LED arrays (sorry!)

Hi,

Although this is an FAQ I haven't really found a definative answer.

Basically, I want to make an array of up to 100 5cd white LEDs and power them *in a car*. Since the range could be anything from 10-14 VDC using series-parallel with a resistor is not a good idea I presume?

Jonathon Edelson's pages talk about the LT1618 which seems to be a very good IC, and the MAX1698 which I have ordered some samples of.

The final panels will be flashed using a PIC16F84 microcontroller, which I could program with various patterns and set the speed etc. Has anyone done this already?

I'm doing it for a replica American cop car and was originally doing it for fun and to save a few hundred quid, but I've already had an email from a voluntary ambulance officer wanting to buy some for his vehicles and his wife who's an emergency doctor! I mentioned that they would probably need to be tested and approved (which I wouldn't have a hope in hell of doing) but he didn't seem concerned.

I got the LEDs from Chi-Wing (before seeing this excellent forum) and paid £30/200 and £1.75ea for 10 white and 10 UV keychains - the UV is very cool and I've sold most of them on!

Gareth

Hi,

Jonathan Edelson here The LT1618 would be capable of driving that many LEDs, if you didn't push them all that hard, but you would be right up at the limits of what it could deal with in terms of switch current. But I presume that you are going to have several sets of LEDs, each of which is being flashed.

In this case, I'd suggest using sets of 6 LEDs in series, and then place these sets in parallel to reach the number of LEDs needed for an individually controlled set. Then you can use the adjust pin on the LT1618 in order to control the LEDs.

Another approach is one that Ned Konz is cooking up, using the ATTINY15 microcontroller as the boost controller chip. Basically this 8 pin chip has a 100KHz PWM channel, and he is using this to drive the switch of a boost converter. You could combine the desired control function and the LED drive component in a single chip.

Finally, since you do have lots of power available, you could consider using linear regulation. You can build a simple current regulator using a standard three terminal regulator, but with a current sense resistor between the output and feedback pins. Or you could use one of the Toshiba LED drive chips that are designed to operate multiple strings of LEDs, each with proper current regulation. Marktech Optoelectronics sells these chips.

Regards,
Jon

It would be **MUCH** simpler and reliable if you just connected them up in series groups, say 3 LEDs per group with a dropping resister

My favorite 12V white LED array method is to divide it into 3X3 (3 paralleled strings of 3 LEDs in series) with a 33 Ohm resistor. Repeat as necessary... For one string of 3, use a 100 Ohm resistor, for two strings of 3, use a 47 or 51 Ohm resistor. The idea is to drop about 2V in the resistor.

/ed in NH