LED's in PCB project - using different LED's

Heya,

I've attached a schematic which is based on what I think you're asking.
I'm assuming you're working with +12Vdc... and I'm not sure what kind of forward current the LEDs you're using should be run at, but those resistor values will give you ~20mA. Depending on the LED, it may rise to ~25mA when everything has reached a thermal stasis. All depends on the LEDs, power consumption, etc. You can monitor current and adjust the resistors as needed. I'd use 1/4W 1% metal film (if through hole) resistors, but you can probably get away w/ 1/8W 5% if you want to go super cheap.

Correct, you will need to drop more voltage across the resistor on the red/red/blue leg.

Things to keep in mind though: LEDs, by nature, should be driven with a constant current since their forward voltages are so varied (even between different lots of the same LED.) This can't always be feasible, such as when all you have is the 12Vdc system in the car. But you can try to mitigate it...

The voltage requirements (referred to as forward voltage) for a certain forward current of the LEDs do change based on a couple of factors.

1.) Junction temperature: The warmer the LED gets, the Vf will start to drop a little, which equates to a rise in forward current, (in this constant voltage example.)
2.) Individual die construction: LEDs usually come in Vf bins, some chemistries require less Vf, some more. This is most apparent in different colors (in this case red vs. blue.) But different individual die constructions will normally have a small difference in Vf even in the same LED between lots.

Hope this isn't too much information, it probably won't be critical. Just make sure you stay under the LEDs max rated current and junction temperature, and they should last forever.

 

Aaah, ok. Makes more sense now.

The squares on your board would be anode, circles cathode. These are... 3mm or 5mm through-hole type LEDs? The other pads are for the resistors. (For some reason I had surface mount LEDs in mind, like the Nichia NESB064.)

The prudent thing to do is plan for the lowest Vf possible, based on the data sheet.
Blue = 3.0Vdc
Red = 2.0 Vdc

So for the 3 blues in series: 3.0 + 3.0 + 3.0 = 9.0Vdc You need to drop the other 3.0V across the resistor. I'm still going to assume 20mA... so 3V / 20mA = 150 Ohms.

2 blue + 1 red in series: 3.0 + 3.0 + 2.0 = 8.0Vdc ... 4V / 20mA = 200 Ohms.

Substitute whatever current you're planning on running these at.

Power dissipated in the resistors:
150 Ohms... 20mA * 3V = 0.06W (Use an 1/8W or 1/4W resistor)
200 Ohms... 20mA * 4V = 0.08W (Use a 1/4W resistor)

1% tolerance metal film resistors should dial in your current pretty close. (Again, all depends on the nuances of the LEDs AND the +12Vdc tolerance. Most supplies will have a +/- 5% or so tolerance, not sure about in the car though.)

If you wanted to get really fancy, you could probably use LM317 (or similar) regulators set up to regulate the current in each leg, but that may be overkill.

 

Using the LM317 should be pretty easy... one for each leg. (They should be pretty cheap in a low power package.) Use the LM317LZ-AP for instance. Should be something like $0.50 a pop. They will do 100mA max, something like 40V max input.

You'd have to re-do your board, obviously, but it would definitely help with the huge tolerance of the 12V in the car. LEDs don't respond well to large increases in Vf (even half a volt will usually shoot the forward current sky high.)

See attached schematic. (If I'm thinking straight, that's all you'd need.) Shouldn't need to worry about filtering the inputs on the LM317's.

You should be able to find 1% metal film resistors in 62 Ohms pretty easily, maybe not from The Shack, but they are out there. A run of the mill 5% carbon film should be OK if you can't find the 1% guys. I'd go higher with the resistance than lower, if you can't find 62 Ohms.

62 Ohms will run the LM317 at ~20mA. (1.25V / 20mA = 62.5) Again, no idea if you're running the LEDs at 20mA, 30mA, etc... alter formulas to taste, minding max If of the LED.

Once you get this finished you should post pics! I can imagine it will look really nice.

Edit: Oh, one more thing... I'm not sure what kind of clearance you have in the Z dimension (from the PCB to the dial's face.) Using the through-hole 3mm LEDs, those are semi-tall. If they just fit behind the dials, you're going to end up with spotting. I'm not sure if you've got that all sorted out already, just thought I'd bring it up in case.

 

Hehe, smd components are the way to go for a lot of reasons. Though only you can decide what type of light output works for you in this case.

As for regulating the 12V, that is possible, but you may have to find a low drop out (LDO) regulator since the voltage overhead on the 317's is a volt or two. If the car's supply goes below 12V, it's another issue.

Best bet if going that way would be to regulate a 10 or 10.5V output anyhow, why waste watts when you don't need the whole 12v for the LED strings. Then you'd set the current limiting resistors based on that voltage. Keep in mind you'd need a regulator rated for a higher current than the ones from my previous post. 200mA plus. The LEDs will draw about 100mA in total. This has the benefit of not needing a board re-design, but now that you're regulating voltage you may need to worry about the filtering caps on the input/output if the supply is a good distance from the regulators, to keep a nice regulated level output.

Even then, it's always best to run LEDs with a regulated current.

At these low power levels, heat shouldn't be an issue.