Building Simple LED circuit

[RedDiode]

Hello,

Im trying to design a LED lighting kit for a model car. I want to use the following LEDs in it:

4 White Super LEDs
Forward Voltage - 3.4 V Typical (max 4.0)
Forward Current - 30 mA

and 4 Super Red LEDs
Forward Voltage - 2.1 V Typical (max 2.5)
Forward Current - 50 mA

I will be running these of a dedicated NiCD battery pack. These LEDs will be continuously running when on. I would greatly appreciate any design help that anyone could offer me.

Thank you for your time and patience.

 

[blcpro]

Well RedDiode, I only wonder if your battery pack will handle the current demands of the LEDs? What kind of battery pack is it (in terms of Volts and its rated life and max continuous current flow)? I'm sure you want the batteries to last, so you will need to do the math to figure out how much life you expect from your battery, how much power is needed from the other components in your circuit, and how bright do you really need the LEDs to be? Experiment with different current levels with the LEDs (using differend resistance values) to find the lowest current level that produces the light intensity that is acceptable to you. This way you will minimize the power demand from your battery.

 

[RedDiode]

For the battery pack I would say either a 4 or 5 cell, 4.8V - 6V resectfully, at a 1500-2000 mAh capacity. I dont really need the lights to last more than an hour or so between charges. The other only components that would be in the circuit would be those necessary to adjust the voltage for the 2 different LEDs.

Thank you for your time and patience.

 

[peebee]

RedDiode, have you searched this site yet for answers to your question? I swear that someone's asked "how do I power up my LED's" in various ways at least twice a week for the last month or two now.

 

[etarhan]

Looking at the FV ratings you have supplied,I would use a 3-cell battery, in series.

Connect all the LEDs of a color in parallel, connect red LEDs between common and the the junction between the 2nd and 3rd batteries, white cells between the common and +ve of the third. Perhaps a low value resistor for each branch to ease the voltage a bit.

Good luck,
Engin

 

[lcsjk]

I don't think you need to run the lamps at their rated current to achieve a good visual brightness. However if you do want maximum brightness, put a resistor in series with each white led to limit the current and drop the voltage from 4.8 to 3.4 volts. For the red leds, put two in series for 4.2 volts and then add a series resistor to drop the voltage from 4.8 to 4.2. I am not sure if a 6 volt battery will give longer operating time or just produce more heat.
The resistor value is R=V/I where V is the voltage across the resistor. V = (Vbatt - Vled) The resistor power is V x I where I is the current through the resistor. Choose a resistor with twice the power rating as the calculated power above. That will keep it cooler, and give the resistor some air to dissipate the heat.

 

[Heydave]

There are several new chips on the market to power LEDS and I would suggest looking at using one. The available voltage drop for the white LEDS seems a little low to make a reliable current sink or even use a dropping resistor. With the chip, you can run all the LEDS in series and save a lot of battery power. The control of the LED current will prevent too much current to the LEDs and maintain constant brightness as the batteries discharge. If you run the LEDs at 20ma, you should be able to get 25+ hours out of 4 AA NiCd cells. You might consider going to 2 cells.
Try Maxim, TI, etc. for LED driver chips, application notes and to request samples. This technology is rapidly growing right now.