hi guys wanted to design a led lights as seen on the bmws headlamp maybe using 15 to 20 leds the maximum voltage across each led is 3.5v and current is 2.5mA and the battery will be a 12v battery plz can anyone draw a circuit plz plz
Are you sure each LED only has 2.5mA going through it? That sounds really low. 25mA would be a more reasonable number. At any rate, you could get away with placing in parallel several strings of 3 LEDs and one resistor. Like this:
You can determine the size of each resistor by using Ohm's Law. You'll need to know Vf (the forward voltage drop) of the LED. It varies between LED colors and is also a function of current. The 3.5V you mentioned sounds like a pretty good number for white LEDs but you can get a more accurate value by either reading the data sheet or measuring it. Then you'll use the equation:
R = (14.5 - 3(Vf))/.025A
You can add more strings of LEDs in parallel or take away some to make the light brighter or dimmer. You could also adjust the value of R to make the LEDs brighter or dimmer. A lower R would allow more current to flow making the LEDs brighter while a higher value of R would have the opposite effect. Just don't exceed the maximum continuous current specified in the data sheet.
The current regulator for those very expensive, very bright, high-power LEDs is much more complicated than a single resistor. Also, the LEDs in question are rated at Amps, not mA like the puny ones used as indicator lamps.
The current regulator for those very expensive, very bright, high-power LEDs is much more complicated than a single resistor. Also, the LEDs in question are rated at Amps, not mA like the puny ones used as indicator lamps.
I don't have any experience with the high power LEDs Mike is talking about. The circuit I described will physically work but I don't know how similar it will look to the stock headlights. Maybe something like **broken link removed** is more of a step in the right direction even though it seems it cant provide 2A+.
Try using a simple op-amp or single MOSFET current source. By far the cheapest and easiest way for a constant current. The current capacity is limited to the size of the MOSFET used. Here is a current sink circuit:
**broken link removed**
The bottom resistor is an Rsense. Just pick a low value that will have the transistor regulate the gate at the current you want. I have used one IRFP250 for a 10A supply before.