same? Obviously 8 1watt LEDs aren't going to use 53watts.

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[Calvin_Skinn]

Now why does this happen. In series 8 x 2.4v = 19.2v
8x0.35amps=2.8amps
19.2v x 2.8amps = 53.76 watts !?

2.4v x .35amps = 0.84watts
0.84watts x 8 = 6.72watts ?

Why aren't they the same? Obviously 8 1watt LEDs aren't going to use 53watts.

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In the first calculation you showed you multiply the current (0.35A) in the series by 8x. BUT, it is the same 0.35A that flows through the entire series. So you multiply the series voltage 19.2V by 0.35A and you get the expected 6.72 watts.

thanks. I mixed it up with resistance. Wooo

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[Calvin_Skinn]

Any recommendations on resistors I will need some thing around 6.9 ohms and over 7 watts. I found a 6.98 ohm 10watt resistor on eBay but it is $15. Any better options?

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[AndyC_772]

This cannot be a good solution. Calculate the LED current when the battery voltage is 12.0V, then 13.2V, then 14.4V, and you'll see why.

You'll also want to include some over-voltage and/or reverse voltage protection. LEDs won't react at all well to the kind of noise found on a vehicle electrical system.

[Neilm]

Not a good idea - just imagine what would happen if one of those LEDs failed - no brake light.

[Zero999]

A constant current source would be a better idea than a resistor.

LEDs tend to fail short circuit, rather than open circuit, so one failing wouldn't stop all of them from working.

[LaserSteve]

You really need a redundant system on a safety of life application like this. One commercially made light and one home made one.  I have a small led light on my bike, and then a larger one. I carefully avoid using the  exact same devices in parallel on safety of life systems.

High power Leds prefer current sources over resistors, they often  have nasty changes in their current vs voltage vs output    curves with respect to temperature.  Good heatsinking is  critical for peak brightness.

Second of all, automotive applications can have spikes in excess of 600V during starting.  Electric or Hybrid vehicles also have dirty electrical systems.  Good designs have spike and reverse protection of some form.

And if I have 8 leds, they are going to be in 4 strings of two or two strings of four, if one string fails, I still have a light.
There will also be a few T1-3/4  red led packages or piranha  packages there as a backup.

I used to be a nationwide field service engineer, constantly on the road.  It was fairly obvious the commercial LED lights for trucks were broken into several strings. The better ones clearly had constant current drivers, and the better ones have multiple brightness levels or motion effects built in.  Really good bike taillights often blink two times on application of braking, in order to better attract attention.

Consider this, in the US, your insurance may not feel obligated to fully pay out in an accident if you used a home made lighting system that did not meet DOT specs.

Steve

[Calvin_Skinn]

This cannot be a good solution. Calculate the LED current when the battery voltage is 12.0V, then 13.2V, then 14.4V, and you'll see why.

You'll also want to include some over-voltage and/or reverse voltage protection. LEDs won't react at all well to the kind of noise found on a vehicle electrical system.

I don't understand what you're saying goes if I have a driver this won't matter.

You really need a redundant system on a safety of life application like this. One commercially made light and one home made one.  I have a small led light on my bike, and then a larger one. I carefully avoid using the  exact same devices in parallel on safety of life systems.

High power Leds prefer current sources over resistors, they often  have nasty changes in their current vs voltage vs output    curves with respect to temperature.  Good heatsinking is  critical for peak brightness.

Second of all, automotive applications can have spikes in excess of 600V during starting.  Electric or Hybrid vehicles also have dirty electrical systems.  Good designs have spike and reverse protection of some form.

And if I have 8 leds, they are going to be in 4 strings of two or two strings of four, if one string fails, I still have a light.
There will also be a few T1-3/4  red led packages or piranha  packages there as a backup.

I used to be a nationwide field service engineer, constantly on the road.  It was fairly obvious the commercial LED lights for trucks were broken into several strings. The better ones clearly had constant current drivers, and the better ones have multiple brightness levels or motion effects built in.  Really good bike taillights often blink two times on application of braking, in order to better attract attention.

Consider this, in the US, your insurance may not feel obligated to fully pay out in an accident if you used a home made lighting system that did not meet DOT specs.

Steve

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[fmaimon]

7 watts are way too much for a led brake light. You will blind everyone behind you!

[Calvin_Skinn]

7 watts are way too much for a led brake light. You will blind everyone behind you!

I made to be the same as what a normal bulb is: 400 lumens.

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[Calvin_Skinn]

[Calvin_Skinn]

Will the leds potentially over drive one another?

[Calvin_Skinn]

Just so it's out there it's a 1976 Honda cb550.

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[AndyC_772]

This cannot be a good solution. Calculate the LED current when the battery voltage is 12.0V, then 13.2V, then 14.4V, and you'll see why.

You'll also want to include some over-voltage and/or reverse voltage protection. LEDs won't react at all well to the kind of noise found on a vehicle electrical system.

I don't understand what you're saying goes if I have a driver this won't matter.

A series resistor isn't needed at all if you have a constant current driver. If, however, you have a constant voltage driver, then a very small value resistor like this will mean the current is strongly dependent on voltage, and that's no good if the voltage is as variable as it will be on a bike's electrical system.

In any case, an LED has series resistance of its own; the forward voltage isn't nearly as constant as some electrical models would have you believe. The ESR of the string of LEDs cannot be ignored.

[fmaimon]

I made to be the same as what a normal bulb is: 400 lumens.

Test it out before fitting. The way LED distributes its light is very different from an incandescent bulb. It will appear way brighter.

[miguelvp]

Just to throw this here.

Normal light bulb brake lights fail and it's not the end of the world. You just have to put a replacement one or you might get a ticket. In the meantime, left hand braking signal (angled downwards) should be sufficient.

That is if people still know what the hand signals mean

[Zero999]

What about a Black regulator style circuit?

They're so simple and efficient. You could build one regulator for each string of LEDs.

http://www.romanblack.com/smps/a04.htm

[Kalvin]

How about using four LM317 in TO-220 package connected as 0.35A constant current source, each feeding two LEDs in series?

[Zero999]

7 watts are way too much for a led brake light. You will blind everyone behind you!

I made to be the same as what a normal bulb is: 400 lumens.

I missed that.

Yes, it'll be too bright and not only because of the light distribution. Incandescent lights output that 400 lumens will do so over the whole of the visible spectrum, a lot of which will be filtered out by the red lens. The LED only produces light over a narrow bandwidth, which means it will be much brighter as the red filter won't block much of the spectrum.