Hey guys,

I am about to install my custom projectors with a led ring around the projector housing. I'm not sure as to how to wire them. Do I wire the positive leg of the LED in parallel and the ground leg in parallel? How many resistors do I need for 12 LEDs, and how do I wire them together with the resistors? Please also explain the type of rating of the resistors I should use.

 

you should wire them in series... find out the rated voltage drop of the LEDs, then wire them in series (positive of one, to the negative of another, and keep going, after doing this, then at each end, one led should have a negative not connected, and the other end the positive should not be connected). Find out the V drop and then do this... if the Vdrop was 3.6V, then divide 14.4V by 3.6V, which is 4. I say 14.4 b/c in cold weather, when you first start, alternators can put out 14.4, and you don't want to have too much current go through them...

If you do the calculations and you can't come up with a number that divides evenly, you will have to do a resistor... here is a site that explains it...

http://www.theledlight.com/ledcircuits.html

 

I used LEDs and didn't use resistors. The LEDs didn't work with the resistors in place so I took them out.

 

yeah, i think most LEDs are good for around 3-5 volts w/o resistors. 4 in series would be just right. if you have 12, you need to make 3 banks of 4 resistors. then each bank can be wired in parallel to a 12v source.

 

it just totally depends on how many LEDs, and what the voltage drop across the LED is.

If you put two LEDs in series, that each have a voltage drop of 3.6V, you would kill them if you hooked them straight to your car voltage.... the current would be way to high... on the same note, if you took five 3.6V leds in series, and wired it to your car voltage, you would not be getting as much light out of them as you could be.

on a side note... Check out Luxeon LEDs. I have a flashlight I made using a 5watt white one. 120 Lumens! A 4D maglite is rated at 60 lumens. Most LEDs they rate in mcds, not lumens. The 5watt LED has a Vdrop of about 6.8V or so, and max current is 700ma... most LEDs are 30-40ma.

Idealy, you wouldn't use resistors to drive LEDs, you would build a current regulator circuit, especially since the voltage on your car can easily be 12.2-14.4. Sometimes, a .5V change, can easily change the current by at least 50%, so if you calculate it out for 12.1V (about what your car would put out not running) and then on a cold morning your alternator puts out 14.4 volts, you could kill the LEDs pretty quickly... but anyhow... it all just depends on the design of the LED. Luxeons for example, if you just used a 6.8V battery, the LED would have about 700mA through it (assuming internal battery resistance of 0ohms), but if it drops to 6.5V, it would only have 400mA through it, and at 400ma, it has about 60% of the light output than 700mA.

take a look at the spec sheet for the Luxeon 5watt and note the last page...
http://elektrolumens.com/pdf_files/L...r_5W_Specs.pdf

BigD, it all just depends on the resistor value you used, the LEDs you used, and how many of them... if you used a 470 ohm resistor with two 5volt blue LEDs, yeah, it probably didnt work.

btw, the current in a series circuit is the same through all elements regardless of the voltage drop across them.

LEDs are not really a linear device like a resistor, the only linear area of the voltage/current relationship is in their operating range, once you go out of this range, very small changes in V can cause a very large change in current... don't get me wrong, there is nothing wrong with using resistors to wire up LEDs... It's how I usually do it as well, just as long as you assume at least 14V or so, and of course it's always easiest if you just use an amount of LEDs that makes it so you don't even have to use a resistor...

 

Yes, thank you. I know how it all works. I did a few things where I followd instructions given on the ORG. When I followed the instructions it didn't work. Took out the resistors and it worked fine.

 

hmm.... who knows.... just curious, what did you use them for?

 

Window switches and for the climate control.