**Newbie starting Project and Needs Help!!

First off I want to let you guys know that I am fairly new at the whole electronics scene so be gentle. /ubbthreads/images/graemlins/grin.gif I need to make a fairly small board filled with 5mm LEDS (roughly 50-100). What would I need to make this work, and how would I wire it? I will eventually be connecting this board to the brake light power supply in my car, so that the board of LEDS light up when the brakes are applied. What kind of resistors would a person need to keep this board of lights from frying? Any wiring ideas or help would be great. Remember that this is new to me but I catch on quickly. I am really stoked about this project so any input is appreciated.

This is similar, only alot smaller.

Did he do that right, or is there a better way to do what I am trying to do?

Since the LEDs don't run on 12v, you need to put them in groups to try and get them as close to 12v as possible

I assume you are going to use RED LEDs, since they are the most cheapest and most eff, they run on approx 2.8v each, therefor - you would need 12/2.8 or 4 of them in series

This comes to approx 11.2v, so you need to "get rid of" approx .8v, actually a little more since most cars system can go as high as 14v

Now, we use good-old Ohms law to calculate the resistor

I=E/R or R=E/I

R(Resistance) = E(voltage) / I(Current)

We need to get rid of approx 1v, and you should always run the typical 5mm LED at 20ma; so the formula would be

R=1/.020
Or
R=50 Ohms

Since 50 ohm resistors might not be available, the next jump should be 52 ohms

You would wire up 4 of the LEDs in a group with one 52 ohm resistor, and connect up that group to your supply - measure the current with a DVM (Digital Volt Meter) on the Current setting and see if you have done your calculations correct, if the current is a little higher than 25ma, increase the resistor - if the current is lower than 20ma and you want more brightness in the LED, lower the value of the resistor

When you have one group of these done, repeat as many times as desired for more brightness

Since you are going to put it in an automobile application, you need to protect it againest "spikes"; although the system runs off the 12v battery - as the starter is turning over the engine and the commutators make/break, there can be as much as 10,000 Volts in your electrical system, and this high voltage could damage the LEDs should you have your foot on the break and start the car; so its best to put a protection device called a Zener Diode across the LED array BACKWARDS - and it will conduct and protect the array againest these surges; a good value would be 16-20v and about 1W

Test your project in stages, get one group working at the correct current and go forward, ALWAYS use a current meter to check the current, don't over drive your LEDs - if you want more light from them, use more LEDs - or brighter LEDs

I agree with Mike's basic method, but have a different idea about some details.

I'm also assuming these are red LEDs, though the link you posted spoke about white. It does make a significant difference. But, again assuming they're normal 5mm red LEDs, I'd think that 2.0 Volts would be a closer approximation to their voltage drop. Do you have a part number for your LEDs? It's best not to guess about the forward voltage drop.

Also, since efficiency is not the major concern here (it's in a car, right?) you should plan to drop a greater percentage of the available 12V supply across the resistor. The voltage dropped here does throw away power, but in a car, who cares about an extra Watt or two? Dropping only 1V as Mike suggested will result in a surprisingly large variation in the LED current as the supply voltage varies from 12-15V.

Assuming that 2.0V is the forward drop, I'd suggest a series-connected string of 4 LEDs, dropping a nominal 8 volts. Assuming a nominal battery voltage of 13.5V, this leaves 5.5V to be dropped across the resistor. R=E/I, so at 20mA, R=5.5V/.02A = 275 ohms (270 is a common value). The resistor power is equal to I^2R or about .1W - so common 1/4W resistors would work out fine.

As Mike suggested, hook as many of these strings together in parallel as you like.

The Zener is a good idea, but there needs to be some series resistance added upstream from it for it to be effective in clamping a large surge. I don't design automotive electrical systems for a living, but I suspect that designing this LED circuit for a 10,000 Volt surge is a bit of overkill.

Do you have a DVM? If so, we can walk you through this. Start by building the string of 4 LEDs and a 270 ohm resistor. Hook it to your battery. Measure the battery voltage and the voltage dropped across the 270 ohm resistor. Report back.

Jim

Yes, the voltage drop for LEDs depends on the color; the longer wavelength devices such as RED can go all the way down to 1.8v - as the wavelength gets shorter (higher frequency), it takes higher voltage - the higher eff RED leds today have a slightly higher voltage, say 2.2v than the original ones

The 10,000v spikes are very short in duration and can happen; on the larger displacement engines - it takes 600-800A to turn them over and these incredibly large currents in the starter can exist when the current is released in the armature windings due to back-emf

The wires themselves will act as inductors to absorb the current clamping effects of the Zener diode

Normal Light bulbs due to their very slow design don't really care about these huge but small spikes, but semiconductor devices such as LEDs can be damaged in much the same way as heavy static electricity

A $1 Zener diode can save a lot of money of LEDs

Super information here!

While I know this wasn't your question, I wanted to make sure you didn't miss out on creating something with a single 1W emitter. There are a few 12V driver options on the market now, making this a VERY simple project. A single 1W is as bright as a handful of the 5mm emitters, so the thing ends up being much smaller.

Here is a brake light I did with two 1W emitters and a single 12V constant-current driver (350mA).

http://www.darelldd.com/ev/rav_brakelight.htm