I am really confused on how to do this. I looked at Bills diagrams and i understand them a little but i have no clue what parts to use at all for mine. I want it to go from 10-16VDC input to 12VDC output with 1 amp still. Please help.

 

OK, back to basics, how many LEDs, what is the Vf, what is the power supply voltage?

I'm going to my youth group meeting (DeMolay, I'm an advisor), but I'll check back.

 

To be able to obtain 12V from a 10-16V source you need a buck-boost switched mode power supply.

What is your application?

 

Sorry i should re-say this because i dont want it to get to complicated. The input will be from 12VDC to around 16VDC (use for car and LEDs) and output the steady 12VDC. This is for the rope lights that i had a post a while back for, so its using 98 LEDs total, 48 on each rope, with each one with around 3.5vf at 20ma.

 

OK, if it is 3.5V Vf, then the total voltage for 3 will be 10.5V. If you use the commercial LM2940CT-X you will get 11.0V at 12V input, if I read that datasheet correctly (and it is hard to read). So we figure it at 12V, as a maximimum voltage.

12V - 10.5V = 1.5V

For 20ma you plug the numbers into ohms law...

1.5V / .02 = 300Ω

Here's the kicker, if I'm right about this chip and it goes to 11.0V with the engine off, then the current goes to...

(11V - 10.5V) / 300Ω = 1.7ma

Not good.

Looks like you're going to have to use 2 LEDs per chain, unless you want to use the current regulator (per leg) I drew up.

The buck boost convertor is looking better all the time, but then, I can't help with one since I haven't built too many of them.

The core problem is the high Vf voltage. If you use the blue LEDs with the 3.8V Vf it only gets worse. Nothing wrong with the diodes per se, but 3 LEDs per chain is just too many for this particular type.

 

I can always change it down to 2 LEDs per chain and have a bigger resistor on it. Im just confused on whats the best part to use for this. Also im not entirely sure what the Vf is of the leds. The site doesn't say what it is or who they are made by. This is what it says: "Super brilliant nice quality blue LED features a see-thru blue lens and operation from 3.8VDC. These produce a nice bright blue light."

 

I understand. That's why a lot of the folks are down on eBay (I'm assuming here).

A contant current source eliminates this issue, but it is a PITA (pain in the assets).

Let's go through some senarios to see if I can help. Remember, when you are designing you go through worst case, which means all the values have to be plugged in.

At the risk of restating the obvious here is the core problem...

0.6VDC just isn't enough to work with worst case. If you design a resistor for 13.7V at 20ma it would be 110Ω. If the voltage drops to 12V then the current drops to 5.5ma. If the Vf of the LEDs is 3.5V then at 13.7 the current would be 29ma, and at 12V it would be 13.6ma.

This constant current regulator could do it...

but it is heavy on parts count. I don't know of any other way to drive 3 LEDs per chain, and mostly eliminate the Vf and variable power source problem.

If you reduce the chain count like so...

then several good things happen. R2 would be 300Ω at 13.7V and 20ma. At 12V the current would be 14.7ma, not to bad a variation. If the Vf of the LEDs were 3.5V the current would be 22.3ma at 13.7V and 16.7ma at 12V. By allowing the resistors to "absorb" more voltage it reduces the extremes in current you saw with three in a chain.

The current regulator shown would work, and would be easy to turn on and off, instead of attaching the 1KΩ resistor to Vcc you could attach it to a control chip, such as a 555.

Hope this helps.

 

Thank you very much bill. It helps alot. Solves a big problem for me.