car interior leds

[doctaq]

i am trying to make a very simple circuit to power leds for cabin lighting, something of the 5mm variety. what kind of parts and how many leds should i use? i already have the leds, 3.3vf and 20ma. i dont know how much a car voltage will vary, i tried 4 leds in series because i was under the impression that car voltage can rise as high as 14v is that true? it flickered a lot, i dont know if it is because of poor wiring or if the voltage was simply not enough at the low end of 11v? i have not tested the lower limit of this led yet but should i expect flickering?
i dont think there is enough room to put any transistors in but if it is a must i will find a way

i cant seem to find how to use a transistor to drop voltage for an led, i already have npn transistors and can buy resistors, thanks

 

[Steve K]

I think that the alternator can float charge the battery as high as 14.4v, but there can be a large variation in the battery voltage depending on whether or not the engine is running or what other electrical devices are turned on. The rule of thumb that I've seen for a 12v lead-acid battery is to assume that the voltage can range from 9v to 18v.

There are other issues to be concerned with too. There can be positive and negative voltage spikes that are quite significant. At a minimum, I'd recommend a diode in series with the power wire to block negative voltages (this is also handy in case someone hooks up the battery cables in reverse). A diode like the common 1N4001 should be adequate. Some filter caps to help absorb the voltage spikes would be good too. Maybe something as simple as 100uF, 35v (aluminum electrolytic capacitor) in parallel with a 0.1uF ceramic capacitor? The 100uF cap may be overkill, though.

You'll need something to regulate the current in the leds. A simple circuit like this has been handy for a lot of stuff:

http://www.flickr.com/photos/kurtsj00/4815596574/
(sorry, I forgot how to insert a photo)

quick design equations:
I_reg is the regulated LED current.
V_batt is the battery voltage (9v minimum, 18v max)
R2 = 0.65/I_reg.
R1 = (V_batt - 1.4)/(I_reg/20), where V_batt is the minimum battery voltage.

For a 20mA current, I get R2 = 32 ohms. R1 = 7.6k ohms.
For the sake of getting standard value resistors, I'd probably use 33 ohms for R2, and 6.8k ohms for R1.

regards,
Steve K.

 

[Nitroz]

I used these to make dome lights and floor lights and used old XR-E LEDs for some bright lighting in my car.

Good Luck!

 

[Alaric Darconville]

http://www.flickr.com/photos/kurtsj00/4815596574/
(sorry, I forgot how to insert a photo)

[IMG]http://farm5.static.flickr.com/4114/4815596574_d5dbd60981.jpg[/IMG]
(Creative use of "bold" to break the UBB code, allowing the code to be pasted in its entirety.) Essentially, use IMG and /IMG in square brackets surrounding the URL of the image.

Done correctly results in this:

 

[Random Guy]

Alaric, you can also use the noparse tags. [noparse]stuff[/noparse]. Used correctly, they do this: [IMG]http://farm5.static.flickr.com/4114/4815596574_d5dbd60981.jpg[/IMG]

 

[Alaric Darconville]

Alaric, you can also use the noparse tags. [noparse]stuff[/noparse]. Used correctly, they do this:
[IMG]http://farm5.static.flickr.com/4114/4815596574_d5dbd60981.jpg[/IMG]

Oh, sweet! There I was thinking "outside the box" but they made tags specifically for that (I guess so then people could give "instructions" like that...)

That'll come in handy for sure!

 

[Steve K]

okay, as long as this thread has derailed into a HTML class... is there a good resource on the web to explain all of these details to a clueless person like me?

thanks,
Steve K.

 

[kingofwylietx]

Great, now I have to go Google 'parse'

 

[doctaq]

thanks guys, i'm going to give it a try tonight

 

[CPF Dude]

Here's how I would do it...

Figure out how many LEDs you want (based on locations where you want illumination, etc), colors, and brightness, then pick your LEDs. Seems you've done that already.

Each LED with need a series resistor, or you can wire up to a few LEDs in a series with one resistor per "string" or series LEDs. If you have more than a few LEDs, you just need to parallel the strings. You adjust that resistor value to match the LED current (leaving a slight buffer just in case).

As SteveK mentions, use a series diode for reverse protection.

During charging, cars usually get up to 13.5 to 13.8 V, but I calculate to 14V. Using your 3.3Vf, with one LED, you get 14V - 0.7V (typical for a basic rectifier diode) - 3.3V (Vf for one LED) = 10.0V. If the LED says 20mA typical, I would drop that by 10% and use 18mA. R=V/I, so 10.0/0.018 = 556ohms. Closest common is 560ohms. Note that the resistor drops 10V, so power through the resistor is 10 x 0.018 = 0.18W, so you should use a 1/4-Watt resistor here.

Is you use 2 resistors in this string, then you get 14 - 0.7 - 3.3 - 3.3 = 6.7V (dropped by the resistor), so R = 372ohms, or I'd just use a 390ohm resistor here, and power works out to 0.12W, so you can get away with a 1/8W resistor.

The voltage can drop to about 12V in certain conditions, or down to 10V or less during cranking (but are you really worried about your LED brightness at that point?). With the resistors calculated above, do the math again for 12V and see what the current is, and you'll see how much brightness variation there will be. The more voltage the resistor drops (compared to the LEDs), the greater the current variation (and hence brightness variation) as the car's voltage varies. But you get to use less resistors this way.

Yes, you can use a regulator to regulate the voltage, or better, use a current regulator. I don't bother unless I'm doing a lot of LEDs, and want to not waste so much power in heat. If I do, I'd use a LM317 current regulator, and the datasheet shows how to calculate the values of that.

BTW, yes I know there's an absolute maximum current for the LED which will be above the typical current, but that's in perfect conditions, and a car can get quite hot.

Cheers,
-Neil.