should i use a resistor, or LM3812 for my tail light

[hizzo3]

I am making an LED tail light for my motorcycle.. I will be using an array of superfluxes (2.14-3.03 VF 70ma), that will PWM controled using a 555 timer.

My aim is highest effacy. My voltage ranges from 12.5 (off) 14.2 (running) 15(at high rpm).

The superflux array will consist of 72 superfluxes in an array of 4 * 18. A 3.3 ohm 2 watt resistor will be place right before the array to control current. I will have the PWM circut operating a transistor (not sure what kind to use) in the line to reduce voltage drop to the array.

I know if i use a LDO LM3812 i should be fine, will i be losing more vs if i used a resistor instead?

also do the LMxxx series go into direct drive if they drop out?

 

[frenzee]

LM3812 is the wrong chip to employ for this application. It converts current to duty cycle in a positive relationship which is probably the opposite of what you are looking for. Besides it's the wrong operating voltage range (2-5.25) and the wrong frequency (12.5-25 Hz), so I doubt it would be much use to you.

What you probably need is a constant current LED driver. National, ON-Semi, Luxdrive, Luxeonics, Catsemi, Linear, Maxim-IC, Melexis, Sipex, TaskLED, Texas Instruments, Xetex and The Sandwich Shoppe are some of the places where you can find LED drivers.

 

[hizzo3]

LM3812 is the wrong chip to employ for this application. It converts current to duty cycle in a positive relationship which is probably the opposite of what you are looking for. Besides it's the wrong operating voltage range (2-5.25) and the wrong frequency (12.5-25 Hz), so I doubt it would be much use to you.

What you probably need is a constant current LED driver. National, ON-Semi, Luxdrive, Luxeonics, Catsemi, Linear, Maxim-IC, Melexis, Sipex, TaskLED, Texas Instruments, Xetex and The Sandwich Shoppe are some of the places where you can find LED drivers.

i thought led drivers were pretty much the same thing.... not sure what you meant by LM3812 is the wrong operating voltage range and frequency... i though all it did was trim off the excess voltage as heat.

Plus i dont need all the electronics in an led driver... i just need something to trim off anything over 12 volts, so that way i have a somewhat steady power source and dont toast the leds.

 

[frenzee]

i thought led drivers were pretty much the same thing....

Ummm...no. There are basically four flavors: Buck, boost, buck/boost and linear. And of course there is the series fixed resisitor setup if you want to call it a "driver". Here's a brief Wikipedia entry: http://en.wikipedia.org/wiki/Switched-mode_power_supply

not sure what you meant by LM3812 is the wrong operating voltage range and frequency... i though all it did was trim off the excess voltage as heat.

Per the **broken link removed**, the Vmax for LM3812 is 5.25V. You put 12V on that chip and you'll basically have a toaster instead. Not sure how you're planning to "trim off" excess voltage. Resistors? Not practical. You have to understand that an LED (and therefore the overall circuit) does not behave like a fixed resistor. For the same fixed voltage you will see significantly variation in current draw depending on temperature and specific properties of each individual LED. That is why the safest way to connect multiple LEDs is in series controlled by a constant current source. That way all the emitters will carry the same exact current and dissipate more or less the same power and that way you will avoid over-drive or under-drive situations.

You CAN use one or more resistors for low power applications, however you have to choose your resisitor value very, very carefully to allow for the full range of voltages you are likely to encounter. Basically the more the voltage variance, the higher the resistor value. Overall, resistors are very inefficient devices when used for this purpose, becasue you have to bleed off a whole lot of the voltage within the resistor in order to minimize the amount of voltage variance in the LED. And of course if you have multople strings to drive, you need to give each of them their own resistor. Running strings of LEDs in parallel is genrally a bad idea becasue of spec variability within the strings. Specifically, if one of the strings happens to have a lower dynamic resistance, it will carry more of the supplied current which will make the LEDs hotter which will reduce their dynamic resistance which will draw even more current, which will make them even hotter and so on. This effect gets magnified even further becasue of the dynamic curve of a typical LED (V vs. I). If you examine that curve you will see that everything is nice and smooth until you reach the Vmax for that specfic LED. Then the current curve takes off like a rocket. For this reason even .1-.2V increase in Vcc could mean the difference between running a nice efficient circuit and smoke coming out of your setup.

On the positive side, resistors are cheap and easy to hook up.

If you must build your own LED driver SMPS, here's a good primer:

**broken link removed**

Here's another good article to peruse:

**broken link removed**

 

[hizzo3]

ok thanks for straiting me out... i wonder if it would just be simpler to use a resistor that will keep the incoming voltage in the vmin-vmax range, with diode protection to keep the nasties out. Then have a transistor set up to control the lighting of the circut from the PWM controler.


I was just thinking i could use the LM3812 to provide a clean 12v source for the leds and then have the PWM controller adjust the brightness... I guess all i really need is a DC-DC converter. Convert the voltage to about 9 volts, and put a small 1.1 ohm resistor(think thats the right one....) to keep them from getting uneven, and have a transistor set up like above....

BTW i was using this datasheet
https://www.electro-tech-online.com/custompdfs/2007/02/LM7805.pdf
according to it, it will burn off the excess and can handle up to 35 volts, with a +1.5 amp draw with proper sinking....