Help with LED circuit

[jsmith24]

Hello all,

Okay, here’s the background and circuit info first:

I'm replacing my aging incandescent tail/brake lights on my old motorcycle with an LED array to try to keep my stator from burning out.

Since the original brake and tail lights use a single bulb with dual filaments, and LEDs have only one conductor, I’ve had to use two different currents to simulate the effect of a second filament. So, when only the tail lights are active, there is a standard current of 20 mA per LED; when the brake light is applied there is a current of 40 mA per LED. The LED array is actually 16 banks of LEDs, so the total tail light current is 320 mA, the brake light current is 640 mA. I have used an LM317T voltage regulator as a current limiter on the tail light “hot lead” and also on the brake light lead, at 320 mA and 640 mA respectively.

Now here’s my question: Since both LM317Ts feed into the same power lead of the LED array, do I *NEED* to place a diode at the output of each LM317T to keep the current from “backflowing”? Or, as I suspect, is the LM317 already biased at the output so that no diode is necessary? The image shows the basic setup (forgive my lack of drawing skill) and necessary current.

In case the image doesn’t show properly, it’s housed at www.falconkits.com/ledbrake/ledbrake.jpg

Thanks!

Jack[/img]

 

[kishore2k4]

How about a simple switch(mosfet,bjt) that increases the reference voltage thereby the output current/voltage.

Most n-channel mostfets can withstand a gate voltage of atleast 15V, so you can pull down the gate in normal condictions and when the brake voltage of 13.5V is applied the mosfet will add/subtract the resistance that is used to control the reference voltage.

Even better, you can use a very small relay to do the same.

Also, are you sure whether your LED's can withstand 40mA each cuz if the brake was helf for long enough they'll burn out.

 

[jsmith24]

Thanks for the reply! I'm going to keep it simple - IF I actually need to make sure that the higher current doesn't back-feed I'll just use diodes...adding another chip or relay (where there really isn't room) is departing from my "KISS" mentality.

These LEDs are the newer 70,000mcd version, and can handle 70mA believe it or not. I'd never go that high, and may not even need 40mA, because at even 20mA they are substantially brighter than the regular bulbs. In fact, I may back both currents down a bit, but want to make sure the circuit is stable first.

Jack

How about a simple switch(mosfet,bjt) that increases the reference voltage thereby the output current/voltage.

Most n-channel mostfets can withstand a gate voltage of atleast 15V, so you can pull down the gate in normal condictions and when the brake voltage of 13.5V is applied the mosfet will add/subtract the resistance that is used to control the reference voltage.

Even better, you can use a very small relay to do the same.

Also, are you sure whether your LED's can withstand 40mA each cuz if the brake was helf for long enough they'll burn out.

 

[IanP]

If pins 2 of both LM317s are floating while unactive, you don't need diodes, if not, diodes will be necessary ..

Regards,
IanP

 

[jsmith24]

IanP,

Pin 2 of the brake light side will be floating until I activate the brakes, but the tail light side is always hot.

Jack

If pins 2 of both LM317s are floating while unactive, you don't need diodes, if not, diodes will be necessary ..

Regards,
IanP

 

[IanP]

If that's the case, you should connect diodes in series with each LM317 current source ..

Regards,
IanP